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LT8609(A, B) Datasheet

Linear Technology/Analog Devices

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Datasheet

LT8609/LT8609A/LT8609B
1
Rev. H
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TYPICAL APPLICATION
FEATURES DESCRIPTION
42V, 3A Synchronous Step-Down
Regulator with 2.5µA Quiescent Current
The LT
®
8609/LT8609A/LT8609B is a compact, high effi-
ciency, high speed synchronous monolithic step-down
switching regulator that consumes only 1.7µA of non-
switching quiescent current. The LT8609/LT8609A/
LT8609B can deliver 3A of continuous current. Burst Mode
operation enables high efficiency down to very low output
currents while keeping the output ripple below 10mVP-P.
A SYNC pin allows synchronization to an external clock, or
spread spectrum modulation for low EMI operation. Internal
compensation with peak current mode topology allows the
use of small inductors and results in fast transient response
and good loop stability. The EN/UV pin has an accurate 1V
threshold and can be used to program VIN UVLO or to shut
down the part. Acapacitor on the TR/SS pin programs the
output voltage ramp rate during start-up while the PG flag
signals when VOUT is within ±8.5% of the programmed
output voltage as well as fault conditions. The LT8609A has
slower switch edges for lower EMI emissions. The LT8609B
operates in pulse-skipping mode only.
APPLICATIONS
n Wide Input Voltage Range: 3.0V to 42V
n Ultralow Quiescent Current Burst Mode
®
Operation:
n <2.5µA IQ Regulating 12VIN to 3.3VOUT
n Output Ripple <10mVP-P
n High Efficiency 2MHz Synchronous Operation:
n >93% Efficiency at 1A, 5VOUT from 12VIN
n 3A Maximum Continuous Output
n Fast Minimum Switch-On Time: 45ns
n Adjustable and Synchronizable: 200kHz to 2.2MHz
n Spread Spectrum Frequency Modulation for Low EMI
n Allows Use of Small Inductors
n Low Dropout
n Peak Current Mode Operation
n Accurate 1V Enable Pin Threshold
n Internal Compensation
n Output Soft-Start and Tracking
n Small 10-Lead or 16-Lead MSOP Package
n GSM Transceivers
n General Purpose Step-Down
n Low EMI Step-Down
All registered trademarks and trademarks are the property of their respective owners.
VIN BST
EN/UV
ON OFF 0.1µF
22µF
10pF
4.7µF
VIN
6V TO 40V
F
VOUT
5V
3A
187k
8609 TA01a
2.2µH
1M
SYNC
INTVCC
TR/SS
RT
LT8609
GND
SW
PG
FB
18.2k
5V, 2MHz Step-Down
12VIN to 5VOUT Efficiency
L = 2.2µH
f
SW
= 2MHz
I
OUT
(A)
0.00
1.00
2.00
2.50
3
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8609 TA01b
PULSE-SKIPPING MODE,
BurstMode OPERATION SWITCH EDGE SPEED
LT8609 Both Fast
LT8609A Both Medium
LT8609B Pulse-Skipping Fast
LT8609/LT8609A/LT8609B
2
Rev. H
For more information www.analog.com
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
VIN, EN/UV, PG ..........................................................42V
FB, TR/SS . .................................................................4V
SYNC Voltage . ............................................................6V
(Note 1)
1
2
3
4
5
BST
SW
INTVCC
RT
SYNC
10
9
8
7
6
EN/UV
VIN
PG
TR/SS
FB
11
GND
MSE PACKAGE
10-LEAD PLASTIC MSOP
JA
= 40°C/W
EXPOSED PAD (PIN 11) IS GND,
MUST BE SOLDERED TO PCB
LT8609/LT8609A
TOP VIEW
1
2
3
4
5
BST
SW
INTVCC
RT
NC
10
9
8
7
6
EN/UV
VIN
PG
TR/SS
FB
LT8609B
TOP VIEW
11
GND
MSE PACKAGE
10-LEAD PLASTIC MSOP
JA
= 40°C/W
EXPOSED PAD (PIN 11) IS GND,
MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
BST
NC
SW
NC
INTVCC
RT
SYNC
GND
16
15
14
13
12
11
10
9
EN/UV
NC
VIN
NC
PG
TR/SS
FB
FB
TOP VIEW
17
GND
MS PACKAGE
16-LEAD PLASTIC MSOP
JA
= 40°C/W,
JC
(PAD) = 10°C/W
EXPOSED PAD (PIN 17) IS GND,
MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8609EMSE#PBF LT8609EMSE#TRPBF LTGRW 10-Lead Plastic MSOP –40°C to 125°C
LT8609IMSE#PBF LT8609IMSE#TRPBF LTGRW 10-Lead Plastic MSOP –40°C to 125°C
LT8609HMSE#PBF LT8609HMSE#TRPBF LTGRW 10-Lead Plastic MSOP –40°C to 150°C
