LT8645S, LT8646S Datasheet by Analog Devices Inc.

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ANALOG DEVICES \ & POWEHEY LINEAR“ LT86458/ LT8é4éS w 5 av T0 55v V'" 4 NF EN/UV 41 2k US$455 sw BIAS V5w=IMHz 2 2w Vnur 5v InfluF EFFICIENCY (m Inn 95 an EFHCIENCV 85 an MA) ssm uaMoa 75 POWER Loss \ a 70 55 — IMHZ.L=22uH 05 —- ZMHZ.L=|UH 50 V 2 3 4 5 E 7 E LOAD CURRENT (A)
LT8645S/LT8646S
1
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL APPLICATION
FEATURES DESCRIPTION
65V, 8A Synchronous
Step-Down Silent Switcher 2
with 2.5µA Quiescent Current
The LT
®
8645S/LT8646S synchronous step-down regulator
features second generation Silent Switcher architecture
designed to minimize EMI/EMC emissions while delivering
high efficiency at high switching frequencies. This includes
the integration of bypass capacitors to optimize all the fast
current loops inside and make it easy to achieve advertised
EMI performance by eliminating layout sensitivity. This
performance makes the LT8645S/LT8646S ideal for noise
sensitive applications and environments.
The fast, clean, low-overshoot switching edges enable
high efficiency operation even at high switching frequen-
cies, leading to a small overall solution size. Peak current
mode control with a 40ns minimum on-time allows high
step-down ratios even at high switching frequencies. The
LT8646S has external compensation via the VC pin to en-
able current sharing and fast transient response at high
switching frequencies. A CLKOUT pin enables synchroniz-
ing other regulators to the LT8645S/LT8646S.
Burst Mode
®
operation enables ultralow standby current
consumption, pulse-skipping mode allows full switching
frequency at lower output loads, or spread spectrum
operation can further reduce EMI/EMC emissions.
5V 8A Step-Down Converter
APPLICATIONS
n Silent Switcher
®
2 Architecture
n Ultralow EMI/EMC Emissions on Any PCB
n Eliminates PCB Layout Sensitivity
n Internal Bypass Capacitors Reduce Radiated EMI
n Optional Spread Spectrum Modulation
n High Efficiency at High Frequency
n Up to 95% Efficiency at 1MHz, 12VIN to 5VOUT
n Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT
n Wide Input Voltage Range: 3.4V to 65V
n Ultralow Quiescent Current Burst Mode Operation
n 2.5μA IQ Regulating 12VIN to 3.3VOUT (LT8645S)
n Output Ripple < 10mVP-P
n External Compensation: Fast Transient Response
and Current Sharing (LT8646S)
n Fast Minimum Switch On-Time: 40ns
n Low Dropout Under All Conditions: 60mV at 1A
n Adjustable and Synchronizable: 200kHz to 2.2MHz
n Peak Current Mode Operation
n Output Soft-Start and Tracking
n Small 32-Lead 6mm × 4mm LQFN Package
n Automotive and Industrial Supplies
n General Purpose Step-Down
n GSM Power Supplies
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
12VIN to 5VOUT Efficiency
LT8645S
8645S TA01a
SW
BIAS
FB
V
IN
5.5V TO 65V
RT
V
IN
EN/UV
V
OUT
5V
8A
100µF
2.2pF
4.7µF
41.2k
1M
243k
2.2µH
f
SW
= 1MHz
EFFICIENCY
POWER LOSS
1MHz, L = 2.2µH
2MHz, L = 1µH
LOAD CURRENT (A)
1
2
3
4
5
6
7
8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFICIENCY (%)
POWER LOSS (W)
8645S TA01b
LT86458/ LT86468 mrwzw m vuzw
LT8645S/LT8646S
2
8645Sfa
For more information www.linear.com/LT8645S
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
VIN, EN/UV ................................................................65V
PG .............................................................................42V
BIAS ..........................................................................25V
FB, TR/SS . .................................................................4V
SYNC/MODE Voltage . ................................................6V
(Note 1)
ORDER INFORMATION
LT8645S LT8646S
LQFN PACKAGE
32-LEAD (6mm × 4mm × 0.94mm)
TOP VIEW
11 12 13 14 15 16
32 31 30 29 28 27
21
17
18
19
20
26
22
23
24
25
6
10
9
8
7
1
5
4
3
2
VIN
GND
GND
GND
NC
BIAS
VIN
VIN
NC
INTV
CC
VIN
GND
GND
GND
NC
RT
VIN
VIN
NC
EN/UV
33
GND
34
GND
35
GND
36
GND
37
GND
38
GND
BST
SW
SW
SW
SW
SW
FB
PG
GND
TR/SS
SYNC/MODE
CLKOUT
JEDEC BOARD: θJA = 31°C/W, θJC(PAD) = 6°C/W (NOTE 3)
DEMO BOARD: θJA = 21°C/W
EXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
LQFN PACKAGE
32-LEAD (6mm × 4mm × 0.94mm)
TOP VIEW
11 12 13 14 15 16
32 31 30 29 28 27
21
17
18
19
20
26
22
23
24
25
6
10
9
8
7
1
5
4
3
2
VIN
GND
GND
GND
NC
BIAS
VIN
VIN
NC
INTV
CC
VIN
GND
GND
GND
NC
RT
VIN
VIN
NC
EN/UV
33
GND
34
GND
35
GND
36
GND
37
GND
38
GND
BST
SW
SW
SW
SW
SW
FB
PG
VC
TR/SS
SYNC/MODE
CLKOUT
JEDEC BOARD: θJA = 31°C/W, θJC(PAD) = 6°C/W (NOTE 3)
DEMO BOARD: θJA = 21°C/W
EXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
http://www.linear.com/product/LT8645S#orderinfo
PART NUMBER PART MARKING* FINISH CODE PAD FINISH PACKAGE TYPE**
MSL
RATING TEMPERATURE RANGE
LT8645SEV#PBF 8645SV
e4 Au (RoHS) LQFN (Laminate Package
with QFN Footprint) 3 –40°C to 125°C
LT8645SIV#PBF
LT8646SEV#PBF 8646SV
LT8646SIV#PBF
Consult Marketing for parts specified with wider operating temperature
ranges. *Device temperature grade is indicated by a label on the shipping
container.
Pad finish code is per IPC/JEDEC J-STD-609.
Terminal Finish Part Marking: www.linear.com/leadfree
Recommended PCB Assembly and Manufacturing Procedures:
www.linear.com/umodule/pcbassembly
Package and Tray Drawings: www.linear.com/packaging
Parts ending with PBF are RoHS and WEEE compliant. **The LT8645S/LT8646S package has the same footprint as a standard 6mm × 4mm QFN Package.
Operating Junction Temperature Range (Note 2)
LT8645SE/LT8646SE ......................... 40°C to 125°C
LT8645SI/LT8646SI........................... –40°C to 125°C
Storage Temperature Range .................. 65°C to 150°C
Maximum Reflow (Package Body) Temperature ... 260°C
LT86458/ LT86468
LT8645S/LT8646S
3
8645Sfa
For more information www.linear.com/LT8645S
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l3.0 3.4 V
VIN Quiescent Current in Shutdown VEN/UV = 0V
l
0.9
0.9
3
10
µA
µA
LT8645S VIN Quiescent Current in Sleep
(Internal Compensation)
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V
l
1.7
1.7
4
10
µA
µA
LT8646S VIN Quiescent Current in Sleep
(External Compensation)
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 0V
l
230
230
290
340
µA
µA
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 5V 16 25 µA
LT8646S BIAS Quiescent Current in Sleep VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 5V 200 260 µA
LT8645S VIN Quiescent Current when Active VEN/UV = 2V, VFB > 0.97V, VSYNC = 2V, RT = 60.4k, VBIAS = 0V 0.4 0.6 mA
LT8646S VIN Quiescent Current when Active VEN/UV = 2V, VFB > 0.97V, VSYNC = 2V, RT = 60.4k, VBIAS = 0V 0.6 0.8 mA
LT8645S VIN Current in Regulation VOUT = 0.97V, VIN = 6V, ILOAD = 100µA, VSYNC = 0V
VOUT = 0.97V, VIN = 6V, ILOAD = 1mA, VSYNC = 0V
l
l
17
200
60
400
µA
µA
Feedback Reference Voltage VIN = 6V
VIN = 6V
l
0.964
0.958
0.970
0.970
0.976
0.982
V
V
Feedback Voltage Line Regulation VIN = 4.0V to 42V l0.004 0.025 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
LT8646S Error Amp Transconductance VC = 1.25V 1.7 mS
LT8646S Error Amp Gain 350 V/V
LT8646S VC Source Current VFB = 0.77V, VC = 1.25V 350 µA
LT8646S VC Sink Current VFB = 1.17V, VC = 1.25V 350 µA
LT8646S VC Pin to Switch Current Gain 8 A/V
LT8646S VC Clamp Voltage 2.6 V
BIAS Pin Current Consumption VBIAS = 3.3V, fSW = 2MHz 22 mA
Minimum On-Time ILOAD = 2A, SYNC = 0V
ILOAD = 2A, SYNC = 2V
l
l
40
35
65
60
ns
ns
Minimum Off-Time 80 110 ns
Oscillator Frequency RT = 221k
RT = 60.4k
RT = 18.2k
l
l
l
180
665
1.8
210
700
1.95
240
735
2.1
kHz
kHz
MHz
Top Power NMOS On-Resistance ISW = 1A 36
Top Power NMOS Current Limit l10.5 14 17.5 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 25
Bottom Power NMOS Current Limit VINTVCC = 3.4V 8.5 11 13.5 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l0.95 1.01 1.07 V
EN/UV Pin Hysteresis 45 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l5 7.5 10 %
PG Lower Threshold Offset from VFB VFB Rising l–10.5 –8 –5.5 %
LT86458/ LT86468
LT8645S/LT8646S
4
8645Sfa
For more information www.linear.com/LT8645S
PARAMETER CONDITIONS MIN TYP MAX UNITS
PG Hysteresis 0.4 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l750 2000 Ω
SYNC/MODE Threshold SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
l
l
l
0.7
2.2
0.9
1.2
2.55
1.4
2.9
V
V
V
Spread Spectrum Modulation
FrequencyRange
RT = 60.4k, VSYNC = 3.3V 24 %
Spread Spectrum Modulation Frequency VSYNC = 3.3V 2.5 kHz
TR/SS Source Current l1.2 1.9 2.6 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 220 Ω
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT8645SE/LT8646SE is guaranteed to meet performance
specifications from 0°C to 125°C junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization, and correlation with statistical process controls.
