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LT8614 Datasheet

Linear Technology/Analog Devices

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Datasheet

LT8614
1
8614fd
For more information www.linear.com/LT8614
Typical applicaTion
FeaTures DescripTion
42V, 4A Synchronous
Step-Down Silent Switcher
with 2.5µA Quiescent Current
The LT
®
8614 step-down regulator features Silent Switcher
architecture designed to minimize EMI/EMC emissions
while delivering high efficiency at frequencies up to 3MHz.
Assembled in a 3mm × 4mm QFN, the monolithic con-
struction with integrated power switches and inclusion of
all necessary circuitry yields a solution with a minimal PCB
footprint. An ultralow 2.5µA quiescent current—with the
output in full regulation— enables applications requiring
highest efficiency at very small load currents. Transient
response remains excellent and output voltage ripple is
below 10mVP-P at any load, from zero to full current.
The LT8614 allows high V
IN
to low V
OUT
conversion at high
frequency with a fast minimum top switch on-time of 30ns.
Operation is safe in overload even with a saturated inductor.
Essential features are included and easy to use: An open-
drain PG pin signals when the output is in regulation. The
SYNC pin allows clock synchronization and choice of Burst
Mode operation or pulse-skipping mode. Soft-start and
tracking functionality is accessed via the TR/SS pin. An
accurate enable threshold can be set using the EN/UV pin
and a resistor at the RT pin programs switch frequency.
5V 4A Step-Down Converter
12VIN to 5VOUT Efficiency
applicaTions
n Silent Switcher
®
Architecture
n Ultralow EMI/EMC Emissions
n High Efficiency at High Frequency
n Up to 96% Efficiency at 1MHz, 12VIN to 5VOUT
n Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT
n Wide Input Voltage Range: 3.4V to 42V
n Ultralow Quiescent Current Burst Mode
®
Operation:
n 2.5μA IQ Regulating 12VIN to 3.3VOUT
n Output Ripple < 10mVP-P
n Fast Minimum Switch On-Time: 30ns
n Low Dropout Under All Conditions: 125mV at 1A
n Safely Tolerates Inductor Saturation in Overload
n Adjustable and Synchronizable: 200kHz to 3MHz
n Peak Current Mode Operation
n Accurate 1V Enable Pin Threshold
n Internal Compensation
n Output Soft-Start and Tracking
n Small 18-Lead 3mm × 4mm QFN
n Automotive and Industrial Supplies
n General Purpose Step-Down
n GSM Power Supplies
L, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode are
registered trademarks and Silent Switcher is a trademark of Analog Devices, Inc. All other
trademarks are the property of their respective owners. Protected by U.S. patents, including
8823345.
VIN2
VIN1
EN/UV
PG LT8614
8614 TA01a
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
F
0.1µF
4.7pF
47µF
1M
VOUT
5V
4A
F
4.7µF
VIN
5.8V TO 42V
10nF
41.2k
F
4.7µH
243k
GND2GND1
fSW = 1MHz
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
90
4.0
8614 TA01b
70
60 1.0 2.0 3.0
0.5 1.5 2.5 3.5
100
1MHz
2MHz
75
85
65
95
LT8614
2
8614fd
For more information www.linear.com/LT8614
pin conFiguraTionabsoluTe MaxiMuM raTings
VIN, EN/UV, PG ..........................................................42V
BIAS .......................................................................... 30V
BST Pin Above SW Pin................................................4V
FB, TR/SS, RT, INTVCC . ..............................................4V
SYNC Voltage . ............................................................6V
Operating Junction Temperature Range (Note 2)
LT8614E ............................................. 40°C to 125°C
LT8614I .............................................. 40°C to 125°C
LT8614H ............................................ 40°C to 150°C
LT8614MP ......................................... 55°C to 150°C
Storage Temperature Range .................. 65°C to 150°C
(Note 1)
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8614EUDC#PBF LT8614EUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 125°C
LT8614IUDC#PBF LT8614IUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 125°C
LT8614HUDC#PBF LT8614HUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 150°C
LT8614MPUDC#PBF LT8614MPUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –55°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l2.9 3.4 V
VIN Quiescent Current VEN/UV = 0V
l
1.0
1.0
3
8
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 0V
l
1.7
1.7
4
10
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 2V 0.26 0.5 mA
VIN Current in Regulation VOUT = 0.97V, VIN = 6V, Output Load = 100µA
VOUT = 0.97V, VIN = 6V, Output Load = 1mA
l
l
21
210
50
350
µA
µA
20 19 18 17
7 8 9 10
TOP VIEW
UDC PACKAGE
18-LEAD (3mm × 4mm) PLASTIC QFN
1
BIAS
INTVCC
BST
VIN1
GND1
TR/SS
RT
EN/UV
VIN2
GND2
GND1
SW
SW
GND2
FB
PG
GND
SYNC/MODE
16
15
14
13
11
2
3
4
6
21
SW
22
SW
θJA = 40°C/W, θJC(PAD) = 12°C/W (Note 3)
EXPOSED PAD (PINS 21, 22) ARE SW, SHOULD BE SOLDERED TO PCB
NOTE: PINS 5, 12 ARE REMOVED. CONFIGURATION DOES NOT MATCH
JEDEC 20-PIN PACKAGE OUTLINE
http://www.linear.com/product/LT8614#orderinfo
LT8614
3
8614fd
For more information www.linear.com/LT8614
elecTrical characTerisTics
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 LT8614E 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
LT8614I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT8614H is guaranteed to meet performance
specifications from –40°C to 150°C operating junction temperature range.