LT8609AEMSE#PBF LT8609AEMSE#TRPBF LTGVR 10-Lead Plastic MSOP –40°C to 125°C
LT8609AIMSE#PBF LT8609AIMSE#TRPBF LTGVR 10-Lead Plastic MSOP –40°C to 125°C
LT8609BEMSE#PBF LT8609BEMSE#TRPBF LTGZY 10-Lead Plastic MSOP –40°C to 125°C
LT8609BIMSE#PBF LT8609BIMSE#TRPBF LTGZY 10-Lead Plastic MSOP –40°C to 125°C
LT8609AHMSE#PBF LT8609AHMSE#TRPBF LTGVR 10-Lead Plastic MSOP –40°C to 150°C
LT8609AJMSE#PBF LT8609AJMSE#TRPBF LTGVR 10-Lead Plastic MSOP –40°C to 150°C
LT8609AHMSE16#PBF LT8609AHMSE16#TRPBF 8609A 16-Lead Plastic MSOP –40°C to 150°C
LT8609AJMSE16#PBF LT8609AJMSE16#TRPBF 8609A 16-Lead Plastic MSOP –40°C to 150°C
Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
Operating Junction Temperature Range (Note 2)
LT8609E/LT8609AE/LT8609BE ......... 40°C to 125°C
LT8609I/LT8609AI/LT8609BI ............ 40°C to 125°C
LT8609H/LT8609AH .......................... 40°C to 150°C
LT8609AJ .......................................... 40°C to 150°C
Storage Temperature Range .................. 65°C to 150°C
LT8609/LT8609A/LT8609B
3
Rev. H
For more information www.analog.com
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
l
2.7
3.0
3.2
V
VIN Quiescent Current LT8609/LT8609A:
VEN/UV = 0V, VSYNC = 0V
VEN/UV = 2V, Not Switching, VSYNC = 0V
l
1
1.7
4
12
µA
µA
LT8609B:
VEN/UV = 0V
VEN/UV = 2V, Not Switching
1
350
4
µA
µA
VIN Current in Regulation LT8609/LT8609A:
VIN = 6V, VOUT = 2.7V, Output Load = 100µA
VIN = 6V, VOUT = 2.7V, Output Load = 1mA
l
l
46
480
90
700
µA
µA
Feedback Reference Voltage LT8609/LT8609A:
VIN = 6V, ILOAD = 100mA
VIN = 6V, ILOAD = 100mA
l
0.778
0.770
0.782
0.782
0.786
0.794
V
V
LT8609B:
VIN = 6V, ILOAD = 100mA
VIN = 6V, ILOAD = 100mA
l
0.772
0.759
0.782
0.782
0.792
0.805
V
V
Feedback Voltage Line Regulation VIN = 4.0V to 40V l0.02 0.04 %/V
Feedback Pin Input Current VFB = 1V l±20 nA
Minimum On-Time ILOAD = 1.5A, SYNC = 0V
ILOAD = 1.5A, SYNC = 1.9V
l
l
45
45
75
60
ns
ns
Minimum Off Time 90 130 ns
Oscillator Frequency LT8609/LT8609A:
RT = 221k, ILOAD = 0.5A
RT = 60.4k, ILOAD = 0.5A
RT = 18.2k, ILOAD = 0.5A
l
l
l
155
640
1.925
200
700
2.00
245
760
2.075
kHz
kHz
MHz
LT8609B:
RT = 18.2k, ILOAD = 0.5A
l
1.875
2.00
2.125
MHz
Top Power NMOS On-Resistance ILOAD = 1A 185
Top Power NMOS Current Limit l3.4 4.5 5.7 A
Bottom Power NMOS On-Resistance 115
SW Leakage Current VIN = 42V, VSW = 40V l15 µA
EN/UV Pin Threshold EN/UV Rising l0.99 1.05 1.11 V
EN/UV Pin Hysteresis 50 mV
EN/UV Pin Current LT8609/LT8609A:
VEN/UV = 2V
l
±20
nA
LT8609B:
VEN/UV = 2V
l
±30
nA
PG Upper Threshold Offset from VFB LT8609/LT8609A:
VFB Rising
l
5.0
8.5
13.0
%
LT8609B:
VFB Rising
l
4.0
8.5
14.0
%
LT8609/LT8609A/LT8609B
4
Rev. H
For more information www.analog.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
PG Lower Threshold Offset from VFB LT8609/LT8609A:
VFB Falling
l
5.0
8.5
13.0
%
LT8609B:
VFB Falling
l
4.0
8.5
14.0
%
PG Hysteresis 0.5 %
PG Leakage VPG = 42V l±200 nA
PG Pull-Down Resistance VPG = 0.1V 550 1200 Ω
Sync Low Input Voltage LT8609/LT8609A l0.4 0.9 V
Sync High Input Voltage LT8609/LT8609A:
INTVCC = 3.5V
l2.7 3.2 V
TR/SS Source Current l1 2 3 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 300 900 Ω
Spread Spectrum Modulation Frequency LT8609/LT8609A:
VSYNC = 3.3V
1
3
6
kHz
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. Absolute Maximum Ratings are those values beyond
which the life of a device may be impaired.
Note 2: The LT8609E/LT8609AE/LT8609BE 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 LT8609I/LT8609AI/LT8609BI is
guaranteed over the full –40°C to 125°C operating junction temperature
range. The LT8609H/LT8609AH/LT8609AJ is guaranteed over the full
–40°C to 150°C operating junction temperature range. High junction
temperatures degrade operating lifetimes. Operating lifetime is derated at
junction temperatures greater than 125°C.
Note 3: 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.