The LT8645SI/LT8646SI is guaranteed over the full –40°C to 125°C
operating junction temperature range.
The junction temperature (TJ, in °C) is calculated from the ambient
temperature (TA in °C) and power dissipation (PD, in Watts) according to
the formula:
TJ = TA + (PD • θJA)
where θJA (in °C/W) is the package thermal impedance.
Note 3: θ values determined per JEDEC 51-7, 51-12. See Applications
Information Section for information on improving the thermal resistance
and for actual temperature measurements of a demo board in typical
operating conditions.
Note 4: This IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
LT86458/ LT864éS mu EEELCLENCV T EFFICIENCV EEELCL ENCY an 75 (M) 55m {BMW 7O 55 5O 123656 00 EEFLCLENCV o E 20% 15:2 5 ‘2' ‘ ./ —vm=12v 5° ,-_ ’/ — vm=24v "3 55 ' vm=asv 04 v =45v 50 PDWERLOSS IN a u T 2 a 4 5 e 7 a LOADCURRENnA) we so so so 50 40 an szzmflkHz L = WE’LHMWEU 4 m e um OT T m ma mm) LOAD CURRENT LmAp 1 EFH CL ENCV EFFICIENCY On A u 95 3 5 9n — ‘ 3 u EEELCLENCV 2 5 an 2 u 75 I 5 POWER L055 7n L=XELBDGO In — SDUkHz L=27UH -1MH1L=15UH 55 O5 — — ZMHz L=082UH 7 5 5n 12345 LOAD CURRENT (A) 5 mu 5 550 E50 40 3° V5w=SDDkHz 20 szErLHMWDSDJNH Om OT T m ma mm) LOAD CURRENT LmAp mu — v.N:T2v —— vmzzw an 20 W: snow L=WE7LHMW§D 47w ‘0 C1 1 ‘0 mn man LOAD CHRRENNmA) % EFH CL ENCV mo 95 EEEICLENCV SDUkHz , ELsuso 2 7UH‘» 75 7o ' 55 so 55 50 POWER Loss 7 z 3 A 5 LOAD CURRENT (A) mu EFFICIENCY (M st : snow L = WE’LHMWEU 4 m TD mu man LOAD CURRENT (MA) EEEIcIENUv vaupaav IL 972A 82 DA 7 L = xELeueu‘ 4 7y" so 04 07 T 13 15 19 SWITCHWG EREoUENCV (MHZ) 22 5
LT8645S/LT8646S
5
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL PERFORMANCE CHARACTERISTICS
12VIN to 5VOUT Efficiency
vs Frequency
12VIN to 3.3VOUT Efficiency
vs Frequency Efficiency at 5VOUT
Efficiency at 3.3VOUT
LT8645S Low Load Efficiency
at 5VOUT
LT8645S Low Load Efficiency at
3.3VOUT Efficiency vs Frequency
LT8646S Low Load Efficiency
at 5VOUT
LT8646S Low Load Efficiency at
3.3VOUT
EFFICIENCY
POWER LOSS
L = XEL6060
500kHz, L = 2.7µH
1MHz, L = 2.2µH
2MHz, L = 1µH
LOAD CURRENT (A)
1
2
3
4
5
6
7
8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFICIENCY (%)
POWER LOSS (W)
8645S G01
EFFICIENCY
POWER LOSS
L = XEL6060
500kHz, L = 2.7µH
1MHz, L = 1.5µH
2MHz, L = 0.82µH
LOAD CURRENT (A)
1
2
3
4
5
6
7
8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFICIENCY (%)
POWER LOSS (W)
8645S G02
f
SW
= 500kHz
L = XEL6060, 2.7µH
EFFICIENCY
POWER LOSS
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
50
55
60
65
70
75
80
85
90
95
100
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
EFFICIENCY (%)
POWER LOSS (W)
8645S G03
f
SW
= 500kHz
L = XEL6060, 2.7µH
EFFICIENCY
POWER LOSS
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
50
55
60
65
70
75
80
85
90
95
100
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
EFFICIENCY (%)
POWER LOSS (W)
8645S G04
V
OUT
= 3.3V
I
LOAD
= 2A
L = XEL6060, 4.7µH
V
IN
= 12V
V
IN
= 24V
SWITCHING FREQUENCY (MHz)
0.4
0.7
1
1.3
1.6
1.9
2.2
80
82
84
86
88
90
92
94
96
EFFICIENCY (%)
8645S G09
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (mA)
0.01
0.1
1
10
100
1000
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8645S G05
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (mA)
0.1
1
10
100
1000
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
OUT
8645S G06
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (mA)
0.01
0.1
1
10
100
1000
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8645S G07
f
SW
= 500kHz
L = WE–LHMI7050, 4.7µH
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
LOAD CURRENT (mA)
0.1
1
10
100
1000
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
LT8646S Low Load Efficiency at 3.3V
OUT
8645S G08
LT86458/ LT864éS EFFICIENCH Vow 5V Tum, : Tom L = WE’LHMWUEU I 2 3 4 5 5 7 E 9 ID INDUCTOR VALUE (um e5 020 (HS um un5 CHANGE IN vnmm 70 n5 70m 0T234557E LOAD CURRENT (A) 030 025 020 f a S 2 S u in 05 in To 5 I5 25 35 45 55 E5 INPUT VOLTAGE (VI 979 REFERENCE VOLTAGE (va 725 n 25 5D 75 Too I25 TEMPERATURE(“C) 0 I 2 3 4 5 E 7 LOAD CHRRENT (A) vmza 3v L: a 75H INVREGULATION INPUT CURRENT (UAI n In 2a an an 50 an INPUT VOLTAGE (V) I03 T02 EN THRESHOLD (VI 095 u 95 750 EN RISING EN FALLING 725 u 25 50 T5 Inn T25 TEMPERATURE 1°C) n20 0T5 0T0 005 nun é” a S 2 S in 05 70m 225 200 INRUT CURRENUUA) I5 25 35 45 55 E5 INPUT VOLTAGE (VI v0“: 5v L = 4 TIM IN REGULATION V0 20 30 40 50 50 INPUT VOLTAGE (V) 6
LT8645S/LT8646S
6
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL PERFORMANCE CHARACTERISTICS
EN Pin Thresholds
Reference Voltage
LT8645S Load Regulation LT8645S Line Regulation
LT8645S No-Load Supply Current
Burst Mode Operation Efficiency
vs Inductor Value (LT8645S)
LT8646S Load Regulation
LT8646S Line Regulation LT8646S No-Load Supply Current
V
OUT
= 5V
I
LOAD
= 10mA
L = WE–LHMI7050
V
IN
= 12V
V
IN
= 24V
INDUCTOR VALUE (µH)
1
2
3
4
5
6
7
8
9
10
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8645S G10
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
961
963
965
967
969
971
973
975
977
979
REFERENCE VOLTAGE (mV)
8645S G11
EN RISING
EN FALLING
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
1.03
EN THRESHOLD (V)
8645S G12
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
–0.10
–0.05
0
0.05
0.10
0.15
0.20
CHANGE IN V
OUT
(%)
8645S G13
V
OUT
= 5V
V
IN
= 12V
V
SYNC
= 0V
V
OUT
= 5V
V
IN
= 12V
V
SYNC
= 0V
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
–0.50
–0.40
–0.30
–0.20
–0.10
0.00
0.10
0.20
0.30
0.40
CHANGE IN V
OUT
(%)
8645S G14
V
OUT
= 5V
I
LOAD
= 2A
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
–0.10
–0.05
0.00
0.05
0.10
0.15
0.20
CHANGE IN V
OUT
(%)
8645S G15
V
OUT
= 5V
I
LOAD
= 2A
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
–0.10
–0.05
0
0.05
0.10
0.15
0.20
0.25
0.30
CHANGE IN V
OUT
(%)
LT8646S Line Regulation
8645S G16
V
OUT
= 5V
L = 4.7µH
IN REGULATION
INPUT VOLTAGE (V)
0
10
20
30
40
50
60
25
50
75
100
125
150
175
200
225
INPUT CURRENT (µA)
8645S G18
V
OUT
= 3.3V
L = 4.7µH
IN-REGULATION
INPUT VOLTAGE (V)
0
10
20
30
40
50
60
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
INPUT CURRENT (µA)
8645S G17
LT86458/ LT8é4éS T50 T45 T40 T35 T30 T25 CURRENTLIMIT (A) T20 H5 TTn DI 350 03 05 DUTY CVCLE 07 09 300 I TOP SWTTCH SWITCH DROP (mV BOTTOM SWITCH 740 730 Hz) 5 720 SWITCHING FREQUENCY 550 750 725 2 3 4 5 5 T SWITCHCURRENTLA) n 25 50 75 TEMPERATURE(“C) T00 I25 DRUPUUT VOLTAGE (mV) SWITCHING FREUUENCV (kHz) I5 I00 SWITCH CURRENT : TA 00 I5 A s E 5%00 é 00 777777 3 s 7 E II E. / w 5 40 TOP SWITCH / I H I _ 8 E / // / / I3 / / BDTIOM sme 20 / I2 0 750 725 0 25 50 75 T00 T25 750 725 0 25 50 75 T00 T25 TEMPERATURE m TEMPERATURE (no) mo v w 50 W = — BURST M00E OPERATTDN 350 MT SET TO REGULATEAT 5V — PULSESKIPPING MODE szELe0e0T NR 45 300 a 250 g 4a 200 S E 35 / / I50 2 a Inn 5 30 lump: 3A 50 ng 037v V , 2MHZ n 25 5W 0 I 2 3 4 5 5 7 ’50 ’25 U 25 50 75 I00 I25 LOAD CURRENT (A) 7 TEMPERATURE m , T200 400 ERONT PAGE APPLICATION 350 V0UT=5V T000 15W =TMH1 A 300 VsVNc=F|aat 000 E g 250 E 000 g 200 3 T50 400 § T00 200 ER0NT RAGEARRUCATTON VW=I2V 50 VuUT=5V 0 T T T 0 0 I00 200 300 400 500 500 5 I5 25 35 45 55 E5 LOAD CURRENT (mA) TNRUT VOLTAGE (V) 7
LT8645S/LT8646S
7
8645Sfa
For more information www.linear.com/LT8645S
Top FET Current Limit
vs Duty Cycle Top FET Current Limit Switch Drop vs Temperature