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
Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A
VIN = 6V, ILOAD = 0.5A
l
0.964
0.958
0.970
0.970
0.976
0.982
V
V
Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 0.5A l0.004 0.02 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
INTVCC Voltage ILOAD = 0mA, VBIAS = 0V
ILOAD = 0mA, VBIAS = 3.3V
3.23
3.25
3.4
3.29
3.57
3.35
V
V
INTVCC Undervoltage Lockout 2.5 2.6 2.7 V
BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz 9 mA
Minimum On-Time ILOAD = 1.5A, SYNC = 0V
ILOAD = 1.5A, SYNC = 3.3V
l
l
15
15
30
30
45
45
ns
ns
Minimum Off-Time 80 110 ns
Oscillator Frequency RT = 221k, ILOAD = 1A
RT = 60.4k, ILOAD = 1A
RT = 18.2k, ILOAD = 1A
l
l
l
180
665
1.85
210
700
2.00
240
735
2.15
kHz
kHz
MHz
Top Power NMOS On-Resistance ISW = 1A 85
Top Power NMOS Current Limit l5.7 8.5 10 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 40
Bottom Power NMOS Current Limit VINTVCC = 3.4V l4.5 6.9 8.5 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l0.94 1.0 1.06 V
EN/UV Pin Hysteresis 40 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l6 9.0 12 %
PG Lower Threshold Offset from VFB VFB Rising l–6 –9.0 –12 %
PG Hysteresis 1.2 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l650 2000 Ω
SYNC Threshold SYNC Falling
SYNC Rising
0.8
1.6
1.1
2.0
1.4
2.4
V
V
SYNC Pin Current VSYNC = 6V –40 40 nA
TR/SS Source Current l1.5 2.2 2.9 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 200 Ω
High junction temperatures degrade operating lifetimes. The LT8614MP
is 100% tested and guaranteed over the full –55°C to 150°C operating
junction temperature range. Operating lifetime is derated at junction
temperatures greater than 125°C.
Note 3: θ values determined per JEDEC 51-7, 51-12. See the Applications
Information section for information on improving the thermal resistance.
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.
LT8614
4
8614fd
For more information www.linear.com/LT8614
Typical perForMance characTerisTics
Efficiency at 3.3VOUT Efficiency vs Frequency
Burst Mode Efficiency
vs Inductor Value
Reference Voltage EN Pin Thresholds Load Regulation
Efficiency at 5VOUT Efficiency at 3.3VOUT Efficiency at 5VOUT
LOAD CURRENT (A)
0
50
EFFICIENCY (%)
55
65
70
75
100
85
122.5
8614 G01
60
90
95
80
0.5 1.5 33.5 4
VIN = 12V
VIN = 24V
VIN = 36V
fSW = 1MHz
L = 4.7µH
LOAD CURRENT (A)
0
50
EFFICIENCY (%)
55
65
70
75
100
85
12
8614 G02
60
90
95
80
3
0.5 1.5 2.5 3.5 4
VIN = 12V
VIN = 24V
VIN = 36V
fSW = 1MHz
L = 2.2µH
LOAD CURRENT (mA)
0.01 0.1
40
EFFICIENCY (%)
50
60
70
80
1 10 100 1000
8614 G03
30
20
10
0
90
100
VIN = 12V
VIN = 24V
VIN = 36V
fSW = 1MHz
L = 4.7µH
LOAD CURRENT (mA)
0.01 0.1
40
EFFICIENCY (%)
50
60
70
80
1 10 100 1000
8614 G04
30
20
10
0
90
100
VIN = 12V
VIN = 24V
VIN = 36V
fSW = 1MHz
L = 4.7µH
SWITCHING FREQUENCY (MHz)
0.25
98
96
94
92
90
88
86
84 1.75 2.75
8614 G05
0.75 1.25 2.25
EFFICIENCY (%)
VOUT = 5V
ILOAD = 1A
L = 8.6µH
VIN = 12V
VIN = 24V
INDUCTOR VALUE (µH)
0
65
EFFICIENCY (%)
70
75
80
85
90
95
2 4 6 8
8614 G06
10
VOUT = 5V
ILOAD = 10mA VIN = 12V
VIN = 24V
TEMPERATURE (°C)
–50
REFERENCE VOLAGE (V)
0.971
0.973
0.975
150
8614 G07
0.969
0.967
0.965
0.961 050 100
–25 25 75 125
0.963
0.979
0.977
TEMPERATURE (°C)
–50
EN THRESHOLD (V)
0.99
1.01
150
8614 G08
0.97
0.95 050 100
–25 25 75 125
1.03
0.98
1.00
0.96
1.02
EN RISING
EN FALLING
LOAD CURRENT (A)
0
–0.15
CHANGE IN VOUT (%)
–0.10
–0.05
0
0.05
1 2 34
8614 G09
0.10
0.15
0.5 1.5 2.5 3.5
VOUT = 5V
VIN = 12V
LT8614
5
8614fd
For more information www.linear.com/LT8614
Typical perForMance characTerisTics
Top FET Current Limit vs Duty Cycle
Top FET Current Limit Bottom FET Current Limit
Switch Drop Minimum On-TimeSwitch Drop
Line Regulation No-Load Supply Current No-Load Supply Current
INPUT VOLTAGE (V)
5
–0.10
CHANGE IN VOUT (%)
