FEATURES
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LT3957
Boost, Flyback, SEPIC and
Inverting Converter
with 5A, 40V Switch
DESCRIPTION
The LT
®
3957 is a wide input range, current mode DC/DC
converter which is capable of generating either positive
or negative output voltages. It can be configured as either
a boost, flyback, SEPIC or inverting converter. It features
an internal low side N-channel power MOSFET rated for
40V at 5A and driven from an internal regulated 5.2V
supply. The fixed frequency, current-mode architecture
results in stable operation over a wide range of supply
and output voltages.
The operating frequency of LT3957 can be set with an
external resistor over a 100kHz to 1MHz range, and can
be synchronized to an external clock using the SYNC pin.
A minimum operating supply voltage of 3V, and a low
shutdown quiescent current of less than 1μA, make the
LT3957 ideally suited for battery-powered systems.
The LT3957 features soft-start and frequency foldback
functions to limit inductor current during start-up.
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No R
SENSE
and ThinSOT are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents pending.
Wide Input Voltage Range: 3V to 40V
Single Feedback Pin for Positive or Negative
Output Voltage
Internal 5A/40V Power Switch
Current Mode Control Provides Excellent Transient
Response
Programmable Operating Frequency (100kHz to
1MHz) with One External Resistor
Synchronizable to an External Clock
Low Shutdown Current < 1μA
Internal 5.2V Low Dropout Voltage Regulator
Programmable Input Undervoltage Lockout with
Hysteresis
Programmable Soft-Start
Thermally Enhanced QFN (5mm
×
6mm) Package
APPLICATIONS
n
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Automotive
Telecom
Industrial
TYPICAL APPLICATION
High Efficiency Output Boost Converter
V
IN
4.5V TO 16V
10μF
200k
V
IN
EN/UVLO
95.3k
SGND
SYNC
RT
41.2k
300kHz
SS
0.33μF
VC
Efficiency vs Output Current
V
OUT
24V
600mA
100
95
EFFICIENCY (%)
90
85
80
75
70
0
100
600
400
500
200
300
OUTPUT CURRENT (mA)
3957 TA01b
V
IN
= 12V
10μH
10μF
2
GND
SW
LT3957
SENSE1
SENSE2
FBX
INTV
CC
4.7μF
15.8k
226k
6.8k
22nF
3957 TA01a
3957f
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LT3957
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SYNC
FBX
NC
NC
SS
VC
28 INTV
CC
27 V
IN
SGND
37
25 EN/UVLO
24 SGND
SENSE1 6
SW 8
SW 9
NC 10
12 13 14 15 16 17
GND
GND
GND
GND
GND
GND
SW
38
23 SGND
21 SW
20 SW
RT
V
IN
, EN/UVLO (Note 5), SW ......................................40V
INTV
CC
......................................................V
IN
+ 0.3V, 8V
SYNC ..........................................................................8V
VC, SS .........................................................................3V
RT
...............................................................................................