LT8609/LT8609A/LT8609B
5
Rev. H
For more information www.analog.com
Efficiency (5V Output, Burst Mode
Operation)
Efficiency (3.3V Output, 2MHz,
Burst Mode Operation)
Efficiency (3.3V Output, 2MHz,
Burst Mode Operation)
Efficiency (5V Output, 2MHz,
Burst Mode Operation)
Efficiency (5V Output, 2MHz,
Burst Mode Operation) FB Voltage
L = 8.2µH
fSW = 700kHz
I
OUT
(mA)
0.001
0.01
0.1
1
10
100
1k
10k
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8609 G04
VIN = 12V
VIN = 24V
I
OUT
(A)
0
0.5
1
1.5
2
2.5
3
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8609 G05
VIN = 12V
VIN = 24V
L = 2.2µH
fSW = 2MHz
I
OUT
(mA)
0.001
0.01
0.1
1
10
100
1k
10k
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8609 G06
VIN = 12V
VIN = 24V
L = 2.2µH
fSW = 2MHz
L = 2.2µH
fSW = 2MHz
I
OUT
(A)
0
0.5
1
1.5
2
2.5
3
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8609 G07
VIN = 12V
VIN = 24V
I
OUT
(mA)
0.001
0.01
0.1
1
10
100
1k
10k
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8609 G08
VIN = 12V
VIN = 24V
L = 2.2µH
fSW = 2MHz
TEMPERATURE (°C)
–50
–10
30
70
110
150
775
776
777
778
779
780
781
782
783
784
785
FB REGULATION VOLTAGE (mV)
8609 G09
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (3.3V Output,
Burst Mode Operation)
Efficiency (3.3V Output,
Burst Mode Operation)
Efficiency (5V Output, Burst
Mode Operation)
L = 8.2µH
fSW = 700kHz
I
OUT
(A)
0
0.5
1
1.5
2
2.5
3
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8609 G01
VIN = 12V
VIN = 24V
L = 8.2µH
fSW = 700kHz
I
OUT
(mA)
0.001
0.01
0.1
1
10
100
1k
10k
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8609 G02
VIN = 12V
VIN = 24V
I
OUT
(A)
0
0.5
1
1.5
2
2.5
3
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
8609 G03
L = 8.2µH
fSW = 700kHz
VIN = 12V
VIN = 24V
LT8609/LT8609A/LT8609B
6
Rev. H
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Load Regulation Line Regulation
No-Load Supply Current
(3.3V Output)
VIN (V)
0
IIN (µA)
5.0
1.5
4.0
2.0
0.5
3.5
3.0
1.0
4.5
2.5
0.0 30
8609 G12
50402010
OUTPUT CURRENT (A)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
–0.500
–0.400
–0.300
–0.200
–0.100
0.000
0.100
0.200
0.300
0.400
0.500
CHANGE IN V
OUT
(%)
8609 G10
INPUT VOLTAGE (V)
4.0
11.6
19.2
26.8
34.4
42.0
–0.20
–0.15
–0.10
–0.05
0.00
0.05
0.10
0.15
0.20
CHANGE IN V
OUT
(%)
Line Regulation
8609 G11
ILOAD = 1A
No-Load Supply Current
vs Temperature
Top FET Current Limit
vs Duty Cycle
Top FET Current Limit
vs Temperature
TEMPERATURE (°C)
–50
–10
30
70
110
150
1.3
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
3.1
3.3
INPUT CURRENT (µA)
No Load Supply Current Vs Temperature
8609 G13
VIN = 12V
VOUT = 3.3V
DUTY CYCLE (%)
0
20
40
60
80
100
3.50
3.75
4.00
4.25
4.50
4.75
5.00
5.25
5.50
TOP FET CURRENT LIMIT (A)
Top Fet Current Limit vs Duty Cycle
8609 G14
TEMPERATURE (°C)
–50
–10
30
70
110
150
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5.0
I
SW
(A)
8609 G15
LT8609/LT8609A/LT8609B
7
Rev. H
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Drop vs Temperature Switch Drop vs Switch Current
Minimum On-Time
vs Temperature
Minimum Off-Time
vs Temperature
Dropout Voltage vs Load Current
Switching Frequency
vs Temperature
L = XFL4020–222MEC
LOAD CURRENT (A)
0
0.5
1
1.5
2
2.5
3
0
200
400
600
800
1000
DROPOUT VOLTAGE (mV)
8609 G20
SWITCH CURRENT = 1A
TOP SW
BOT SW
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
150
0
50
100
150
200
250
300
350
SWITCH DROP (mV)
Switch Drop vs Temperature
8609 G16
TOP SW
BOT SW
SWITCH CURRENT (A)
0
0.5
1
1.5
2
2.5
3
0
100
200
300
400
500
600
700
800
SWITCH DROP (mV)
Switch Drop vs Switch Current
8609 G17
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
150
50
60
70
80
90
100
110
120
130
140
150
MINIMUM OFF–TIME (ns)
Minimum Off–Time Vs Temperature
8609 G19
VIN = 6V
ILOAD = 1A
R
T
= 18.2kΩ
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
150
1.975
1.980
1.985
1.990
1.995
2.000
2.005
SWITHCING FREQUENCY (MHz)
Switching Frequency Vs Temperature
8609 G21
SYNC = 2V, 1.5A OUT
SYNC = 0V, 1.5A OUT
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
150
0
10
20
30
40
50
60
70
MINIMUM ON-TIME (ns)
Minimum On-Time Vs Temperature
8609 G18
LT8609/LT8609A/LT8609B
8
Rev. H
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Frequency vs Load Current
Minimum Load to Full Frequency
(SYNC Float to 1.9V) Frequency Foldback
Soft-Start Tracking
Case Temperature vs
Load Current
Soft-Start Current vs Temperature
Case Temperature vs
3A Pulsed Load
VIN UVLO
LOAD CURRENT (mA)
0
FREQUENCY (kHz)
2500
1000
1500
500
2000
0600200 400
L = 2.2µH
VOUT = 3.3V
VIN = 12V
SYNC = 0V
8609 G22
INPUT VOLTAGE (V)
8609 G23
0
LOAD CURRENT (mA)
100
40
60
20
80
90
30
50
10
70
030 5020 4010
VOUT = 5V
fSW = 700kHz
SYNC = FLOAT
FB VOLTAGE (V)
0
FREQUENCY (kHz)
2500
1000
1500
500
2000
010.4 0.80.2 0.6
8609 G24
VIN = 12V
VOUT = 3.3V
RT = 18.2k
TEMPERATURE (°C)
–55
VIN UVLO (V)
3.5
1
2
1.5
2.5
0.5
3
01555 125–25 6535 95
8609 G30
SS VOLTAGE (V)
0
0.1
0.2
0.4
0.5
0.6
0.7
0.8
1.0
1.1
1.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
FB VOLTAGE (V)
Soft-Start Tracking
8609 G25
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
150
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
SOFT START CURRENT (µA)
Soft Start Current Vs Temperature
8609 G26
VSS = 0.1V
LOAD CURRENT (A)
0
0.5
1
1.5
2
2.5
3
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
8609 G31
VIN = 12V
VIN = 24V
VOUT = 5V
fSW = 2MHz
STANDBY LOAD = 50mA
PULSED LOAD = 3A
V
OUT = 5V
f
SW = 2MHz
V
IN
= 12V
V
IN
= 24V
DUTY CYCLE (%)
0
10
20
30
40
50
60
70
80
90
100
0
5
10
15
20
25
30
35
40
45
50
CASE TEMPERATURE RISE (°C)
Case Temperature vs 3A Pulsed Load
8609 G32
LT8609/LT8609A/LT8609B
9
Rev. H
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response Transient Response
Start-Up Dropout Start-Up Dropout
Switching Waveforms Switching Waveforms Switching Waveforms
10µs/DIV
200mA/DIV
5V/DIV
8609 G34
12VIN TO 5VOUT AT 10mA
SYNC = 0
500ns/DIV
1A/DIV
10V/DIV
8609 G35
36VIN TO 5VOUT AT 1A
500ns/DIV
1A/DIV
5V/DIV
8609 G33
12VIN TO 5VOUT AT 1A
V
IN
V
OUT
R
LOAD
= 25Ω
INPUT VOLTAGE (V)
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
Start-Up Dropout
8609 G38
V
IN
V
OUT
R
LOAD
= 2.5Ω
INPUT VOLTAGE (V)
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
Start-Up Droupout
8609 G39
50µs/DIV
500mA/DIV
100mV/DIV
8609 G36
50mA TO 1A TRANSIENT
12VIN TO 5VOUT
COUT = 47µF
20µs/DIV
500mA/DIV
100mV/DIV
8609 G37
0.5A TO 1.5A TRANSIENT
12VIN TO 5VOUT
COUT = 47µF
LT8609/LT8609A/LT8609B
10
Rev. H
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PIN FUNCTIONS
BST: 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. Do
not place a resistor in series with this pin.