–0.08
–0.04
–0.02
0
0.10
0.04
15 25 30
8614 G10
–0.06
0.06
0.08
0.02
10 20 35 40 45
VOUT = 5V
ILOAD = 1A
INPUT VOLTAGE (V)
0
0
INPUT CURRENT (µA)
0.5
1.5
2.0
2.5
5.0
3.5
10 20 25 45
8614 G11
1.0
4.0
4.5
3.0
5 15 30 35 40
VOUT = 3.3V
IN REGULATION
TEMPERATURE (°C)
55 –25
0
INPUT CURRENT (µA)
10
25
565 95
8614 G12
5
20
15
35 125 155
VOUT = 3.3V
VIN = 12V
IN REGULATION
TEMPERATURE (°C)
–50
CURRENT LIMIT (A)
6.0
6.5
7.0
100 12525 50 75
8614 G15
5.5
5.0
–25 0 150
4.5
4.0
7.5
DUTY CYCLE
0
6.0
CURRENT LIMIT (A)
6.5
7.0
7.5
8.0
8.5
9.0
0.2 0.4 0.6 0.8
1.0
TEMPERATURE (°C)
–50
6.0
CURRENT LIMIT (A)
6.5
7.5
8.0
8.5
9.5
–25 75 125
8614 G14
7.0
9.0
50 150
025 100
5% DC
TEMPERATURE (°C)
–50
SWITCH DROP (mV)
100
150
150
8614 G16
50
0050 100
–25 25 75 125
200
75
125
25
175
SWITCH CURRENT = 1A
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT (A)
0
0
SWITCH DROP (mV)
50
150
200
250
500
350
12
8614 G17
100
400
450
300
34
TOP SWITCH
BOTTOM SWITCH
TEMPERATURE (°C)
50
20
MINIMUM ON-TIME (ns)
22
26
28
30
40
34
050 75 100
8614 G18
24
36
38
32
25 25 125
ILOAD = 1A, VSYNC = 0V
ILOAD = 1A, VSYNC = 3V
ILOAD = 2A
ILOAD = 4A
LT8614
6
8614fd
For more information www.linear.com/LT8614
Typical perForMance characTerisTics
Dropout Voltage Switching Frequency Burst Frequency
Frequency Foldback
Minimum Load to Full Frequency
(SYNC DC High) Soft-Start Tracking
Soft-Start Current PG High Thresholds PG Low Thresholds
LOAD CURRENT (A)
0
DROPOUT VOLTAGE (mV)
400
600
4
8614 G19
200
0123
0.5 1.5 2.5 3.5
800
300
500
100
700
TEMPERATURE (°C)
–50
SWITCHING FREQUENCY (kHz)
730
25
8614 G20
700
680
–25 0 50
670
660
740 RT = 60.4k
720
710
690
75 100 150125
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
200
400
600
800
1000
1200
50 100 150 200
8614 G21
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
INPUT VOLTAGE (V)
5
LOAD CURRENT (mA)
60
80
100
20 30 45
8614 G22
40
20
0
10 15 25 35 40
FRONT PAGE APPLICATION
VOUT = 5V
fSW = 1MHz
FB VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
300
400
500
0.6 1
8614 G23
200
100
00.2 0.4 0.8
600
700
800 VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k
TR/SS VOLTAGE (V)
0
FB VOLTAGE (V)
0.8
1.0
1.2
0.6 1.0
8614 G24
0.6
0.4
0.2 0.4 0.8 1.2 1.4
0.2
0
TEMPERATURE (°C)
–50
SS PIN CURRENT (µA)
2.2
2.4
150
8614 G25
2.0
1.8 050 100
–25 25 75 125
2.6
2.1
2.3
1.9
2.5
VSS = 0.5V
TEMPERATURE (°C)
50
7.0
PG THRESHOLD OFFSET FROM VREF (%)
7.5
8.5
9.0
9.5
12.0
10.5
050 75 100
8614 G26
8.0
11.0
11.5
10.0
25 25 150125
FB RISING
FB FALLING
TEMPERATURE (°C)
50
–12.0
PG THRESHOLD OFFSET FROM VREF (%)
–11.5
–10.5
–10.0
–9.5
–7.0
–8.5
050 75 100 125
8614 G27
–11.0
–8.0
–7.5
–9.0
25 25 150
FB RISING
FB FALLING
LT8614
7
8614fd
For more information www.linear.com/LT8614
Typical perForMance characTerisTics
RT Programmed Switching
Frequency VIN UVLO Bias Pin Current
Bias Pin Current
Switching Waveforms, Full
Frequency Continuous Operation
Transient Response; Load Current
Stepped from 1A to 2A
Transient Response; Load Current
Stepped from 100mA (Burst Mode
Operation) to 1.1A
Switching Waveforms, Burst
Mode Operation
Switching Waveforms
SWITCHING FREQUENCY (MHz)
0.2
RT PIN RESISTOR (kΩ)
150
200
250
1.8
8614 G27
100
50
125
175
225
75
25
00.6 11.4 2.2 2.6 3
TEMPERATURE (°C)
–55
INPUT VOLTAGE (V)
3.4
35
8614 G29
2.8
2.4
–25 5 65
2.2
2.0
3.6
3.2
3.0
2.6
95 125 155
INPUT VOLTAGE (V)
5
BIAS PIN CURRENT (mA)
4.0
5.0
45
8614 G30
3.0
15 25 35
10 20 30 40
6.0
3.5
4.5
2.5
5.5
VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 700kHz
SWITCHING FREQUENCY (MHz)
0.2
8
10
14
1.4 2.2
8614 G31
6
4
0.6 1 1.8 2.6 3
2
0
12
BIAS PIN CURRENT (mA)
VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A
IL
500mA/DIV
VSW
5V/DIV
VOUT
5mV/DIV
500ns/DIV
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 1A
8614 G32
IL
500mA/DIV
VSW
5V/DIV
VOUT
10mV/DIV