Switch Drop vs Switch Current
TYPICAL PERFORMANCE CHARACTERISTICS
Dropout Voltage Minimum On-Time
Switching Frequency Burst Frequency
Minimum Load to Full Frequency
(Pulse-Skipping Mode)
DUTY CYCLE
0.1
0.3
0.5
0.7
0.9
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
CURRENT LIMIT (A)
8645S G19
5% DC
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
12
13
14
15
16
CURRENT LIMIT (A)
8645S G20
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT = 1A
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
0
20
40
60
80
100
SWITCH DROP (mV)
8645S G21
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT (A)
0
1
2
3
4
5
6
7
0
50
100
150
200
250
300
350
SWITCH DROP (mV)
8645S G22
V
IN
= 5V
V
OUT
SET TO REGULATE AT 5V
L = XEL6060, 1µH
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
0
50
100
150
200
250
300
350
400
DROPOUT VOLTAGE (mV)
8645S G23
I
LOAD
= 3A
V
OUT
= 0.97V
f
SW
= 2.2MHz
BURST MODE OPERATION
PULSE–SKIPPING MODE
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
25
30
35
40
45
50
MINIMUM ON-TIME (ns)
8645S G24
R
T
= 60.4k
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
660
670
680
690
700
710
720
730
740
SWITCHING FREQUENCY (kHz)
8645S G25
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
LOAD CURRENT (mA)
0
100
200
300
400
500
600
0
200
400
600
800
1000
1200
SWITCHING FREQUENCY (kHz)
8645S G26
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
V
SYNC
= Float
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
0
50
100
150
200
250
300
350
400
LOAD CURRENT (mA)
8645S G27
LT86458/ LT864éS van PIN CURRENTWM RT RIM RESISTOR (km I 2 V 2 22 2 V V 0 / i 20 E u 8 E V 9 U. K :5 K g u 5 8 V a g g V 7 E u a g 5 V5 0 2 V 5 0 V 4 0 02 04 DE DE V V2 V4 V5 ’50 ’25 U 25 50 75 V00 V25 TR/ss VOLTAGE (V) TEMPERATURE ("0) son me 375 95 250 5‘ 90 >1 2 s E 8 5 8 I25 EL LL E k Ea mm a $ 8 0 FB RVSING § ”5 E 425 3 7 5 3 FE FALLING % Ea FALLVNG g 7250 $ 70 3 § § 7375 S 55 5 VC =V 25v ”L ”L 7500 so 7200 400 a mm mm 750 725 n 25 an 75 V00 V25 fan 725 n 25 so 75 mn V25 FB PW ERROR VOLTAGE (mV) TEMPERATURE (Tn) TEMPERATURE (Tm , 25a as V5 34 A V4 / q E 32 \ E E \ E V3 5 an % > (.7 S \ a V2 % 23 5 _ \ E 25 “ VBTA5=5V , VDUI 5V sz= VMHZ 24 VD ’53 ’25 U 25 50 75 V30 V25 5 V5 25 35 45 55 55 TEMPERATURE m VNPUT VOLTAGE (V)
LT8645S/LT8646S
8
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start Current
PG High Thresholds PG Low Thresholds
RT Programmed Switching
Frequency Minimum Input Voltage Bias Pin Current
LT8645S Soft-Start Tracking LT8646S Soft-Start Tracking
LT8646S Error Amp
Output Current
TR/SS VOLTAGE (V)
0
FB VOLTAGE (V)
0.8
1.0
1.2
0.6 1.0
8645S G28
0.6
0.4
0.2 0.4 0.8 1.2
1.4
0.2
0
TR/SS VOLTAGE (V)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0
0.2
0.4
0.6
0.8
1.0
1.2
FB VOLTAGE (V)
LT8646S Soft-Start Tracking
8645S G29
V
SS
= 0.5V
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
TR/SS PIN CURRENT (µA)
8645S G30
V
C
= 1.25V
FB PIN ERROR VOLTAGE (mV)
–200
–100
0
100
200
–500
–375
–250
–125
0
125
250
375
500
V
CC
PIN CURRENT (µA)
LT8646S Error Amp Output Current
8645S G31
FB RISING
FB FALLING
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
PG THRESHOLD OFFSET FROM V
REF
(%)
8645S G32
FB RISING
FB FALLING
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
–10.0
–9.5
–9.0
–8.5
–8.0
–7.5
–7.0
–6.5
–6.0
PG THRESHOLD OFFSET FROM V
REF
(%)
8645S G33
SWITCHING FREQUENCY (MHz)
0.2
RT PIN RESISTOR (kΩ)
150
200
250
1.8
8645S G34
100
50
125
175
225
75
25
00.6 11.4
2.2
TEMPERATURE (°C)
–50
–25
0
25
50
75
100
125
2.4
2.6
2.8
3.0
3.2
3.4
3.6
INPUT VOLTAGE (V)
8645S G35
V
BIAS
= 5V
V
OUT
= 5V
f
SW
= 1MHz
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
10
11
12
13
14
15
BIAS PIN CURRENT (mA)
8645S G36
LT86458/ LT8é4éS 30 80 ucmaA DEMO BOARD \ 4 7n zmsw=5uumz ’ 25 D 4V15w=500kHz ,' E :50 2vvsw=2mz , In 4v: :2 z ¢ V 20 — . SW 4 E E 50 (A ' E‘ 5 i, g ‘5 E an , 2 K ’ . a g an , ' m w _, V w A . a E g 20 ’ ’ 5 I0 0 n 02 fl 5 I V 4 V E 2 2 0 I 2 3 4 5 5 7 E smewG 9&quch 1an) LOAD CURRENT (A) > ‘L W IA/mv sz 2WD“! sz 5V/mv 2mm “155:1: mus/DIV was an mom PAGE APPUCATION wva w 5an, AT 2A IL ‘1 SflflmA/DW wow V sz WEEK! zuvnmv mus/aw “‘5” scans/w “'5” FRONT PAGE APPLICAT‘DN FRONT PAGE APPUCAT‘ON 1va m 5VWAT mmA 48qu TD 5VauT N M VSVNC = W
LT8645S/LT8646S
9
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL PERFORMANCE CHARACTERISTICS
Bias Pin Current Case Temperature Rise
Switch Rising Edge
Switching Waveforms, Full
Frequency Continuous Operation
Switching Waveforms, Burst
Mode Operation Switching Waveforms
V
BIAS
= 5V
V
OUT
= 5V
V
IN
= 12V
I
LOAD
= 1A
SWITCHING FREQUENCY (MHz)
0.2
0.6
1
1.4
1.8
2.2
0
5
10
15
20
25
30
BIAS PIN CURRENT (mA)
8645S G37
DC2468A DEMO BOARD
V
IN
= 12V, f
SW
= 500kHz
V
IN
= 24V, f
SW
= 500kHz
V
IN
= 12V, f
SW
= 2MHz
V
IN
= 24V, f
SW
= 2MHz
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
0
10
20
30
40
50
60
70
80
CASE TEMPERATURE RISE (°C)
8645S G38
V
IN
= 12V
I
LOAD
= 3A
2ns/DIV
V
SW
2V/DIV
8645S G39
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 2A
500ns/DIV
V
SW
5V/DIV
I
L
1A/DIV
8645S G40
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 10mA
V
SYNC
= 0V
10µs/DIV
V
SW
5V/DIV
I
L
500mA/DIV
8645S G41
FRONT PAGE APPLICATION
48V
IN
TO 5V
OUT
AT 2A
500ns/DIV
V
SW
20V/DIV
I
L
1A/DIV
8645S G42
LT86458/ LT864éS ILDAD zA/mv VauT momV/mv 2mm 2AT04ATRANSIENT ‘2vw,5vmtsw MHz CuuT=|00uECLEAu “HF ILOAD IA/mv Vam, wumwmv éuus/DW auumA Tm 3A mmswgm ‘va svam tsw, MHz cuwmwrcm nor ‘Lmn 2mm Vnur J—l_~___ Inan/Dlv' f—J ‘Lmn VA/DIV L Vnun_r___{\._——_ Inan/Dlv Zuus/DW 2A 10 4A mmsmm 1sz 5v“ vsw = 2M H1 CE = 3m R5 = 7 5k Cum OWE 0mm = 4 W fiuus/DW annmA m I 3A TRANS‘ENT 1sz 5v“ vsw = ZMHZ CD: SUnERc=75k Cnu UWECLEAD=47DF 2m LOAD m w aseuwmm “IO 2m LOAD (250m m REGULATION)
LT8645S/LT8646S
10
8645Sfa
For more information www.linear.com/LT8645S
TYPICAL PERFORMANCE CHARACTERISTICS
Start-Up Dropout Performance Start-Up Dropout Performance
LT8646S Transient Response;
300mA (Burst Mode Operation) to
1.3A Transient
LT8645S Transient Response;
Internal Compensation
LT8645S Transient Response;
300mA (Burst Mode Operation) to
1.3A Transient
LT8646S Transient Response;
External Compensation
Internal Compensation
2A TO 4A TRANSIENT
12V
IN
, 5V
OUT
, f
SW
= 2MHz
C
OUT
= 100µF, C
LEAD
= 4.7pF
20µs/DIV
V
OUT
100mV/DIV
I
LOAD
2A/DIV
8645S G43
2A TO 4A TRANSIENT
12V
IN
, 5V
OUT
, f
SW
= 2MHz
C
C
= 330pF, R
C
= 7.5k
C
OUT
= 100µF, C
LEAD
= 4.7pF
20µs/DIV
V
OUT
100mV/DIV
I
LOAD
2A/DIV
8645S G44
to 1.3A Transient
300mA TO 1.3A TRANSIENT
12V
IN
, 5V
OUT
, f
SW
= 2MHz
C
OUT
= 100µF, C
LEAD
= 4.7pF
50µs/DIV
V
OUT
100mV/DIV
I
LOAD
1A/DIV
8645S G45
300mA TO 1.3A TRANSIENT
12V
IN
, 5V
OUT
, f
SW
= 2MHz
C
C
= 330pF, R
C
= 7.5k
C
OUT
= 100µF, C
LEAD
= 4.7pF
50µs/DIV
V
OUT
100mV/DIV
I
LOAD
1A/DIV
8645S G46
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8645S G47
VIN
VOUT
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8645S G48
VIN
VOUT
LT86458/ LT864éS AMPLITUDE (uaDwmI 4D — FIXED FREQUENCV MODE — SPREAD SPECTRUM MODE u359T215I52I24273O FREQUENCV (MHZ) DCZABBA DEMO BOARD mm. (WITH EMT FILTER INSTALLED) 14v IIIPOT To 5v OUTPUT AT 4A, 15w=2MHz VERTICAL POLARIZATION PEAK DETECTOR AMPLITUDE (HEW/m) — CLASS 5 PEAK LIMIT n — SPREAD SPECTRUM MODE 75 — EIREDEREOOEIIOV MODE O IOO zuu saw 400 am son mu m you man EREOOENOV (MHz) HORIZONTAL POLARIZATION PEAK DETECTOR E35 >1 E 3 'z E < —="" class="" 5="" peak="" limit="" —="" spread="" spectrum="" mode="" —="" fixed="" freduencv="" mode="" o="" mu="" mm="" 300="" ann="" son="" sun="" mu="" sou="" sun="" moo="" ereouech="" imrzi="" dczma="" demo="" board="" "155:5="" (with="" emi="" eilter="" installed)="" mv="" input="" to="" 5v="" output="" at="" 4a="" tsw="2MIIz" ‘i‘i="">