SW: The SW pin is the output of the internal power
switches. Connect this pin to the inductor and boost
capacitor. This node should be kept small on the PCB for
good performance.
INTVCC: Internal 3.5V Regulator Bypass Pin. The inter-
nal power drivers and control circuits are powered from
this voltage. INTVCC max output current is 20mA. Voltage
on INTVCC will vary between 2.8V and 3.5V. Decouple
this pin to power ground with at least a 1μF low ESR
ceramic capacitor. Do not load the INTVCC pin with exter-
nal circuitry.
RT: A resistor is tied between RT and ground to set the
switching frequency.
SYNC: External Clock Synchronization Input. Ground
this pin for low ripple Burst Mode operation at low out-
put loads. Tie to a clock source for synchronization to
an external frequency. Leave floating for pulse-skipping
mode with no spread spectrum modulation. Tie to INTVCC
or tie to a voltage between 3.2V and 5.0V for pulse-skip-
ping mode with spread spectrum modulation. When in
pulse-skipping mode, the IQ will increase to several mA. In
the LT8609B, the SYNC pin is replaced with a no connect;
the LT8609B operates in pulse-skipping mode without
spread spectrum.
FB: The LT8609/LT8609A/LT8609B regulates the FB pin
to 0.782V. Connect the feedback resistor divider tap to
this pin.
TR/SS: Output Tracking and Soft-Start Pin. This pin allows
user control of output voltage ramp rate during start-up.
A TR/SS voltage below 0.782V forces the LT8609/
LT8609A/LT8609B to regulate the FB pin to equal the
TR/SS pin voltage. When TR/SS is above 0.782V, the
tracking function is disabled and the internal reference
resumes control of the error amplifier. An internal 2μA
pull-up current from INTV
CC
on this pin allows a capacitor
to program output voltage slew rate. This pin is pulled
to ground with a 300Ω MOSFET during shutdown and
fault conditions; use a series resistor if driving from a low
impedance output.
PG: The PG pin is the open-drain output of an internal
comparator. PG remains low until the FB pin is within
±8.5% of the final regulation voltage, and there are no
fault conditions. PG is valid when VIN is above 3.2V,
regardless of EN/UV pin state.
V
IN
: The V
IN
pin supplies current to the LT8609/LT8609A/
LT8609B internal circuitry and to the internal topside
power switch. This pin must be locally bypassed. Be sure
to place the positive terminal of the input capacitor as
close as possible to the V
IN
pins, and the negative capaci-
tor terminal as close as possible to the GND pins.
EN/UV: The LT8609/LT8609A/LT8609B is shut down
when this pin is low and active when this pin is high. The
hysteretic threshold voltage is 1.05V going up and 1.00V
going down. Tie to VIN if the shutdown feature is not used.
An external resistor divider from VIN can be used to pro-
gram a VIN threshold below which the LT8609/LT8609A/
LT8609B will shut down.
GND: Exposed Pad Pin. The exposed pad must be con-
nected to the negative terminal of the input capacitor
and soldered to the PCB in order to lower the thermal
resistance.
LT8609/LT8609A/LT8609B
11
Rev. H
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BLOCK DIAGRAM
+
+
+
SLOPE COMP
INTERNAL 0.782V REF
OSCILLATOR
200kHz TO 2.2MHz
BURST
DETECT
3.5V
REG
M1
M2
CBST
COUT
V
OUT
8609 BD
SW L
BST
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±8.5%
VC
SHDN
TSD
INTVCC UVLO
VIN UVLO
SHDN
TSD
VIN UVLO
EN/UV
1V +
INTVCC
GND
PG
FB
R1
RPG
R2
RT
CSS
VOUT
TR/SS
2µA
RT
SYNC
VIN
VIN
CIN
CVCC
R3
OPT
R4
OPT
CFF
LT8609/LT8609A/LT8609B
12
Rev. H
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OPERATION
The LT8609/LT8609A/LT8609B 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 0.782V 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 excess current 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 LT8609/LT8609A/LT8609B is
shut down and draws 1µA from the input. When the EN/UV
pin is above 1V, the switching regulator becomesactive.
To optimize efficiency at light loads, the LT8609/LT8609A
enters Burst Mode operation during 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 opera-
tion and can be floated 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 cur-
rent will be several mA. The SYNC pin may be tied high
for spread spectrum modulation mode, and the LT8609/
LT8609A will operate similar to pulse-skipping mode but
vary the clock frequency to reduce EMI.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±8.5% (typi
-
cal) from the set point, or if a fault condition is present.