5µs/DIV
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
8614 G33
IL
500mA/DIV
VSW
5V/DIV
200ns/DIV
FRONT PAGE APPLICATION
36VIN TO 5VOUT AT 1A
8614 G34
ILOAD
1A/DIV
VOUT
100mV/DIV
50µs/DIV
FRONT PAGE APPLICATION
1A TO 2A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8614 G35
ILOAD
1A/DIV
VOUT
200mV/DIV
50µs/DIV
FRONT PAGE APPLICATION
100mA (Burst Mode OPERATION) TO
1.1A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8614 G36
LT8614
8
8614fd
For more information www.linear.com/LT8614
FREQUENCY (MHz)
0
AMPLITUDE (dBµV/m)
30
40
50
800
20
10
0100 200 300 400 500 600 700 900 1000
FREQUENCY (MHz)
DC2019A DEMO BOARD (WITH EMI FILTER INSTALLED)
14VIN TO 5VOUT AT 4A, fSW = 2MHz
0
AMPLITUDE (dBµV/m)
30
40
50
800
8614 G39
20
10
0100 200 300 400 500 600 700 900 1000
VERTICAL POLARIZATION
HORIZONTIAL POLARIZATION
LT8614
CLASS 5 PEAK
LT8614
CLASS 5 PEAK
Typical perForMance characTerisTics
Start-Up Dropout Performance
Radiated EMI Performance (CISPR25 Radiated Emission Test with
Class 5 Peak Limits)
Start-Up Dropout Performance
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8614 G37
VIN
VOUT
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8614 G38
VIN
VOUT
LT8614
9
8614fd
For more information www.linear.com/LT8614
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 30V 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.
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
20mA. 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. Decouple this pin to power
ground with at least a 1μF low ESR ceramic capacitor
placed close to the IC.
BST (Pin 3): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
Place a 0.1µF boost capacitor as close as possible to the
IC.
V
IN1
(Pin 4): The LT8614 requires two 1µF small input
bypass capacitors. One 1µF capacitor should be placed
between VIN1 and GND1. A second 1µF capacitor should
be placed between VIN2 and GND2. These capacitors must
be placed as close as possible to the LT8614. A third
larger capacitor of 2.2µF or more should be placed close
to the LT8614 with the positive terminal connected to VIN1
and VIN2, and the negative terminal connected to ground.
See applications section for sample layout.
GND1 (6, 7): Power Switch Ground. These pins are the
return path of the internal bottom side power switch and
must be tied together. Place the negative terminal of the
input capacitor as close to the GND1 pins as possible.
Also be sure to tie GND1 to the ground plane. See the
Applications Information section for sample layout.
SW (Pins 8, 9): 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.
GND2 (10, 11): Power Switch Ground. These pins are the
return path of the internal bottom side power switch and
must be tied together. Place the negative terminal of the
input capacitor as close to the GND2 pins as possible.
Also be sure to tie GND2 to the ground plane. See the
Applications Information section for sample layout.
VIN2 (Pin 13): The LT8614 requires two 1µF small input
bypass capacitors. One 1µF capacitor should be placed
between VIN1 and GND1. A second 1µF capacitor should
be placed between VIN2 and GND2. These capacitors
must be placed as close as possible to the LT8614. A
third larger capacitor of 2.2µF or more should be placed
close to the LT8614 with the positive terminal connected
to V
IN1
and V
IN2
, and the negative terminal connected
to ground. See the Applications Information section for
sample layout.
EN/UV (Pin 14): The LT8614 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 LT8614 will shut down.
RT (Pin 15): A resistor is tied between RT and ground to
set the switching frequency.
TR/SS (Pin 16): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. A TR/SS voltage below 0.97V forces the LT8614
to regulate the FB pin to equal the TR/SS pin voltage.