LT8645S/LT8646S
11
8645Sfa
For more information www.linear.com/LT8645S
Radiated EMI Performance
(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
TYPICAL PERFORMANCE CHARACTERISTICS
Conducted EMI Performance
DC2468A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, f
SW
= 2MHz
FIXED FREQUENCY MODE
SPREAD SPECTRUM MODE
FREQUENCY (MHz)
0
3
6
9
12
15
18
21
24
27
30
–40
–30
–20
–10
0
10
20
30
40
50
60
AMPLITUDE (dBµV/m)
8645S G49
VERTICAL POLARIZATION
PEAK DETECTOR
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
FREQUENCY (MHz)
0
100
200
300
400
500
600
700
800
900
1000
–5
0
5
10
15
20
25
30
35
40
45
50
AMPLITUDE (dBµV/m)
8645S G50
DC2468A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, f
SW
= 2MHz
HORIZONTAL POLARIZATION
PEAK DETECTOR
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
FREQUENCY (MHz)
0
100
200
300
400
500
600
700
800
900
1000
–5
0
5
10
15
20
25
30
35
40
45
50
AMPLITUDE (dBµV/m)
8645S G51
LT86458/ LT86468 12
LT8645S/LT8646S
12
8645Sfa
For more information www.linear.com/LT8645S
PIN FUNCTIONS
BIAS (Pin 1): The internal regulator will draw current from
BIAS instead of VIN when BIAS is tied to a voltage higher
than 3.1V. For output voltages of 3.3V to 25V this pin
should be tied to VOUT. If this pin is tied to a supply other
than VOUT use a 1µF local bypass capacitor on this pin.
If no supply is available, tie to GND. However, especially
for high input or high frequency applications, BIAS should
be tied to output or an external supply of 3.3V or above.
INTVCC (Pin 2): Internal 3.4V Regulator Bypass Pin. The
internal power drivers and control circuits are powered
from this voltage. INTVCC maximum output current is
25mA. Do not load the INTVCC pin with external circuitry.
INTVCC current will be supplied from BIAS if BIAS > 3.1V,
otherwise current will be drawn from VIN. Voltage on INTVCC
will vary between 2.8V and 3.4V when BIAS is between
3.0V and 3.6V. This pin should be floated.
NC (Pins 3, 7, 20, 24): No Connect. This pin is not con-
nected to internal circuitry and can be tied anywhere on
the PCB, typically ground.
VIN (Pins 4, 5, 6, 21, 22, 23): The VIN pins supply cur-
rent to the LT8645S/LT8646S internal circuitry and to
the internal topside power switch. These pins must be
tied together and be locally bypassed with a capacitor of
4.7µF or more. Be sure to place the positive terminal of
the input capacitor as close as possible to the VIN pins,
and the negative capacitor terminal as close as possible
to the GND pins. See the Applications Information section
for a sample layout.
GND (Pins 8, 9, 10, 17, 18, 19, Exposed Pad Pins
33–38): Ground. Place the negative terminal of the input
capacitor as close to the GND pins as possible. See the
Applications Information section for a sample layout. The
exposed pads should be soldered to the PCB for good
thermal performance. If necessary due to manufacturing
limitations Pins 33 to 38 may be left disconnected, however
thermal performance will be degraded.
BST (Pin 11): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
This pin should be floated.
SW (Pins 12, 13, 14, 15, 16): The SW pins are the outputs
of the internal power switches. Tie these pins together and
connect them to the inductor and boost capacitor. This node
should be kept small on the PCB for good performance
and low EMI.
EN/UV (Pin 25): The LT8645S/LT8646S is shut down
when this pin is low and active when this pin is high. The
hysteretic threshold voltage is 1.00V going up and 0.96V
going down. Tie to VIN if the shutdown feature is not
used. An external resistor divider from VIN can be used
to program a VIN threshold below which the LT8645S/
LT8646S will shut down.
RT (Pin 26): A resistor is tied between RT and ground to
set the switching frequency.
CLKOUT (Pin 27): In pulse-skipping mode, spread spec-
trum, and synchronization modes, the CLKOUT pin will
provide a ~200ns wide pulse at the switch frequency. The
low and high levels of the CLKOUT pin are ground and
INTVCC respectively, and the drive strength of the CLKOUT
pin is several hundred ohms. In Burst Mode operation,
the CLKOUT pin will be low. Float this pin if the CLKOUT
function is not used.
SYNC/MODE (Pin 28): This pin programs four different
operating modes: 1) Burst Mode. Tie this pin to ground
for Burst Mode operation at low output loads—this will
result in ultralow quiescent current. 2) Pulse-skipping
mode. This mode offers full frequency operation down to
low output loads before pulse skipping occurs. Float this
pin for pulse-skipping mode. When floating, pin leakage
currents should be <1µA. 3) Spread spectrum mode. Tie
this pin high to INTVCC (~3.4V) or an external supply of
3V to 4V for pulse-skipping mode with spread spectrum
modulation. 4) Synchronization mode. Drive this pin with
a clock source to synchronize to an external frequency.
During synchronization the part will operate in pulse-
skipping mode.
LT86458/ LT86468 13
LT8645S/LT8646S
13
8645Sfa
For more information www.linear.com/LT8645S
PIN FUNCTIONS
TR/SS (Pin 29): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. For the LT8645S, a TR/SS voltage below 0.97V
forces it to regulate the FB pin to equal the TR/SS pin volt-
age. When TR/SS is above 0.97V, the tracking function is
disabled and the internal reference resumes control of the
error amplifier. For the LT8646S, a TR/SS voltage below
1.6V forces it to regulate the FB pin to a function of the
TR/SS pin voltage. See plot in the Typical Performance
Characteristics section. When TR/SS is above 1.6V, the
tracking function is disabled and the internal reference
resumes control of the error amplifier. An internal 1.9μA
pull-up current from INTVCC on this pin allows a capacitor
to program output voltage slew rate. This pin is pulled to
ground with an internal 200Ω MOSFET during shutdown
and fault conditions; use a series resistor if driving from
a low impedance output. This pin may be left floating if
the tracking function is not needed.
GND (Pin 30 LT8645S Only): Ground. Connect this pin to
system ground and to the ground plane. This pin is also
connected to ground internally, and can be left floating on
PCB to be pin compatible with the LT8646S.
VC (Pin 30, LT8646S Only): The VC pin is the output of
the internal error amplifier. The voltage on this pin controls
the peak switch current. Tie an RC network from this pin
to ground to compensate the control loop.
PG (Pin 31): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±8% of the final regulation voltage, and there are
no fault conditions. PG is pulled low when EN/UV is below
1V, INTVCC has fallen too low, VIN is too low, or thermal
shutdown. PG is valid when VIN is above 3.4V.
FB (Pin 32): The LT8645S/LT8646S regulates the FB pin
to 0.97V. Connect the feedback resistor divider tap to this
pin. Also, connect a phase lead capacitor between FB and
VOUT. Typically, this capacitor is 1pF to 10pF.
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground.