The oscillator reduces the LT8609/LT8609A’s operating
frequency when the voltage at the FB pin is low. This fre-
quency foldback helps to control the inductor current when
the output voltage is lower than the programmed value
which occurs during start-up. When a clock is applied to
the SYNC pin the frequency foldback is disabled.
The LT8609B has no SYNC pin and only operates in pulse-
skipping mode.
LT8609/LT8609A/LT8609B
13
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APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8609/LT8609A
enters into 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
LT8609/LT8609A 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 LT8609/LT8609A consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure1) and the percentage
of time the LT8609/LT8609A is in sleep mode increases,
resulting in much higher light load efficiency than for typi
-
cal converters. By maximizing the time between pulses,
INPUT VOLTAGE (V)
8609 G23
0
LOAD CURRENT (mA)
100
40
60
20
80
90
30
50
10
70
030 5020 4010
VOUT = 5V
fSW = 700kHz
SYNC = FLOAT
Figure1b. Full Switching Frequency Minimum Load
vs VIN in Pulse Skipping Mode
Figure2. Burst Mode Operation
2.00µs/DIV
200mA/DIV
10mV/DIV
8609 F02
the converter quiescent current approaches 2.5µA for a
typical 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.
While in Burst Mode operation the current limit of the
top switch is approximately 600mA resulting in output
voltage ripple shown in Figure2. 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 pro-
grammed by the resistor at the RT pin as shown in Table1.
The output load at which the LT8609/LT8609A reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice.
For some applications, it is desirable for the LT8609/
LT8609A to operate in pulse-skipping mode, which is the
only mode available to the LT8609B. Pulse-skipping mode
offers two major differences from Burst Mode operation.
First is the clock stays awake at all times and all switch-
ing 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 as shown in Figure1b. To
enable pulse-skipping mode the SYNC pin is floated. To
achieve spread spectrum modulation with pulse-skipping
mode, the SYNC pin is tied high. While a clock is applied
to the SYNC pin the LT8609/LT8609A will also operate in
pulse-skipping mode.
LOAD CURRENT (mA)
0
FREQUENCY (kHz)
2500
1000
1500
500
2000
0600200 400
L = 2.2µH
VOUT = 3.3V
VIN = 12V
SYNC = 0V
8609 G22
Figure 1a. SW Burst Mode Frequency vs Load
LT8609/LT8609A/LT8609B
14
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APPLICATIONS INFORMATION
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
0.782V – 1
1% resistors are recommended to maintain output volt-
age accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible when good low load
efficiency is desired: The resistor divider generates a
small load on the output, which should be minimized to
optimize the quiescent current at low loads.
When using large FB resistors, a 10pF phase lead capaci-
tor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8609/LT8609A/LT8609B 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. When in spread
spectrum modulation mode, the frequency is modulated
upwards of the frequency set by RT.
Table1. SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
0.2 221
0.300 143
0.400 110
0.500 86.6
0.600 71.5
0.700 60.4
0.800 52.3
0.900 46.4
1.000 40.2
1.200 33.2
1.400 27.4
1.600 23.7
1.800 20.5
2.000 18.2
2.200 16.2
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
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) =
V
OUT +
V
SW(BOT)
tON(MIN) VIN – VSW(TOP) +VSW(BOT)
( )
where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch drops
(~0.4V, ~0.25V, respectively at max load) and tON(MIN) is
the minimum top switch on-time (see Typical Applications).
This equation shows that slower switching frequency is
necessary to accommodate a high VIN/VOUTratio.
For transient operation VIN may go as high as the Abs Max
rating regardless of the RT value, however the LT8609/
LT8609A/LT8609B will reduce switching frequency as nec-
essary to maintain control of inductor current to assure
safe operation.
The LT8609/LT8609A/LT8609B is capable of maximum
duty cycle approaching 100%, and the VIN to VOUT
dropout is limited by the RDS(ON) of the top switch. In
this mode the LT8609/LT8609A /LT8609B skips switch
cycles, resulting in a lower switching frequency than pro-
grammed 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) =VOUT +VSW(BOT)
1– fSW • tOFF(MIN)
– VSW(BOT) +VSW(TOP)
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 (~0.4V, ~0.25V,
respectively at max load), fSW is the switching frequency
(set by RT), and t
OFF(MIN)
is the minimum switch off-time.
LT8609/LT8609A/LT8609B
15
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APPLICATIONS INFORMATION
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 LT8609/LT8609A/LT8609B is designed to minimize solu-
tion size by allowing the inductor to be chosen based on the
output load requirements of the application. During overload
or short circuit conditions the LT8609/LT8609A/LT8609B
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=
V
OUT +
V
SW(BOT)
fSW
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, VSW(BOT) is the bottom switch drop
(~0.25V) and L is the inductor value in μH.
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 applica-
tion. 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
ΔL
where ∆IL is the inductor ripple current as calculated sev-
eral paragraphs below and I
LOAD(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. 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 LT8609/LT8609A/LT8609B limits the peak switch current
in order to protect the switches and the system from overload
faults. The top switch current limit (ILIM) is typically 4.75A at
low duty cycles and decreases linearly to 4.0A at D = 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 Δ
I
L
2
The peak-to-peak ripple current in the inductor can be
calculated as follows:
ΔIL=VOUT
L • fSW
1VOUT
VIN(MAX)
where fSW is the switching frequency of the LT8609/
LT8609A/LT8609B, and L is the value of the inductor.
Therefore, the maximum output current that the LT8609/
LT8609A/LT8609B will deliver depends on minimum 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.