When TR/SS is above 0.97V, the tracking function is dis-
abled and the internal reference resumes control of the
error amplifier. An internal 2.2μA 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 230Ω MOSFET during shutdown and fault condi-
tions; use a series resistor if driving from a low impedance
output. This pin may be left floating if the tracking function
is not needed.
LT8614
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block DiagraM
+
+
+
SLOPE COMP
INTERNAL 0.97V REF
OSCILLATOR
200kHz TO 3MHz
BURST
DETECT
3.4V
REG
M1
M2
C
BST
C
OUT
V
OUT
8614 BD
SW L
BST
8, 9, 21, 22
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±9%
V
C
SHDN
TSD
INTV
CC
UVLO
V
IN
UVLO
SHDN
TSD
V
IN
UVLO
EN/UV
1V
+
14
4
3
18
GND
INTV
CC
2
BIAS 1
V
IN2
13
GND1
6, 7
GND2
10, 11
PG
19
FB
R1C1
R3
OPT
R4
OPT
R2
R
T
C
SS
OPT
V
OUT
20
TR/SS
2.2µA
16
RT
15
SYNC/MODE
17
V
IN1
V
IN
C
IN1
C
IN3
C
VCC
C
IN2
pin FuncTions
SYNC/MODE (Pin 17): External Clock Synchronization
Input. Ground this pin for low ripple Burst Mode operation
at low output loads. Tie to a clock source for synchroniza-
tion to an external frequency. Apply a DC voltage of 3V or
higher or tie to INTVCC for pulse-skipping mode. When
in pulse-skipping mode, the IQ will increase to several
hundred µA. Do not float this pin.
GND (Pins 18): LT8614 Ground Pin. Connect this pin to
system ground and to the ground plane.
PG (Pin 19): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±9% of the final regulation voltage, and there are
no fault conditions. PG is valid when VIN is above 3.4V,
regardless of EN/UV pin state.
FB (Pin 20): The LT8614 regulates the FB pin to 0.970V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and V
OUT
.
Typically, this capacitor is 4.7pF to 22pF.
SW (Exposed Pad Pins 21, 22): The exposed pads should
to connected and soldered to the SW trace for good ther-
mal performance. If necessary due to manufacturing limi-
tations Pins 21 and 22 may be left disconnected, however
thermal performance will be degraded.
LT8614
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operaTion
The LT8614 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 inter-
nal 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 volt-
age 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 6.9A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8614 is shut down and
draws 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 LT8614 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 supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be tied to a logic high to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize
to an external clock frequency and operate in pulse-skip-
ping 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 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 V
IN
. The BIAS pin should be connected
to VOUT if the LT8614 output is programmed at 3.3V or
above.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±9% (typi-
cal) from the set point, or if a fault condition is present.
The oscillator reduces the LT8614’s operating 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 conditions. When
a clock is applied to the SYNC pin or the SYNC pin is
held DC high, the frequency foldback is disabled and the
switching frequency will slow down only during overcur-
rent conditions.
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Low EMI PCB Layout
The LT8614 is specifically designed to minimize EMI/EMC
emissions and also to maximize efficiency when switching
at high frequencies. For optimal performance the LT8614
requires the use of multiple VIN bypass capacitors.
Two small 1µF capacitors should be placed as close as
possible to the LT8614: One capacitor should be tied to
VIN1/GND1; a second capacitor should be tied to VIN2/
GND2. A third capacitor with a larger value, 2.2µF or
higher, should be placed near VIN1 or VIN2.
See Figure1 for a recommended PCB layout.
For more detail and PCB design files refer to the Demo
Board guide for the LT8614.
Note that large, switched currents flow in the LT8614
V
IN1
, V
IN2
, GND1, and GND2 pins and the input capacitors
(CIN1, CIN2). The loops formed by the input capacitors
should be as small as possible by placing the capacitors
adjacent to the VIN1/2 and GND1/2 pins. Capacitors with
small case size such as 0603 are optimal due to lowest
parasitic inductance.
The input capacitors, along with the inductor and out-
put 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
Figure1. Recommended PCB Layout for the LT8614
V
V
V
V
VV
1
611
16
GROUND PLANE
ON LAYER 2
20
R1
R2
CVCC
CBST
CIN1 CIN2
CIN3
COUT
L
RT
CSS
C1 RPG
17
7 10
22
21
GROUND VIA VIN VIA VOUT VIA OTHER SIGNAL VIAS
9614 F01
V
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traces will shield them from the SW and BOOST nodes.
The exposed pad on the bottom of the package should
be soldered to SW to reduce thermal resistance to ambi-
ent. To keep thermal resistance low, extend the ground
plane from GND1 and GND2 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
To enhance efficiency at light loads, the LT8614 oper-
ates in low ripple Burst Mode operation, which keeps the
output capacitor charged to the desired output voltage
while minimizing the input quiescent current and mini-
mizing output voltage ripple. In Burst Mode operation the
LT8614 delivers single small pulses of current to the out-
put capacitor followed by sleep periods where the output
power is supplied by the output capacitor. While in sleep
mode the LT8614 consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure2a) and the percentage
of time the LT8614 is in sleep mode increases, result-
ing in much higher light load efficiency than for typical
converters. By maximizing the time between pulses, the
converter quiescent current approaches 2.5µA for a typi-
cal application when there is no output load. Therefore,
to optimize the quiescent current performance at light
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 LT8614 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 switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing 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 600mA resulting in output
voltage ripple shown in Figure3. Increasing the output
capacitance will decrease the output ripple proportionally.