1.5V
SENSE1, SGND .................. Internally Connected to GND
SENSE2..................................................................±0.3V
FBX ................................................................. –6V to 6V
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range .................. –65°C to 125°C
36 35 34 33 32 31 30
NC 1
NC 2
SENSE2 3
SGND 4
UHE PACKAGE
36-LEAD (5mm 6mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 42°C/W,
θ
JC
= 3°C/W
EXPOSED PAD (PIN 37) IS SGND, MUST BE SOLDERED TO SGND PLANE
EXPOSED PAD (PIN 38) IS SW, MUST BE SOLDERED TO SW PLANE
ORDER INFORMATION
LEAD FREE FINISH
LT3957EUHE#PBF
LT3957IUHE#PBF
TAPE AND REEL
LT3957EUHE#TRPBF
LT3957IUHE#TRPBF
PART MARKING*
3957
3957
PACKAGE DESCRIPTION
36-Lead (5mm
×
6mm) Plastic QFN
36-Lead (5mm
×
6mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°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/
3957f
2
LT3957
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Operating Range
V
IN
Shutdown I
Q
V
IN
Operating I
Q
V
IN
Operating I
Q
with Internal LDO Disabled
SW Pin Current Limit
SW Pin On Voltage
SENSE2 Input Bias Current
Error Amplifier
FBX Regulation Voltage (V
FBX(REG)
)
FBX Overvoltage Lockout
FBX Pin Input Current
Transconductance g
m
(ΔI
VC
/ΔFBX)
VC Output Impedance
V
FBX
Line Regulation (ΔV
FBX
/[ΔV
IN
• V
FBX(REG)
])
VC Current Mode Gain (ΔV
VC
/ΔV
SENSE
)
VC Source Current
VC Sink Current
Oscillator
Switching Frequency
R
T
= 140k to SGND, FBX = 1.6V, V
C
= 1.5V
R
T
= 41.2k to SGND, FBX = 1.6V, V
C
= 1.5V
R
T
= 10.5k to SGND, FBX = 1.6V, V
C
= 1.5V
FBX = 1.6V
80
270
850
100
300
1000
1.2
220
240
1.5
SS = 0V, Current Out of Pin
l
The
l
denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at T
A
≈ T
J
= 25°C. V
IN
= 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted.
CONDITIONS
EN/UVLO = 0V
EN/UVLO = 1.15V
VC = 0.3V, R
T
= 41.2k
VC = 0.3V, R
T
= 41.2k, INTV
CC
= 5.5V
l
MIN
3
TYP
0.1
1.7
350
MAX
40
1
6
2.3
400
6.8
UNITS
V
μA
μA
mA
μA
A
mV
μA
5
5.9
100
–65
I
SW
= 3A
Current Out of Pin
FBX > 0V (Note 3)
FBX < 0V (Note 3)
FBX > 0V (Note 4)
FBX < 0V (Note 4)
FBX = 1.6V (Note 3)
FBX = –0.8V (Note 3)
(Note 3)
(Note 3)
FBX > 0V, 3V < V
IN
< 40V (Notes 3, 6)
FBX < 0V, 3V < V
IN
< 40V (Notes 3, 6)
VC = 1.5V, FBX = 0V, Current Out of Pin
FBX = 1.7V
FBX = –0.85V
l
l
1.569
–0.816
6
7
–10
1.6
–0.800
8
11
70
230
5
0.04
0.03
10
–15
12
11
1.631
–0.784
10
14
100
10
V
V
%
%
nA
nA
μS
MΩ
0.06
0.06
%/V
%/V
V/V
μA
μA
μA
120
330
1200
275
320
0.4
kHz
kHz
kHz
V
ns
ns
RT Voltage
SW Minimum Off-Time
SW Minimum On-Time
SYNC Input Low
SYNC Input High
SS Pull-Up Current
Low Dropout Regulator
INTV
CC
Regulation Voltage
INTV
CC
Undervoltage Lockout Threshold
–10
5
2.6
5.2
2.7
0.15
5.45
2.85
μA
V
V
V
Falling INTV
CC
UVLO Hysteresis
3957f
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LT3957
ELECTRICAL CHARACTERISTICS
PARAMETER
INTV
CC
Current Limit
INTV
CC
Load Regulation (ΔV
INTVCC
/ V
INTVCC
)
Dropout Voltage (V
IN
– V
INTVCC
)
INTV
CC
Current in Shutdown
INTV
CC
Voltage to Bypass Internal LDO
Logic Inputs
EN/UVLO Threshold Voltage Falling
EN/UVLO Voltage Hysteresis
EN/UVLO Input Low Voltage
EN/UVLO Pin Bias Current Low
EN/UVLO Pin Bias Current High
I
VIN
Drops Below 1μA
EN/UVLO = 1.15V
EN/UVLO = 1.33V
1.7
2
20
V
IN
= INTV
CC
= 6V
l
The
l
denotes the specifications which apply over the full operating temp-
erature range, otherwise specifications are at T
A
≈ T
J
= 25°C. V
IN
= 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted.