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 LT8609/LT8609A/LT8609B 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 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 Capacitor
Bypass the input of the LT8609/LT8609A/LT8609B circuit
with a ceramic capacitor of X7R or X5R type. Y5V types have
poor performance over temperature and applied voltage,
LT8609/LT8609A/LT8609B
16
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APPLICATIONS INFORMATION
and should not be used. A 4.7μF to 10μF ceramic capacitor
is adequate to bypass the LT8609/LT8609A/LT8609B and
will easily handle the ripple current. 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.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT8609/LT8609A/LT8609B and to force this very
high frequency switching current into a tight local loop,
minimizing EMI. A 4.7μF capacitor is capable of this task,
but only if it is placed close to the LT8609/LT8609A/
LT8609B (see the PCB Layout section). A second precau-
tion regarding the ceramic input capacitor concerns the
maximum input voltage rating of the LT8609/LT8609A/
LT8609B. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
tank circuit. If the LT8609/LT8609A/LT8609B circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8609/
LT8609A/LT8609B’s voltage rating. This situation is eas-
ily avoided (see Linear Technology 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
LT8609/LT8609A/LT8609B 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 stabi-
lize the LT8609/LT8609A/LT8609Bs control loop. Ceramic
capacitors have very low equivalent series resistance (ESR)
and provide the best ripple performance. A good starting
value is:
COUT =
100
VOUT • fSW
where fSW is in MHz, and COUT is the recommended output
capacitance in μF. 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 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 prob-
lems when used with the LT8609/LT8609A/LT8609B
due to their piezoelectric nature. When in Burst Mode
operation, the LT8609/LT8609A/LT8609B’s switch-
ing frequency depends on the load current, and at very
light loads the LT8609/LT8609A/LT8609B can excite
the ceramic capacitor at audio frequencies, generat-
ing audible noise. Since the LT8609/LT8609A/LT8609B
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.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8609/LT8609A/
LT8609B. As previously mentioned, a ceramic input
capacitor combined with trace or cable inductance forms
a high quality (under damped) tank circuit. If the LT8609/
LT8609A/LT8609B circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8609/LT8609A/LT8609B’s rating. This
situation is easily avoided (see Application Note 88).
Enable Pin
The LT8609/LT8609A/LT8609B is in shutdown when the
EN pin is low and active when the pin is high. The rising
threshold of the EN comparator is 1.05V, with 50mV of
hysteresis. The EN pin can be tied to VIN if the shutdown
LT8609/LT8609A/LT8609B
17
Rev. H
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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
LT8609/LT8609A/LT8609B to regulate the output only
when VIN is above a desired voltage (see Block Diagram).
Typically, this threshold, VIN(EN), is used in situations
where the input supply is current limited, or has a rel-
atively high source resistance. A switching regulator
draws constant power from the source, so source cur-
rent 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
• 1V
where the LT8609/LT8609A/LT8609B will remain off until V
IN
is above VIN(EN). Due to the comparators hysteresis, switch-
ing will not stop until the input falls slightly below VIN(EN).
When in Burst Mode operation for light-load currents,
the current through the VIN(EN) resistor network can eas-
ily be greater than the supply current consumed by the
LT8609/LT8609A/LT8609B. Therefore, the VIN(EN) resis-
tors should be large to minimize their effect on efficiency
at low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the
3.5V supply from VIN that powers the drivers and the
internal bias circuitry. The INTVCC can supply enough
current for the LT8609/LT8609A/LT8609B’s circuitry
and must be bypassed to ground with a minimum of
1μF ceramic capacitor. Good bypassing is necessary to
supply the high transient currents required by the power
MOSFET gate drivers. Applications with high input voltage
and high switching frequency will increase die tempera-
ture because of the higher power dissipation across the
LDO. Do not connect an external load to the INTVCC pin.
APPLICATIONS INFORMATION
Output Voltage Tracking and Soft-Start
The LT8609/LT8609A/LT8609B allows the user to
program its output voltage ramp rate by means of
the TR/SS pin. An internal 2μA pulls up the TR/SS
pin to INTVCC. Putting an external capacitor on TR/
SS enables soft-starting the output to prevent cur-
rent 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 0.782V, the TR/SS voltage will override the
internal 0.782V reference input to the error amplifier, thus
regulating the FB pin voltage to that of TR/SS pin. When
TR/SS is above 0.782V, tracking is disabled and the feed-
back voltage will regulate to the internal reference voltage.
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 LT8609/LT8609A/LT8609B’s output voltage is
within the ±8.5% window of the regulation point, which is
a VFB voltage in the range of 0.716V to 0.849V (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 drain pull-
down device will pull the PG pin low. To prevent glitch-
ing both the upper and lower thresholds include 0.5% 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 LT8609/LT8609A oscillator to
an external frequency connect a square wave (with 20%
to 80% duty cycle) to the SYNC pin. The square wave
LT8609/LT8609A/LT8609B
18
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APPLICATIONS INFORMATION
amplitude should have valleys that are below 0.9V and
peaks above 2.7V (up to 5V).
The LT8609/LT8609A will not enter Burst Mode opera-
tion at low output loads while synchronized to an exter-
nal clock, but instead will pulse skip to maintain regu-
lation. The LT8609/LT8609A may be synchronized over
a 200kHz to 2.2MHz range. The R
T
resistor should be
chosen to set the LT8609/LT8609A 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 oscil-
lations is established by the inductor size, input voltage,
and output voltage. Since the synchronization frequency
will not change the slopes of the inductor current wave-
form, 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.
For some applications, it is desirable for the LT8609/
LT8609A to operate in pulse-skipping mode, which is the
only mode available to the LT8609B. Pulse-skipping mode
offers two major differences from Burst Mode operation.
First is the clock stays awake at all times and all switch-
ing cycles are aligned to the clock. Second is that full
switching frequency is reached at lower output load than
in Burst Mode operation as shown in Figure1b in an ear-
lier section. These two differences come at the expense
of increased quiescent current. To enable pulse-skipping
mode the SYNC pin is floated.