As load ramps upward from zero the switching frequency
will increase but only up to the switching frequency
programmed by the resistor at the RT pin as shown in
Figure2a. The output load at which the LT8614 reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice.
Figure2. SW Frequency vs Load Information in Burst
Mode Operation (2a) and Pulse-Skipping Mode (2b)
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(2a)
(2b)
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
200
400
600
800
1000
1200
50 100 150 200
8614 F02a
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
INPUT VOLTAGE (V)
5
LOAD CURRENT (mA)
60
80
100
20 30 45
8614 F02b
40
20
0
10 15 25 35 40
FRONT PAGE APPLICATION
VOUT = 5V
fSW = 1MHz
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For some applications it is desirable for the LT8614 to
operate in pulse-skipping mode, offering two major differ-
ences 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 current 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, the SYNC
pin is tied high either to a logic output or to the INTVCC
pin. When a clock is applied to the SYNC pin the LT8614
will also operate in pulse-skipping mode.
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
V
OUT
0.970V 1
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load effi-
ciency 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
VIN
1
n
(2)
where 1.7µA is the quiescent current of the LT8614 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 4.7pF to 22pF phase-lead
capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8614 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 3MHz
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 R
T
resistor required for a desired switching frequency
can be calculated using:
RT=
46.5
f
SW
– 5.2
(3)
where RT is in kΩ and fSW is the desired switching fre-
quency in MHz.
Figure3. Burst Mode Operation
IL
500mA/DIV
VSW
5V/DIV
VOUT
10mV/DIV
5µs/DIV
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
8614 F03
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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 18.2
2.2 15.8
3.0 10.7
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
inductor and capacitor values may be used. The disad-
vantages 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) =
V
OUT
+V
SW(BOT)
tON(MIN) VIN – VSW(TOP) +VSW(BOT)
( )
(4)
where VIN is the typical input voltage, VOUT is the output
voltage, V
SW(TOP)
and V
SW(BOT)
are the internal switch
drops (~0.45V, ~0.15V, respectively at maximum load)
and tON(MIN) is the minimum top switch on-time (see the
Electrical 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 42V regardless of the RT value,
however the LT8614 will reduce switching frequency
as necessary to maintain control of inductor current to
assure safe operation.
The LT8614 is capable of a maximum duty cycle of greater
than 99%, and the VIN-to-VOUT dropout is limited by the
RDS(ON) of the top switch. In this mode the LT8614 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)
1– fSW tOFF(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 (~0.45V, ~0.15V,
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 LT8614 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8614 safely tolerates opera-
tion 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=
V
OUT
+V
SW(BOT)
f
SW
(6)
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, VSW(BOT) is the bottom switch drop
(~0.15V) and L is the inductor value in μH.
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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
ΔIL
(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 1A output
should use an inductor with an RMS rating of greater than
1A and an ISAT of greater than 1.3A. 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 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8614 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (ILIM) is at least 8.5A at low
duty cycles and decreases linearly to 7.2A at DC = 0.8.
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
ΔI
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
ΔIL=VOUT
LfSW
1– VOUT
VIN(MAX)
(9)
where fSW is the switching frequency of the LT8614, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8614 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 current does not allow
sufficient maximum output current (IOUT(MAX)) given the
switching frequency, and maximum input voltage used in
the desired application.
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 LT8614 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 switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing 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 requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8614 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 Linear Technology’s
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 LT8614 should be bypassed with at least
three ceramic capacitors for best performance. Two small
ceramic capacitors of 1µF should be placed close to the
part; one at the VIN1/GND1 pins and a second at VIN2/
GND2 pins. These capacitors should be 0402 or 0603 in
size. For automotive applications requiring 2 series input
capacitors, two small 0402 or 0603 may be placed at
each side of the LT8614 near the VIN1/GND1 and VIN2/
GND2 pins.
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A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to VIN1 or VIN2. 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 cir-
cuit. If the LT8614 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8614’s voltage rating. This situation is
easily avoided (see 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 LT8614 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 sta-
bilize the LT8614’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 out-
put ripple and good transient response. Transient perfor-
mance can be improved with a higher value output capaci-
tor 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 capaci-
tance 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 LT8614 due to their piezoelectric
nature. When in Burst Mode operation, the LT8614’s
switching frequency depends on the load current, and
at very light loads the LT8614 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8614 operates at a lower current limit during
Burst Mode operation, 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 LT8614. As
previously mentioned, a ceramic input capacitor com-
bined with trace or cable inductance forms a high qual-
ity (underdamped) tank circuit. If the LT8614 circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8614’s
rating. This situation is easily avoided (see Application
Note 88).