LT86458/ LT86468 14
LT8645S/LT8646S
14
8645Sfa
For more information www.linear.com/LT8645S
BLOCK DIAGRAM
+
+
+
SLOPE COMP
INTERNAL 0.97V REF
OSCILLATOR
200kHz TO 2.2MHz
BURST
DETECT
3.4V
REG
M1
M2
CBST
0.22µF
COUT
V
OUT
8645s BD
SW L
BST
12-16
21-23
ERROR
AMP
SHDN
±8%
VC
SHDN
THERMAL SHDN
INTVCC UVLO
VIN UVLO
SHDN
THERMAL SHDN
VIN UVLO
EN/UV
1V +
25
11
GND
33-38
INTVCC 2
BIAS 1
VIN 13
GND
GND
8-10
17-19
CLKOUT
PG
31
FB
R1
27
C1
R3
OPT
R4
OPT
R2
RT
CSS
OPT
VOUT
32
TR/SS
1.9µA
60k
INTVCC
29
RT
26
SYNC/MODE
28
VIN
4-6
V
IN
COPT1
CIN3 CIN1
20nF
CVCC
2.2µF
COPT2
CIN2
20nF
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
600k
LT8645S ONLY
30
LT8645S ONLY
VC
LT8646S ONLY 30
CF
CC
RC
LT86458/ LT86468 15
LT8645S/LT8646S
15
8645Sfa
For more information www.linear.com/LT8645S
OPERATION
The LT8645S/LT8646S is a monolithic, constant frequency,
current mode step-down DC/DC converter. An oscillator,
with frequency set using a resistor on the RT pin, turns
on the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on
the internal VC node. The error amplifier servos the VC
node by comparing the voltage on the VFB pin with an
internal 0.97V reference. When the load current increases
it causes a reduction in the feedback voltage relative to
the reference leading the error amplifier to raise the VC
voltage until the average inductor current matches the new
load current. When the top power switch turns off, the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 11A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
The “S” in LT8645S/LT8646S refers to the second genera-
tion Silent Switcher technology. This technology allows
fast switching edges for high efficiency at high switching
frequencies, while simultaneously achieving good EMI/
EMC performance. This includes the integration of ceramic
capacitors into the package for VIN, BST, and INTVCC (see
Block Diagram). These caps keep all the fast AC current
loops small, which improves EMI/EMC performance.
If the EN/UV pin is low, the LT8645S/LT8646S is shut
down and draws approximately 1µA from the input. When
the EN/UV pin is above 1V, the switching regulator will
become active.
To optimize efficiency at light loads, the LT8645S/LT8646S
operates in Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input sup-
ply current to 1.7μA (LT8645S) or 230μA (LT8646S with
BIAS= 0). In a typical application, 2.5μA (LT8645S) or
120μA (LT8646S with BIAS = 5VOUT) will be consumed
from the input supply when regulating with no load. The
SYNC/MODE pin is tied low to use Burst Mode operation
and can be floated to use pulse-skipping mode. If a clock is
applied to the SYNC/MODE pin, the part will synchronize to
an external clock frequency and operate in pulse-skipping
mode. While in pulse-skipping mode the oscillator operates
continuously and positive SW transitions are aligned to
the clock. During light loads, switch pulses are skipped
to regulate the output and the quiescent current will be
several hundred µA.
To improve EMI/EMC, the LT8645S/LT8646S can operate in
spread spectrum mode. This feature varies the clock with
a triangular frequency modulation of +20%. For example,
if the LT8645S/LT8646S’s frequency is programmed to
switch at 2MHz, spread spectrum mode will modulate the
oscillator between 2MHz and 2.4MHz. The SYNC/MODE
pin should be tied high to INTVCC (~3.4V) or an external
supply of 3V to 4V to enable spread spectrum modulation
with pulse-skipping mode.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will
draw current from VIN. The BIAS pin should be connected
to VOUT if the LT8645S/LT8646S output is programmed
at 3.3V to 25V.
The VC pin optimizes the loop compensation of the
switching regulator based on the programmed switching
frequency, allowing for a fast transient response. The VC
pin also enables current sharing and a CLKOUT pin enables
synchronizing other regulators to the LT8646S.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±8%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8645S/LT8646S’s operat-
ing frequency when the voltage at the FB pin is low. This
frequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent condi-
tions. When a clock is applied to the SYNC/MODE pin, the
SYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.
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LT8645S/LT8646S
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APPLICATIONS INFORMATION
Low EMI PCB Layout
The LT8645S/LT8646S is specifically designed to minimize
EMI/EMC emissions and also to maximize efficiency when
switching at high frequencies. For optimal performance
the LT8645S/LT8646S should use multiple VIN bypass
capacitors.
Two small 0.47µF capacitors can be placed as close as
possible to the LT8645S/LT8646S: One capacitor on each
side of the device (COPT1, COPT2). A third capacitor with a
larger value, 4.7µF or higher, should be placed near COPT1
or COPT2.
See Figure1 for a recommended PCB layouts.
Figure 1. Recommended PCB Layouts for the LT8645S and LT8646S
8645S F01a
GROUND VIA VIN VIA VOUT VIA OTHER SIGNAL VIAS
V
V
V
V
V
C1
R2
R1
CSS
RT
COPT1 CIN3
COUT
L
a) LT8645S
COPT2
8645S F01b
GROUND VIA VIN VIA VOUT VIA OTHER SIGNAL VIAS
V
V
V
V
C1
R2
R1
CSS
RT
COPT1 CIN3
COUT
L
b) LT8646S
COPT2
V
CF
RC
CC
SWITCHING FREDUENCV (kHz) mm mun Hun Hun Ann znu mu FRONT PAGE APPUCA‘HON vwazv vam=5v znn sun Ann 500 600 LOADCURRENTLMA) LOAD CURRENT(mA> 400 350 300 LT86458/ LT86463 FRONT PAGE APPLICAT‘ON va = 5v qsw = VMHZ szc = Float \5 25 35 45 WPUT VOLTAGE (v) 55 55 17
LT8645S/LT8646S
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For more information www.linear.com/LT8645S
APPLICATIONS INFORMATION
For more detail and PCB design files refer to the Demo
Board guide for the LT8645S/LT8646S.
Note that large, switched currents flow in the LT8645S/
LT8646S VIN and GND pins and the input capacitors. The
loops formed by the input capacitors should be as small as
possible by placing the capacitors adjacent to the VIN and
GND pins. Capacitors with small case size such as 0603
or 0805 are optimal due to lowest parasitic inductance.
The input capacitors, along with the inductor and output
capacitors, should be placed on the same side of the
circuit board, and their connections should be made on
that layer. Place a local, unbroken ground plane under the
application circuit on the layer closest to the surface layer.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and RT nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The exposed pads on the bottom of the package should
be soldered to the PCB to reduce thermal resistance to
ambient. To keep thermal resistance low, extend the ground
plane from the GND pins as much as possible, and add
thermal vias to additional ground planes within the circuit
board and on the bottom side.
Achieving Ultralow Quiescent Current (Burst Mode
Operation)
To enhance efficiency at light loads, the LT8645S/LT8646S
operates in low ripple Burst Mode operation, which keeps
the output capacitor charged to the desired output voltage
while minimizing the input quiescent current and minimiz-
ing output voltage ripple. In Burst Mode operation the
LT8645S/LT8646S delivers single small pulses of current
to the output capacitor followed by sleep periods where
the output power is supplied by the output capacitor.
While in sleep mode the LT8645S consumes 1.7μA. and
the LT8646S consumes 230μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure2a) and the percentage
of time the LT8645S/LT8646S is in sleep mode increases,
resulting in much higher light load efficiency than for typi-
cal converters. By maximizing the time between pulses,
the LT8645S’s quiescent current approaches 2.5µA for a
typical application when there is no output load. Therefore,
to optimize the quiescent current performance at light
Figure 2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)
(2a) (2b)
Burst Frequency
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
LOAD CURRENT (mA)
0
100
200
300
400
500
600
0
200
400
600
800
1000
1200
SWITCHING FREQUENCY (kHz)
8645S F02a
Minimum Load to Full Frequency
(Pulse-Skipping Mode)
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
V
SYNC
= Float
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
0
50
100
150
200
250
300
350
400
LOAD CURRENT (mA)
8645S F02b
LT86458/ LT86468 M SflflmNDIV sz SV’DIV 18 LL H ‘UUS’DIV FRONT PAGE APPLICAHON WW T0 svwm WmA stNc = W
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APPLICATIONS INFORMATION
loads, the current in the feedback resistor divider must
be minimized as it appears to the output as load current.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8645S/
LT8646S can stay in sleep mode longer between each pulse.
This can be achieved by using a larger value inductor (i.e.,
4.7µH), and should be considered independent of switch-
ing frequency when choosing an inductor. For example,
while a lower inductor value would typically be used for
a high switching frequency application, if high light load
efficiency is desired, a higher inductor value should be
chosen. See curve in Typical Performance Characteristics.
While in Burst Mode operation the current limit of the top
switch is approximately 1.25A (as shown in Figure3),
resulting in low output voltage ripple. Increasing the output
capacitance will decrease output ripple proportionally. As
load ramps upward from zero the switching frequency
will increase but only up to the switching frequency pro-
grammed by the resistor at the RT pin as shown in Figure2a.
The output load at which the LT8645S/LT8646S reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice. To select low ripple
Burst Mode operation, tie the SYNC/MODE pin below 0.4V
(this can be ground or a logic low output).
Pulse-Skipping Mode
For some applications it is desirable for the LT8645S/
LT8646S to operate in pulse-skipping mode, offering two
major differences from Burst Mode operation. First is the
clock stays awake at all times and all switching cycles are
aligned to the clock. In this mode much of the internal
circuitry is awake at all times, increasing quiescent cur-
rent to several hundred µA. Second is that full switching
frequency is reached at lower output load than in Burst
Mode operation (see Figure2b). To enable pulse-skipping
mode, float the SYNC/MODE pin. Leakage current in this
pin should be <1µA. See Block Diagram for internal pull-up
and pull-down resistance.
Spread Spectrum Mode
The LT8645S/LT8646S features spread spectrum op-
eration to further reduce EMI/EMC emissions. To enable
spread spectrum operation, the SYNC/MODE pin should
be tied high to INTVCC (~3.4V) or an external supply of
3V to 4V. In this mode, triangular frequency modulation
is used to vary the switching frequency between the value
programmed by RT to approximately 20% higher than that
value. The modulation frequency is approximately 3kHz.