For some applications, reduced EMI operation may be
desirable, which can be achieved through spread spec-
trum modulation. This mode operates similar to pulse
skipping mode operation, with the key difference that the
switching frequency is modulated up and down by a 3kHz
triangle wave. The modulation has the frequency set by RT
as the low frequency, and modulates up to approximately
20% higher than the frequency set by RT. To enable spread
spectrum mode, tie SYNC to INTVCC or drive to a voltage
between 3.2V and 5V.
The LT8609/LT8609A/LT8609B does not operate in forced
continuous mode regardless of SYNC signal.
Shorted and Reversed Input Protection
The LT8609/LT8609A/LT8609B 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 cur-
rent control. Second, the bottom switch current is moni-
tored 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. This allows
for tailoring the LT8609/LT8609A/LT8609B to individual
applications and limiting thermal dissipation during short
circuit conditions.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low or high, or floated the
switching frequency will slow while the output voltage is
lower than the programmed level. If the SYNC pin is con-
nected to a clock source, the LT8609/LT8609A/LT8609B
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 LT8609/
LT8609A/LT8609B 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
LT8609/LT8609A/LT8609B’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 LT8609/
LT8609A/LT8609B’s internal circuitry will pull its quies-
cent 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 0.7µA.
However, if the V
IN
pin is grounded while the output is
held high, regardless of EN, parasitic body diodes inside
the LT8609/LT8609A/LT8609B can pull current from the
output through the SW pin and the VIN pin. Figure3 shows
a connection of the VIN and EN/UV pins that will allow the
LT8609/LT8609A/LT8609B to run only when the input
voltage is present and that protects against a shorted or
reversed input.
LT8609/LT8609A/LT8609B
19
Rev. H
For more information www.analog.com
VIN
VIN
LT8609
GND
D1
8609 F03
EN/UV
Figure3. Reverse VIN Protection
APPLICATIONS INFORMATION
8609 F04
GND VIA VIN VIA VOUT VIA EN/UV VIA OTHER SIGNAL VIA
COUT
CIN
CBST
CVCC
GROUND PLANE ON LAYER 2
RT
L
1
CIN(OPT)
R1
CFF R2
R4
R3
RPG
CSS
Figure4. PCB Layout
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure4 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT8609/LT8609A/LT8609B’s VIN
pins, GND pins, and the input capacitor (CIN). The loop
formed by the input capacitor should be as small as
possible by placing the capacitor adjacent to the V
IN
and
GND pins. When using a physically large input capacitor
the resulting loop may become too large in which case
using a small case/value capacitor placed close to the
VIN and GND pins plus a larger capacitor further away
is preferred. These components, along with the induc-
tor and output capacitor, 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 pad on the
bottom of the package must be soldered to ground so
that the pad is connected to ground electrically and also
acts as a heat sink thermally. To keep thermal resistance
low, extend the ground plane as much as possible, and
add thermal vias under and near the LT8609/LT8609A/
LT8609B to additional ground planes within the circuit
board and on the bottom side.
LT8609/LT8609A/LT8609B
20
Rev. H
For more information www.analog.com
Thermal Considerations and Peak Current Output
For higher ambient temperatures, care should be taken
in the layout of the PCB to ensure good heat sinking of
the LT8609/LT8609A/LT8609B. The exposed pad on the
bottom of the package must 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 LT8609/LT8609A/LT8609B. 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 LT8609/LT8609A/LT8609B
can be estimated by calculating the total power loss from
an efficiency measurement and subtracting the induc-
tor loss. The die temperature is calculated by multiplying
the LT8609/LT8609A/LT8609B power dissipation by the
thermal resistance from junction to ambient. The LT8609/
LT8609A/LT8609B will stop switching and indicate a fault
condition if safe junction temperature is exceeded.
Temperature rise of the LT8609/LT8609A/LT8609B is
worst when operating at high load, high VIN, and high
switching frequency. If the case temperature is too high
for a given application, then either VIN, switching fre-
quency or load current can be decreased to reduce the
temperature to an acceptable level. Figure5 shows how
case temperature rise can be managed by reducing VIN.
Figure6 shows an example of how case temperature
rise changes with the duty cycle of a 10Hz pulsed 3A
load. Junction temperature will be higher than case
temperature.
LOAD CURRENT (A)
0
0.5
1
1.5
2
2.5
3
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
8609 F05
VIN = 12V
VIN = 24V
VOUT = 5V
fSW = 2MHz
Figure5. Case Temperature Rise vs Load Current
STANDBY LOAD = 50mA
PULSED LOAD = 3A
V
IN
=12V
V
IN
=24V
DUTY CYCLE (%)
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
8609 F06
V
OUT
= 5V
SW
= 2MHz
f
Figure6. Case Temperature Rise vs 3A Pulsed Load
APPLICATIONS INFORMATION
LT8609/LT8609A/LT8609B
21
Rev. H
For more information www.analog.com
TYPICAL APPLICATIONS
3.3V Step-Down
5V Step-Down
C1
0.1µF
C2
4.7µF
C3
F
C4
22µF
1206
C5
10pF
L1
2.2µH
R1
18.2k
R2
1M