Enable Pin
The LT8614 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV 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 V
IN
to EN programs the
LT8614 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
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 V
IN(EN)
threshold prevents the regulator from operating at source
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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
1.0V
(10)
where the LT8614 will remain off until VIN is above VIN(EN).
Due to the comparators hysteresis, switching 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 LT8614. 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 INTV
CC
can supply enough cur-
rent for the LT8614’s circuitry and must be bypassed to
ground with a minimum of 1μF ceramic capacitor. Good
bypassing is necessary to supply the high transient cur-
rents required by the power MOSFET gate drivers. To
improve efficiency the internal LDO can also draw cur-
rent 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 LT8614, 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 V
IN
. Applications with high input voltage and
high switching frequency where the internal LDO pulls
current from V
IN
will increase die temperature because
of the higher power dissipation across the LDO. Do not
connect an external load to the INTVCC pin.
Output Voltage Tracking and Soft-Start
T
he LT8614 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
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.
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 feed-
back voltage will regulate to the internal reference 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 LT8614s output voltage is within the ±9% win
-
dow of the regulation point, which is a VFB voltage in the
range of 0.883V to 1.057V (typical), 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 1.2% of hysteresis.
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.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic low out-
put). To synchronize the LT8614 oscillator to an external
frequency connect a square wave (with 20% to 80% duty
cycle) to the SYNC pin. The square wave amplitude should
have valleys that are below 0.4V and peaks above 2.4V
(up to 6V).
The LT8614 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8614
may be synchronized over a 200kHz to 3MHz range. The
RT resistor should be chosen to set the LT8614 switching
frequency equal to or below the lowest synchronization
LT8614
19
8614fd
For more information www.linear.com/LT8614
applicaTions inForMaTion
input. For example, if the synchronization signal will be
500kHz and higher, the R
T
should be selected for 500kHz.
The slope compensation is set by the RT value, while the
minimum slope compensation required to avoid subhar-
monic oscillations is established by the inductor size,
input voltage, and output voltage. Since the synchroniza-
tion 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 R
T
, then
the slope compensation will be sufficient for all synchro-
nization frequencies.
For some applications it is desirable for the LT8614 to
operate in pulse-skipping mode, offering two major dif-
ferences from Burst Mode operation. First is the clock
stays awake at all times and all switching cycles are
aligned to the clock. Second is that full switching fre-
quency is reached at lower output load than in Burst Mode
operation. These two differences come at the expense
of increased quiescent current. To enable pulse-skipping
mode, the SYNC pin is tied high either to a logic output
or to the INTVCC pin.
The LT8614 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
The LT8614 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 switching of the top switch
will be delayed until such time as the inductor current
falls to safe levels.
Frequency foldback behavior depends on the state of
the SYNC pin: If the SYNC pin is low the switching fre-
quency will slow while the output voltage is lower than
the programmed level. If the SYNC pin is connected to
a clock source or tied high, the LT8614 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 LT8614 is
absent. This may occur in battery charging applications or
in battery-backup systems where a battery or some other
supply is diode ORed with the LT8614’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
LT8614’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 LT8614
can pull current from the output through the SW pin and
the VIN pin. Figure4 shows a connection of the VIN and
EN/UV pins that will allow the LT8614 to run only when
the input voltage is present and that protects against a
shorted or reversed input.
Figure4. Reverse VIN Protection
VIN
VIN
D1
LT8614
EN/UV
8614 F04
GND
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8614. 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 LT8614.
Placing additional vias can reduce thermal resistance fur-
ther. The maximum load current should be derated as the
ambient temperature approaches the maximum junction
rating. Power dissipation within the LT8614 can be esti-
mated by calculating the total power loss from an efficiency
measurement and subtracting the inductor loss. The die
temperature is calculated by multiplying the LT8614 power
dissipation by the thermal resistance from junction to
ambient. The LT8614 will stop switching and indicate a
fault condition if safe junction temperature is exceeded.
LT8614
20
8614fd
For more information www.linear.com/LT8614
5V 4A Step-Down Converter
3.3V, 4A Step-Down Converter
VIN2
VIN1
EN/UV
PG LT8614
8614 TA08
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 47µF
1210
X5R/X7R
1M
VOUT
5V
4A
F
0603
F
0603
4.7µF
VIN
5.8V TO 42V
10nF
41.2k
F
4.7µH
243k
GND2GND1
fSW = 1MHz
L: IHLP2525CZ-01
VIN2
VIN1
EN/UV
PG LT8614
8614 TA05
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 47µF
1210
X5R/X7R
1M
VOUT
3.3V
4A
F
0603
F
0603
4.7µF
VIN
4.1V TO 42V
10nF
41.2k
F
4.7µH
412k
GND2GND1
fSW = 1MHz
L: IHLP2525CZ-01
Typical applicaTions
Ultralow EMI 5V, 4A Step-Down Converter
VIN2
VIN1
EN/UV
PG LT8614
8614 TA02
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 1M 47µF
1210
X5R/X7R
VOUT
5V
4A
F
0603
F
0603
4.7µF
1206
VIN
5.8V TO 42V
10nF
18.2k
F
fSW = 2MHz
FB1 BEAD: MPZ2012S300A
L: IHLP2525CZ-01
L2: IHLP1616AB-01
2.2µH
L2
0.22µH
FB1
BEAD
243k
10µF
1210
4.7µF
1206
GND2GND1
LT8614
21
8614fd
For more information www.linear.com/LT8614
2MHz 5V, 4A Step-Down Converter
2MHz 3.3V, 4A Step-Down Converter
VIN2
VIN1
EN/UV
PG LT8614
8614 TA03
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 47µF
1210
X5R/X7R
1M
VOUT
5V
4A
F
0603
F
0603
4.7µF
VIN
5.8V TO 42V
10nF
18.2k
F
2.2µH
243k
GND2GND1
fSW = 2MHz
L: IHLP2525CZ-01
VIN2
VIN1
EN/UV
PG LT8614
8614 TA06
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 47µF
1210
X5R/X7R
1M
VOUT
3.3V
4A
F
0603
F
0603
4.7µF
VIN
4.1V TO 42V
10nF
18.2k
F
1.5µH
412k
GND2GND1
fSW = 2MHz
L: IHLP2020CZ-01
Typical applicaTions
LT8614
22
8614fd
For more information www.linear.com/LT8614
package DescripTion
Please refer to http://www.linear.com/product/LT8614#packaging for the most recent package drawings.