For example, when the LT8645S/LT8646S is programmed
to 2MHz, the frequency will vary from 2MHz to 2.4MHz at
Figure 3. Burst Mode Operation
Switching Waveforms,
Burst Mode Operation
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 10mA
V
SYNC
= 0V
10µs/DIV
V
SW
5V/DIV
I
L
500mA/DIV
8645S F03
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APPLICATIONS INFORMATION
a 3kHz rate. When spread spectrum operation is selected,
Burst Mode operation is disabled, and the part will run in
pulse-skipping mode.
Synchronization
To synchronize the LT8645S/LT8646S oscillator to an
external frequency, connect a square wave to the SYNC/
MODE pin. The square wave amplitude should have valleys
that are below 0.4V and peaks above 1.5V (up to 6V), with
a minimum on-time and off-time of 50ns.
The LT8645S/LT8646S will not enter Burst Mode opera-
tion at low output loads while synchronized to an external
clock, but instead will pulse-skip to maintain regulation.
The LT8645S/LT8646S may be synchronized over a 200kHz
to 2.2MHz range. The RT resistor should be chosen to set
the LT8645S/LT8646S switching frequency equal to or
below the lowest synchronization input. For example, if the
synchronization signal will be 500kHz and higher, the RT
should be selected for 500kHz. The slope compensation is
set by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage, and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, then the slope compensation will be
sufficient for all synchronization frequencies.
The LT8645S/LT8646S does not operate in forced continu-
ous mode regardless of SYNC/MODE signal.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=R2 VOUT
0.97V 1
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
For the LT8645S, if low input quiescent current and good
light-load efficiency are desired, use large resistor values
for the FB resistor divider. The current flowing in the divider
acts as a load current, and will increase the no-load input
current to the converter, which is approximately:
IQ=1.7µA +VOUT
R1+R2
VOUT
V
IN
1
n
(2)
where 1.7µA is the quiescent current of the LT8645S and
the second term is the current in the feedback divider
reflected to the input of the buck operating at its light
load efficiency n. For a 3.3V application with R1 = 1M and
R2 = 412k, the feedback divider draws 2.3µA. With VIN =
12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent
current resulting in 2.5µA no-load current from the 12V
supply. Note that this equation implies that the no-load
current is a function of VIN; this is plotted in the Typical
Performance Characteristics section.
When using large FB resistors, a 1pF to 10pF phase-lead
capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8645S/LT8646S uses a constant frequency PWM
architecture that can be programmed to switch from
200kHz to 2.2MHz by using a resistor tied from the RT
pin to ground. A table showing the necessary RT value for
a desired switching frequency is in Table 1.
The RT resistor required for a desired switching frequency
can be calculated using:
RT=
46.5
fSW
5.2
(3)
where RT is in kΩ and fSW is the desired switching fre-
quency in MHz.
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Table 1. SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
1.4 28.0
1.6 23.7
1.8 20.5
2.0 17.8
2.2 15.8
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller induc-
tor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows:
fSW(MAX) =VOUT +VSW(BOT)
tON(MIN) VIN VSW(TOP) +VSW(BOT)
( )
(4)
where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch
drops (~0.3V, ~0.2V, respectively at maximum load) and
tON(MIN) is the minimum top switch on-time (see the Elec-
trical Characteristics). This equation shows that a slower
switching frequency is necessary to accommodate a high
VIN/VOUT ratio.
For transient operation, VIN may go as high as the abso-
lute maximum rating of 65V regardless of the RT value,
however the LT8645S/LT8646S will reduce switching
frequency as necessary to maintain control of inductor
current to assure safe operation.
The LT8645S/LT8646S is capable of a maximum duty
cycle of approximately 99%, and the VIN-to-VOUT dropout
is limited by the RDS(ON) of the top switch. In this mode
the LT8645S/LT8646S skips switch cycles, resulting in a
lower switching frequency than programmed by RT.
For applications that cannot allow deviation from the pro-
grammed switching frequency at low VIN/VOUT ratios use
the following formula to set switching frequency:
VIN(MIN) =
V
OUT
+V
SW(BOT)
1fSW •t
OFF(MIN)
VSW(BOT) +VSW(TOP)
(5)
where VIN(MIN) is the minimum input voltage without
skipped cycles, VOUT is the output voltage, VSW(TOP) and
VSW(BOT) are the internal switch drops (~0.3V, ~0.2V,
respectively at maximum load), fSW is the switching fre-
quency (set by RT), and tOFF(MIN) is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8645S/LT8646S is designed to minimize solution
size by allowing the inductor to be chosen based on the
output load requirements of the application. During over-
load or short-circuit conditions the LT8645S/LT8646S
safely tolerates operation with a saturated inductor through
the use of a high speed peak-current mode architecture.
A good first choice for the inductor value is:
L=VOUT +VSW(BOT)
fSW
• 0.4
(6)
where fSW is the switching frequency in MHz, VOUT is the
output voltage, VSW(BOT) is the bottom switch drop (~0.2V)
and L is the inductor value in μH.
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APPLICATIONS INFORMATION
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled ISAT)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
IL(PEAK) =ILOAD(MAX) +
1
2
IL
(7)
where ∆IL is the inductor ripple current as calculated in
Equation 9 and ILOAD(MAX) is the maximum output load
for a given application.
As a quick example, an application requiring 2A output
should use an inductor with an RMS rating of greater than
2A and an ISAT of greater than 3A. During long duration
overload or short-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.02Ω, and the core material
should be intended for high frequency applications.
The LT8645S/LT8646S limits the peak switch current in
order to protect the switches and the system from overload
faults. The top switch current limit (ILIM) is 14A at low duty
cycles and decreases linearly to 11.5A at DC = 0.9. The
inductor value must then be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) =ILIM IL
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
IL=VOUT
L•f
SW
•1VOUT
VIN(MAX)
(9)
where fSW is the switching frequency of the LT8645S/
LT8646S, and L is the value of the inductor. Therefore, the
maximum output current that the LT8645S/LT8646S will
deliver depends on the switch current limit, the inductor
value, and the input and output voltages. The inductor
value may have to be increased if the inductor ripple cur-
rent does not allow sufficient maximum output current
(IOUT(MAX)) given the switching frequency, and maximum
input voltage used in the desired application.
When operating at high VIN (greater than 40V) and at a
frequency and duty cycle that would require a switch on-
time of less than 100ns, choose an inductor such that the
∆IL is greater than 1.5A in order to prevent duty cycle jitter.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8645S/
LT8646S can stay in sleep mode longer between each pulse.
This can be achieved by using a larger value inductor (i.e.,
4.7µH), and should be considered independent of switch-
ing frequency when choosing an inductor. For example,
while a lower inductor value would typically be used for
a high switching frequency application, if high light load
efficiency is desired, a higher inductor value should be
chosen. See curve in Typical Performance Characteristics.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. For applications
requiring smaller load currents, the value of the inductor
may be lower and the LT8645S/LT8646S may operate
with higher ripple current. This allows use of a physically
smaller inductor, or one with a lower DCR resulting in
higher efficiency. Be aware that low inductance may result
in discontinuous mode operation, which further reduces
maximum load current.
For more information about maximum output current and
discontinuous operation, see Analog Devices’ Application
Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitors
The VIN of the LT8645S/LT8646S should be bypassed
with at least three ceramic capacitors for best perfor-
mance. Two small ceramic capacitors of 0.47µF can be
placed close to the part; one on each side of the device
(COPT1, COPT2). These capacitors should be 0603 or 0805
in size. For automotive applications requiring 2 series
LT86458/ LT86468
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APPLICATIONS INFORMATION
input capacitors, two small 0603 or 0805 may be placed
at each side of the LT8645S/LT8646S.
A third, larger ceramic capacitor of 4.7µF or larger should
be placed close to COPT1 or COPT2. See layout section for
more detail. X7R or X5R capacitors are recommended for
best performance across temperature and input voltage
variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT8645S/LT8646S circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8645S/LT8646S’s voltage
rating. This situation is easily avoided (see Analog Devices
Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT8645S/LT8646S to produce the DC output. In this
role it determines the output ripple, thus low impedance at
the switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8645S/LT8646S’s control loop. Ceramic
capacitors have very low equivalent series resistance (ESR)
and provide the best ripple performance. For good starting
values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feedforward capacitor placed between
VOUT and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capacitance
under the relevant operating conditions of voltage bias and
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8645S/LT8646S due to their
piezoelectric nature. When in Burst Mode operation, the
LT8645S/LT8646S’s switching frequency depends on the
load current, and at very light loads the LT8645S/LT8646S
can excite the ceramic capacitor at audio frequencies,
generating audible noise. Since the LT8645S/LT8646S
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output. Low noise ceramic
capacitors are also available.
A final precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT8645S/LT8646S. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank circuit. If the LT8645S/LT8646S circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8645S/
LT8646S’s rating. This situation is easily avoided (see
Analog Devices Application Note 88).
Enable Pin
The LT8645S/LT8646S is in shutdown when the EN pin is
low and active when the pin is high. The rising threshold
of the EN comparator is 1.01V, with 45mV of hysteresis.
The EN pin can be tied to VIN if the shutdown feature is not
used, or tied to a logic level if shutdown control is required.
Adding a resistor divider from VIN to EN programs the
LT8645S/LT8646S to regulate the output only when VIN is
above a desired voltage (see the Block Diagram). Typically,
this threshold, VIN(EN), is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
LT86458/ LT86468 4w 23
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APPLICATIONS INFORMATION
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The VIN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
VIN(EN) =R3
R4 +1
• 1.01V
(10)
where the LT8645S/LT8646S will remain off until VIN is
above VIN(EN). Due to the comparator’s hysteresis, switch-
ing will not stop until the input falls slightly below VIN(EN).
When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8645S/LT8646S. Therefore, the VIN(EN) resistors
should be large to minimize their effect on efficiency at
low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internal
bias circuitry. The INTVCC can supply enough current for
the LT8645S/LT8646S’s circuitry. To improve efficiency
the internal LDO can also draw current from the BIAS pin
when the BIAS pin is at 3.1V or higher. Typically the BIAS
pin can be tied to the output of the LT8645S/LT8646S,
or can be tied to an external supply of 3.3V or above. If
BIAS is connected to a supply other than VOUT, be sure
to bypass with a local ceramic capacitor. If the BIAS pin is
below 3.0V, the internal LDO will consume current from
VIN. Applications with high input voltage and high switching
frequency where the internal LDO pulls current from VIN
will increase die temperature because of the higher power
dissipation across the LDO. Do not connect an external
load to the INTVCC pin.