R3
309k
R4
100k
C6
10nF
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
4.3V
TO 42V
POWER
GOOD
fSW = 2MHz
L1 = COILCRAFT XFL4020-222ME
VOUT
3.3V
3A
8609 TA02
C1
0.1µF
C2
4.7µF
C3
F
C4
22µF
1206
C5
10pF
L1
2.2µH
R1
18.2k
R2
1M
R3
187k
R4
100k
C6
10nF
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
6V
TO 42V
POWER
GOOD
fSW = 2MHz
L1 = COILCRAFT XFL4020-222ME
VOUT
5V
3A
8609 TA03
12V Step-Down
C1
0.1µF
C2
4.7µF
C3
F
C4
22µF
1210
C5
10pF
L1
10µH
R1
40.2k
R2
1M
R3
69.8k
R4
100k
C6
10nF
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
13V
TO 42V
POWER
GOOD
F
SW
= 1MHz
L1 = COILCRAFT XAL4040-103ME
VOUT
12V
3A
8609 TA04
LT8609/LT8609A/LT8609B
22
Rev. H
For more information www.analog.com
TYPICAL APPLICATIONS
1.8V 2MHz Step-Down Converter
Ultralow EMI 5V 2A Step-Down Converter
C1
0.1µF
C2
4.7µF
C3
F
C4
47µF
1210
C5
10pF
L1
2.2µH
R1
18.2k
R2
1M
R3
768k
R4
100k
C6
10nF
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
2.8V
TO 20V
(42V TRANSIENT)
POWER
GOOD
fSW = 2MHz
L1 = COILCRAFT XFL4020-222ME
VOUT
1.8V
3A
8609 TA05
PSKIP M1
NFET
C1
0.1µF
C2
4.7µF
C3
F
C4
47µF
1210
C5
10pF
L1
8.2µH
R1
110k
R2
1M
R3
301k
R4
100K
C6
10nF
FB1
BEAD
L3
4.7µH
C7
4.7µF
C8
4.7µF
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
6V TO 40V
POWER GOOD
fSW = 400kHz
L1 = COILCRAFT XAL4040-822
FB1 = TDK MPZ2012S221AT000
C9 = OS-CON 63SXV33M
VOUT
3.3V
3A
8609 TA06
C9
33μF
LT8609/LT8609A/LT8609B
23
Rev. H
For more information www.analog.com
PACKAGE DESCRIPTION
MSOP (MSE) 0213 REV I
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
1234 5
4.90 ±0.152
(.193 ±.006)
0.497 ±0.076
(.0196 ±.003)
REF
8910
10
1
76
3.00 ±0.102
(.118 ±.004)
(NOTE 3)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
1.68 ±0.102
(.066 ±.004)
1.88 ±0.102
(.074 ±.004)
0.50
(.0197)
BSC
0.305 ± 0.038
(.0120 ±.0015)
TYP
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.68
(.066)
1.88
(.074)
0.1016 ±0.0508
(.004 ±.002)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.05 REF
0.29
REF
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev I)
LT8609/LT8609A/LT8609B
24
Rev. H
For more information www.analog.com
PACKAGE DESCRIPTION
MSOP (MSE16) 0213 REV F
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
16
16151413121110
12345678
9
9
18
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ±0.038
(.0120 ±.0015)
TYP
0.50
(.0197)
BSC
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
0.1016 ±0.0508
(.004 ±.002)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0.280 ±0.076
(.011 ±.003)
REF
4.90 ±0.152
(.193 ±.006)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.12 REF
0.35
REF
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
LT8609/LT8609A/LT8609B
25
Rev. H
For more information www.analog.com
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 01/16 Added the LT8609A Version to Header
Added the LT8609A Version to Description
Clarified Description
Clarified Electrical Specifications
Clarified Load Regulation, Line Regulation, No-Load Supply Current vs Temperature, Minimum On-Time vs
Temperature Graphs, Frequency Foldback and Soft-Start vs Temperature Graphs
Clarified Block Diagram with Optional Input Resistors
Replaced Figure 1 with Table 1 in text
Clarified CIN Capacitor in Text and PCB Layout
Clarified Typical Application
All
1
1
3
5, 6, 7, 8
10
12
18
24
B 06/16 Clarified Switch Drop vs Switch Current Graph axis units
Clarified Switching Waveforms conditions
6
8
C 10/16 Added LT8609B Option
Added LT8609B Option to Absolute Maximum Ratings, Added LT8609B Package Drawing and Ordering Information
Clarified Electrical Parameters and Notes for LT8609B Option
Clarified Top FET Current Limit vs Temperature Graph
Clarified Pin Functions to Include LT8609B
Clarified Operation Section to Include LT8609B
Clarified Applications Information Section to Include LT8609B
Clarified Graphs in Figure 5 and 6
Clarified Typical Applications Schematics
All
2
3
6
10
12
13 – 20
20
21, 22, 23, 24
D 01/17 Clarified Graphs
Clarified Schematics
6, 7, 20
23, 26
E 06/17 Add H-grade to the A version
Modified application circuits
2, 4
21, 22, 23
F 11/17 Clarified Oscillator Frequency RT Conditions
Clarified Minimum Off Time
Clarified Efficiency Graph
Clarified Frequency Foldback Graph
Clarified Block Diagram
Clarified Maximum Duty Cycle
Clarified Figure 4
3
3
5
8
11
14
19
G 01/18 Added MSOP-16E Package option
Clarified the Pin Functions
1, 2, 24
10
H 02/19 Changed 2A/3A to 3A
Replaced all graphs with IOUT to 3A
Added LT8609AJMSE16 and LT8609AJMSE options
Eliminated “The internal circuitry…” paragraph
Clarified Figure 4 PCB Layout
Clarified Figure 5
Eliminated “The LT8609/LT8609A/LT8609B’s…” paragraph
Updated Typical Applications
1
1, 5, 6, 7, 8
2
15
19
20
20
21, 22
LT8609/LT8609A/LT8609B
26
Rev. H
For more information www.analog.com
ANALOG DEVICES, INC. 2015-2018
02/19
www.analog.com
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C7
0.1µF
C8
4.7µF
R9
10k
R10
31.6k
C10
47µF
1210
C11
10pF
L2
2.2µH
R5
18.2k
R6
1M
R7
768k
R8
100k
C12
F
VIN
EN/UV
SYNC
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
POWER
GOOD
fSW = 2MHz
VOUT
1.8V
3A
8609 TA07
C1
0.1µF
C2
4.7µF
C3
F
C4
47µF
1210
C5
10pF
L1
2.2µH
R1
18.2k
R2
1M
R3
309k
R4
100k
C6
10nF
VIN
EN/UV
LT8609
INTVCC
TR/SS
RT
GND
FB
PG
SW
BST
VIN
4.3V
TO 20V
(42V TRANSIENT)
POWER
GOOD
fSW = 2MHz
VOUT
3.3V, 3A
SYNC
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