UDC Package
Variation: UDC20(18)
20(18)-Lead Plastic QFN (3mm × 4mm)
(Reference LTC DWG # 05-08-1956 Rev C)
Exposed Pad Variation AA
3.00 ±0.10 1.50 REF
4.00 ±0.10
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 5)
0.40 ±0.10
PIN 1 ID
0.12 × 45°
0.356 ±0.05
0.220 ±0.05
0.400 ±0.05
0.770
BSC
0.770
BSC
1
2
BOTTOM VIEW—EXPOSED PAD
2.50 REF
2.127
±0.10
0.75 ±0.05
R = 0.110
TYP
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UDC20(18)) QFN 1116 REV C
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
2.50 REF
3.10 ±0.05
4.50 ±0.05
1.50 REF
0.356 ±0.05
0.055 BSC
0.400 ±0.05
0.220 ±0.05
2.10 ±0.05
3.50 ±0.05
PACKAGE OUTLINE
0.50 BSC
UDC Package
Variation: UDC20(18)
20(18)-Lead Plastic QFN (3mm × 4mm)
(Reference LTC DWG # 05-08-1956 Rev C)
Exposed Pad Variation AA
LT8614
23
8614fd
For more information www.linear.com/LT8614
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 03/14 Clarified package description.
Clarified Applications Information.
Clarified applications components.
Clarified rev of package drawing.
Clarified Related Parts list.
2
13
20, 21, 24
22
24
B 05/15 Added H- Grade version 2, 3
C 08/16 Clarified Order Information
Clarified Minimum On-Time Condition
Clarified RT Programming Switching Frequency Graph
Clarified Bias Pin Description
Clarified Components on Typical Applications
2
3
7
9
20, 21
D 06/17 Clarified the Features section.
Added MP Grade.
Added MP Grade (Notes 2 and 3).
Clarified the Operating Frequency Selection and Trade-Offs section.
Clarified components in the Typical Applications section.
1
2
3
15
24
LT8614
24
8614fd
For more information www.linear.com/LT8614
LINEAR TECHNOLOGY CORPORATION 2013
LT 0617 REV D • PRINTED IN USA
www.linear.com/LT8614
relaTeD parTs
Typical applicaTions
PART NUMBER DESCRIPTION COMMENTS
LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower
Step-Down DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E Package
LT8610A/LT8610AB 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower
Step-Down DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E Package
LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower
Step-Down DC/DC Converter with IQ = 2.5µA and Input/Output
Current Limit/Monitor
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, 3mm × 5mm QFN-24 Package
LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower
Step-Down DC/DC Converter with IQ = 3µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, 3mm × 6mm QFN Package
LT3971 38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.8µA
VIN: 4.2V to 38V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3991 55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.8µA
VIN: 4.2V to 55V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
ISD < 1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
LT3990 62V, 350mA, 2.2MHz High Efficiency MicroPower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-6E Packages
12V, 4A Step-Down Converter
2MHz 1.8V, 4A Step-Down Converter
VIN2
VIN1
EN/UV
PG LT8614
8614 TA04
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
4.7pF 47µF
1210
X5R/X7R
1M
VOUT
12V
4A
F
0603
F
0603
4.7µF
VIN
12.8V TO 42V
10nF
41.2k
F
4.7µH
88.7k
GND2GND1
fSW = 1MHz
L: IHLP2525CZ-01
VIN2
VIN1
EN/UV
PG LT8614
8614 TA07
BST
SYNC/MODE SW
TR/SS BIAS
INTVCC FB
RT GND
0.1µF
EXTERNAL
SOURCE >3.1V
OR GND 100µF
1210
X5R/X7R
10pF 866k
VOUT
1.8V
4A
F
0603
F
0603
4.7µF
VIN
3.4V TO 22V
(42V TRANSIENT)
10nF
18.2k
F
H
1M
GND2GND1
fSW = 2MHz
L: IHLP2020CZ-01

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