Frequency Compensation (LT8646S Only)
Loop compensation determines the stability and transient
performance, and is provided by the components tied to
the VC pin. Generally, a capacitor (CC) and a resistor (RC)
in series to ground are used. Designing the compensation
network is a bit complicated and the best values depend
on the application. A practical approach is to start with
one of the circuits in this data sheet that is similar to your
application and tune the compensation network to opti-
mize the performance. LTspice
®
simulations can help in
this process. Stability should then be checked across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load.
Figure 4 shows an equivalent circuit for the LT8646S
control loop. The error amplifier is a transconductance
amplifier with finite output impedance. The power section,
consisting of the modulator, power switches, and inductor,
is modeled as a transconductance amplifier generating an
output current proportional to the voltage at the VC pin.
Note that the output capacitor integrates this current, and
that the capacitor on the VC pin (CC) integrates the error
amplifier output current, resulting in two poles in the loop.
A zero is required and comes from a resistor RC in series
with CC. This simple model works well as long as the value
VC
200k
FB
0.97V
8645S F04
CPL
LT8646S
CURRENT MODE
POWER STAGE
+
C1
R1
R2
OUTPUT
CF
CC
RC
Gm = 8S
gm = 1.7mS
Figure 4. Model for Loop Response
LT86458/ LT86468 it? ”H w I— -I|—ww f
LT8645S/LT8646S
24
8645Sfa
For more information www.linear.com/LT8645S
APPLICATIONS INFORMATION
of the inductor is not too high and the loop crossover
frequency is much lower than the switching frequency. A
phase lead capacitor (Cpl) across the feedback divider can
be used to improve the transient response and is required
to cancel the parasitic pole caused by the feedback node
to ground capacitance.
Output Voltage Tracking and Soft-Start
T
he LT8645S/LT8646S allows the user to program its out-
put voltage ramp rate by means of the TR/SS pin. An internal
1.9μA pulls up the TR/SS pin to INTVCC. Putting an external
capacitor on TR/SS enables soft starting the output to
prevent current surge on the input supply. During the soft-
start ramp the output voltage will proportionally track the
TR/SS pin voltage.
For output tracking applications, TR/SS can be externally
driven by another voltage source. For the LT8645S, from
0V to 0.97V, the TR/SS voltage will override the internal
0.97V reference input to the error amplifier, thus regulating
the FB pin voltage to that of TR/SS pin. When TR/SS is
above 0.97V, tracking is disabled and the feedback volt-
age will regulate to the internal reference voltage. For the
LT8646S, from 0V to 1.6V, the TR/SS voltage will override
the internal 0.97V reference input to the error amplifier,
thus regulating the FB pin voltage to a function of the TR/
SS pin. See plot in the Typical Performance Characteristics
section. When TR/SS is above 1.6V, tracking is disabled
and the feedback voltage will regulate to the internal ref-
erence voltage. The TR/SS pin may be left floating if the
function is not needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8645S/LT8646S’s output voltage is within the
±8% window of the regulation point, the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 0.4% of hysteresis. PG is valid
when VIN is above 3.4V
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTVCC has fallen too
low, VIN is too low, or thermal shutdown.
Paralleling (LT8646S Only)
To increase the possible output current, two LT8646Ss can
be connected in parallel to the same output. To do this, the
VC and FB pins are connected together, and each LT8646S’s
SW node is connected to the common output through its
own inductor. The CLKOUT pin of one LT8646S should be
connected to the SYNC/MODE pin of the second LT8646S
to have both devices operate in the same mode. During
pulse-skipping, Spread Spectrum, and Synchronization
modes, both devices will operate at the same frequency.
Figure 5 shows an application where two LT8646S are
paralleled to get one output capable of up to 16A.
Figure 5. Paralleling Two LT8646S
LT8646S
8645S F05
SW
CLKOUT
VC
FB
LT8646S
FB
VC
SYNC/MODE
SW
COUT
R1 C1
V
OUT
16A
L1
L2
R2
RC
CC
Shorted and Reversed Input Protection
The LT8645S/LT8646S will tolerate a shorted output.
Several features are used for protection during output
short-circuit and brownout conditions. The first is the
switching frequency will be folded back while the output
is lower than the set point to maintain inductor current
control. Second, the bottom switch current is monitored
such that if inductor current is beyond safe levels switch-
ing of the top switch will be delayed until such time as the
inductor current falls to safe levels.
CASETEMPEHATURE RISE LT86458/ LT86468 DCZAEEA DEMU sum , —v =‘2VV5w=5UUkHz ' 4v 'sw = snukhz .‘ 2VV5w=2MHz , v =24VVSW=ZMH1 x / X / .~ , / ., , 2” ‘, /’ ,y/ ‘ 2 3 a 5 5 7 s LUADCURRENTLA) 25
LT8645S/LT8646S
25
8645Sfa
For more information www.linear.com/LT8645S
APPLICATIONS INFORMATION
Figure 7. Case Temperature Rise
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro-
grammed level. If the SYNC pin is connected to a clock
source, floated, or tied high, the LT8645S/LT8646S will stay
at the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where the
output will be held high when the input to the LT8645S/
LT8646S is absent. This may occur in battery charg-
ing applications or in battery-backup systems where
a battery or some other supply is diode ORed with the
LT8645S/LT8646S’s output. If the VIN pin is allowed to
float and the EN pin is held high (either by a logic signal
or because it is tied to VIN), then the LT8645S/LT8646S’s
internal circuitry will pull its quiescent current through
its SW pin. This is acceptable if the system can tolerate
several μA in this state. If the EN pin is grounded the SW
pin current will drop to near 1µA. However, if the VIN pin
is grounded while the output is held high, regardless of
EN, parasitic body diodes inside the LT8645S/LT8646S
can pull current from the output through the SW pin and
the VIN pin. Figure6 shows a connection of the VIN and
EN/UV pins that will allow the LT8645S/LT8646S to run
only when the input voltage is present and that protects
against a shorted or reversed input.
the LT8645S/LT8646S. Placing additional vias can reduce
thermal resistance further. The maximum load current
should be derated as the ambient temperature approaches
the maximum junction rating. Power dissipation within the
LT8645S/LT8646S can be estimated by calculating the total
power loss from an efficiency measurement and subtract-
ing the inductor loss. The die temperature is calculated by
multiplying the LT8645S/LT8646S power dissipation by
the thermal resistance from junction to ambient.
The internal overtemperature protection monitors the junc-
tion temperature of the LT8645S/LT8646S. If the junction
temperature reaches approximately 180°C, the LT8645S/
LT8646S will stop switching and indicate a fault condition
until the temperature drops about 10°C cooler.
Temperature rise of the LT8645S/LT8646S is worst when
operating at high load, high VIN, and high switching fre-
quency. If the case temperature is too high for a given
application, then either VIN, switching frequency, or load
current can be decreased to reduce the temperature to
an acceptable level. Figure 7 shows examples of how
case temperature rise can be managed by reducing VIN,
switching frequency, or load.
Figure 6. Reverse VIN Protection
VIN
V
IN
D1
LT8645S/
LT8646S
EN/UV
8645S F06
GND
DC2468A DEMO BOARD
V
IN
= 12V, f
SW
= 500kHz
V
IN
= 24V, f
SW
= 500kHz
V
IN
= 12V, f
SW
= 2MHz
V
IN
= 24V, f
SW
= 2MHz
LOAD CURRENT (A)
0
1
2
3
4
5
6
7
8
0
10
20
30
40
50
60
70
80
CASE TEMPERATURE RISE (°C)
8645S F07
Thermal Considerations
For higher ambient temperatures, care should be taken
in the layout of the PCB to ensure good heat sinking of
the LT8645S/LT8646S. The ground pins on the bottom of
the package should be soldered to a ground plane. This
ground should be tied to large copper layers below with
thermal vias; these layers will spread heat dissipated by
The LT8645S/LT8646S’s top switch current limit decreases
with higher duty cycle operation for slope compensation.
This also limits the output current the LT8645S/LT8646S
can deliver for a given application. See curve in the Typical
Performance Characteristics section.
LT86458/ LT86468 PWS NOT USED PM MDT USED ‘Vg PIN AND COMPONEN DNLV APPLY To US$455 m 55V T0 55v PINS NOT USED LTBEASS/ 3 W V cm 5 av T0 343?: VI“ 5'” 5" EN/UV EA 4 M I <— clkdut="" pg="" :3:="" —="" —=""> svu MODE 5 W v . W ms mr c _ x2 m/ss ”"1 — 1210 R1 xsmxm «SW = 5mm : L LHmanAn mesS/ a 314" VW VW Vm sw m 3 3v 3 evm 55v I L 4 m EM/UV max 1.— 4—CLKDUT PG w - —> svmc MODE 549k ’ Mr W W x2 mm mm w w m FE xswxm : IDnF as 7x GND “2k 1 mm «W: mm L LHmanAn Ell/UV Vm VIN n mr messx 0 mr I—m m {g = MN — ‘l‘H Vam I: W arm 3/ svmcmons ms 3A 7 5k IM— v ' IDDuF RT FR 1213 kw: mm L‘ XELEDSD
LT8645S/LT8646S
26
8645Sfa
For more information www.linear.com/LT8645S
Figure 8. 5V 8A Step-Down Converter with Soft-Start and Power Good
Figure 10. Ultralow EMI 5V, 8A Step-Down Converter with Spread Spectrum
TYPICAL APPLICATIONS
Figure 9. 3.3V, 8A Step-Down Converter with Soft Start and Power Good
LT8645S/
LT8646S
PG
BIAS
FB
V
IN
3.8V TO 65V
GND
RT
V
IN
EN/UV
V
OUT
3.3V
8A
2.2pF
4.7µF
10nF
1M
412k
f
SW
= 500kHz
L: LHMI-8040
88.7k
TR/SS
SYNC/MODE
CLKOUT
SW
100k
3.3µH
8645S F09
47µF
×
2
1210
X5R/X7R
VC*
6.49k
PINS NOT USED
IN THIS CIRCUIT:
BST, INTV
CC
*V
C
PIN AND COMPONENTS
ONLY APPLY TO LT8646S.
1.5nF
LT8645S/
LT8646S
V
IN
SW
BIAS
FB
V
IN
5.5V TO 65V
PINS NOT USED
IN THIS CIRCUIT:
BST, CLKOUT, PG,
TR/SS
*V
C
PIN AND COMPONENTS
ONLY APPLY TO LT8646S.
RT
V
IN
EN/UV
V
OUT
5V
8A
100µF
1210
X5R/X7R
4.7pF
0.47µF
0805
0.47µF
0805
4.7µF
1210
17.8k
1M
243k
8645S F10
H
f
SW
= 2MHz
L: XEL6030
FB1 BEAD: WE-MPSB 100Ω 8A 1812
GND
FB1
BEAD
2.2µF
1210
2.2µF
1210
INTVCC
SYNC/MODE
VC*
7.5k
330pF
LT8645S/
LT8646S
PG
BIAS
FB
V
IN
5.5V TO 65V
PINS NOT USED
IN THIS CIRCUIT:
BST, INTV
CC
*V
C
PIN AND COMPONENTS
ONLY APPLY TO LT8646S.
RT
V
IN
EN/UV
V
OUT
5V
8A
47µF
×
2
1210
X5R/X7R
2.2pF
4.7µF
88.7k
1M
243k
f
SW
= 500kHz
L: LHMI-8040
10nF
TR/SS
VC*
SYNC/MODE
CLKOUT
SW
100k
3.3µH
8645S F08
6.04k
1nF
LT86458/ LT86468 um V W am 5 av 10 55v ‘1‘" W W NF 8A Ell/UV ms LTBEASS/ J—nnr §VM 33:32? 75k mus NOT USED M v ' ra I as m m :aaunr 32m 3 17 Bk GNU V 1 W: ZMHZ L mango 032w v v am 3 av T0 55% V'" W 3 W I 4 NF EN/LIV ms EA VOOMF Pms um usw mesS/ mu 387k Vc' n3 RT 4le ‘V5 PIN AND COMPONEN DNLV APPLY To US$455 PM MDT USED “2k V5w=2MHz L XELEDSD 4 7W ng Vm 5w 12v EA EN/UV ms 47% 2 2 r “6455 n 3 WM 1210 FE . m g 88 7k GND st : IMHZ L xELsuso 27
LT8645S/LT8646S
27
8645Sfa
For more information www.linear.com/LT8645S
Figure 11. 2MHz 5V, 8A Step-Down Converter
Figure 13. 12V, 8A Step-Down Converter
TYPICAL APPLICATIONS
Figure 12. 2MHz 3.3V, 8A Step-Down Converter
LT8645S/
LT8646S
SW
BIAS
FB
V
IN
5.5V TO 65V
PINS NOT USED
IN THIS CIRCUI
T:
BST, CLKOUT,
INTV
CC, PG,
SYNC/MODE, TR/SS
*V
C
PIN AND COMPONENTS
ONLY APPLY TO LT8646S
.
RT
V
IN
EN/UV
V
OUT
5V
8A
100µF
1210
X5R/X7R
4.7pF
8645S F11
4.7µF
17.8k
1M
243k
H
f
SW
= 2MHz
L: XEL6030
VC*
7.5k
330pF
LT8645S/
LT8646S
SW
BIAS
FB
V
IN
3.8V TO 65V
PINS NOT USED
IN THIS CIRCUI
T:
BST, CLKOUT,
INTV
CC, PG,
SYNC/MODE, TR/SS
*V
C
PIN AND COMPONENTS
ONLY APPLY TO LT8646S.
RT
V
IN
EN/UV
V
OUT
3.3V
8A
100µF
1210
X5R/X7R
4.7pF
8645S F12
4.7µF
17.8k
1M
412k
0.82µH
f
SW
= 2MHz
L: XEL6030
VC*
8.87k
330pF
LT8645S
SW
BIAS
FB
VIN
12.5V TO 65V
GND
RT
V
IN
EN/UV
V
OUT
12V
8A
47µF
1210
X5R/X7R
2.2pF
8645S F13
4.7µF
41.2k
1M
88.7k
4.7µH
f
SW
= 1MHz
L: XEL6060
PINS NOT USED
IN THIS CIRCUI
T:
BST, CLKOUT,
INTV
CC, PG,
SYNC/MODE, TR/SS
LT86458/ LT86468 |_L :t I: Q HEB , 1114],, a new; -H-‘ in; in: in: Luz! E v ,7 \3/ J H is? 17 i \\ / w , , WWEJ E} \ ,\ ’1/ , N 7 \\1 a: E \\\ \\\ E j, 47 u T l m Fran 28
LT8645S/LT8646S
28
8645Sfa
For more information www.linear.com/LT8645S
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LT8645S#packaging for the most recent package drawings.
LQFN Package
32-Lead (6mm × 4mm × 0.94mm)
(Reference LTC DWG # 05-08-1512 Rev B)
DETAIL B
A
PACKAGE TOP VIEW
5
PAD “A1”
CORNER
Y
X
aaa Z2×
ddd Z
32×
32b PACKAGE BOTTOM VIEW
4
6
SEE NOTES
E
D
b
0.375
e
e
e
b
0.375
0.20
0.20
E1
1.355
1.355
1.33
D1
1.125
LQFN 32 0317 REV B
TRAY PIN 1
BEVEL PACKAGE IN TRAY LOADING ORIENTATION
COMPONENT
PIN “A1
SYMBOL
A
A1
L
b
D
E
D1
E1
e
H1
H2
aaa
bbb
ccc
ddd
eee
fff
MIN
0.85
0.01
0.30
0.22
NOM
0.94
0.02
0.40
0.25
4.00
6.00
2.40
4.40
0.50
0.24
0.70
MAX
1.03
0.03
0.50
0.28
0.10
0.10
0.10
0.10
0.15
0.08
NOTES
DIMENSIONS
DETAIL B
SUBSTRATE
MOLD
CAP
// bbb Z
Z
Z
H2
H1
A1
DETAIL A
DETAIL C
DETAIL C
SUGGESTED PCB LAYOUT
TOP VIEW
0.0000
0.0000
0.2500
0.2500
0.7500
0.7500
1.2500
1.2500
1.7500
1.7500
2.2500
2.2500
1.2500
0.7500
0.2500
0.2500
1.2500
0.7500
DETAIL A
7
PIN 1 NOTCH
0.25 × 45°
LTXXXXXX
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. PRIMARY DATUM -Z- IS SEATING PLANE
METAL FEATURES UNDER THE SOLDER MASK OPENING NOT SHOWN
SO AS NOT TO OBSCURE THESE TERMINALS AND HEAT FEATURES
5
4
DETAILS OF PAD #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE
LOCATED WITHIN THE ZONE INDICATED. THE PAD #1 IDENTIFIER
MAY BE EITHER A MOLD OR MARKED FEATURE
6 THE EXPOSED HEAT FEATURE IS SEGMENTED AND ARRANGED
IN A MATRIX FORMAT. IT MAY HAVE OPTIONAL CORNER RADII
ON EACH SEGMENT
7 CORNER SUPPORT PAD CHAMFER IS OPTIONAL
27 32
16 11
1
10
26
17
aaa Z
2×
MX YZccc
MX YZccc
MX YZeee
MZfff
0.20
0.20
1.355
1.355
1.33 1.125
PACKAGE
OUTLINE
0.25 ±0.05
0.70 ±0.05
6.50 ±0.05
4.50 ±0.05
L
e/2
LT86458/ LT86468 29
LT8645S/LT8646S
29
8645Sfa
For more information www.linear.com/LT8645S
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 01/18 Added LT8646S All
LT86458/ LT86468 Vw ”2w VW 34V 10 auv vm sw ”W“ 1 av (s5vwmmswmn I 8A —_L— 4 NF EN/UV ms EXTERNAL _ I IMF saunas): w - LTBEASS on GND - :I: = g$sssk WF x2 Pms um USED m n; —— 12m \BNS % XSR/Xm W 4‘ 2K GNU ‘M 5 J. mm V5w=IMHz L mama ‘ 3O SEGLc‘ES
LT8645S/LT8646S
30
8645Sfa
For more information www.linear.com/LT8645S
LT 0118 REV A • PRINTED IN USA
www.linear.com/LT8645S
ANALOG DEVICES, INC. 2017
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VIN(MIN) = 3.4V, VIN(MAX) = 65V, VOUT(MIN) = 0.97V,
IQ = 2.5μA, ISD < 1μA, MSOP-16E, 3mm × 5mm QFN-24
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DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V,
IQ = 3.0μA, ISD < 1μA, 3mm × 6mm QFN-28
LT8613 42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with Current Limiting
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V,
IQ = 3.0μA, ISD < 1μA, 3mm × 6mm QFN-28
LT8602 42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA
VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V,
IQ = 2.5μA, ISD < 1μA, 6mm × 6mm QFN-40
LT8645S
SW
BIAS
FB
V
IN
3.4V TO 30V
(65V TRANSIENT)
GND
RT
V
IN
EN/UV
V
OUT
1.8V
8A
4.7pF
4.7µF
41.2k
866k
1M
0.82µH
f
SW
= 1MHz
L: XEL6030
F
EXTERNAL
SOURCE >3.1V
OR GND
8645S TA02
47µF
×
2
1210
X5R/X7R
PINS NOT USED
IN THIS CIRCUI
T:
BST, CLKOUT,
INTV
CC, PG,
SYNC/MODE, TR/SS

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LT8646S DEMO BOARD 65V, 8A SYN