CONDITIONS
V
IN
= 40V
V
IN
= 15V
0 < I
INTVCC
< 20mA, V
IN
= 8V
V
IN
= 5V, I
INTVCC
= 20mA, VC = 0V
EN/UVLO = 0V, INTV
CC
= 6V
MIN
32
–1
TYP
40
95
–0.5
0.02
450
17
5.5
1.17
1.22
20
0.4
2.5
100
1.27
0.05
MAX
55
UNITS
mA
mA
%
%/V
mV
μA
V
V
mV
V
μA
nA
INTV
CC
Line Regulation (ΔV
INTVCC
/ [ΔV
IN
• V
INTVCC
]) 6V < V
IN
< 40V
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 LT3957E is guaranteed to meet performance specifications
from the 0°C to 125°C operating 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 LT3957I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 3:
The LT3957 is tested in a feedback loop which servos V
FBX
to the
reference voltages (1.6V and –0.8V) with the VC pin forced to 1.3V.
Note 4:
FBX overvoltage lockout is measured at V
FBX(OVERVOLTAGE)
relative
to regulated V
FBX(REG)
.
Note 5:
For 3V ≤ V
IN
< 6V, the EN/UVLO pin must not exceed V
IN
.
Note 6:
EN/UVLO = 1.33V when V
IN
= 3V.
TYPICAL PERFORMANCE CHARACTERISTICS
Positive Feedback Voltage
vs Temperature, V
IN
1605
REGULATED FEEDBACK VOLTAGE (mV)
REGULATED FEEDBACK VOLTAGE (mV)
V
IN
= 40V
1600
V
IN
= 24V
–788
–790
–792
–794
–796
–798
–800
–802
–804
–50
–25
50
25
0
75
TEMPERATURE (°C)
V
IN
= 24V
V
IN
= 40V
V
IN
= 8V
V
IN
= INTV
CC
= 3V
SHDN/UVLO = 1.33V
T
A
≈ T
J
= 25°C, unless otherwise noted.
Quiescent Current
vs Temperature, V
IN
1.8
Negative Feedback Voltage
vs Temperature, V
IN
QUIESCENT CURRENT (mA)
1.7
V
IN
= 40V
V
IN
= 24V
1590
V
IN
= 8V
1.6
1585
V
IN
= INTV
CC
= 3V,
SHDN/UVLO
= 1.33V
1.5
V
IN
= INTV
CC
= 3V
1580
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
100
125
1.4
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3957 G01
3957 G02
3957 G03
3957f
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LT3957
TYPICAL PERFORMANCE CHARACTERISTICS
Dynamic Quiescent Current
vs Switching Frequency
12
10
8
6
4
2
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING FREQUENCY (kHz)
3957 G04
T
A
≈ T
J
= 25°C, unless otherwise noted.
Normalized Switching
Frequency vs FBX
120
NORMALIZED FREQUENCY (%)
100
80
60
40
20
0
–0.8
R
T
vs Switching Frequency
1000
R
T
(kΩ)
I
Q
(mA)
100
10
0 100 200 300 400 500 600 700 800 900 1000
SWITCHING FREQUENCY (kHz)
3957 G05
–0.4
0
0.4
0.8
FBX VOLTAGE (V)
1.2
1.6
3957 G06
Switching Frequency
vs Temperature
325
320
SWITCHING FREQUENCY (kHz)
315
310
305
300
295
290
285
280
275
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
R
T
= 41.2k
SW PIN CURRENT LIMIT (A)
6.6
6.4
6.2
6.0
5.8
5.6
SW Pin Current Limit
vs Temperature
6.6
6.4
6.2
6.0
5.8
5.6
5.4
–25
50
25
0
75
TEMPERATURE (°C)
100
125
SW Pin Current Limit
vs Duty Cycle
5.4
–50
SW PIN CURRENT LIMIT (A)
0
20
40
60
DUTY CYCLE (%)
80
100
3957 G09
3957 G07
3957 G08
EN/UVLO Threshold
vs Temperature
1.28
40
EN/UVLO Current vs Voltage
2.4
EN/UVLO Hysteresis Current
vs Temperature
1.26
EN/UVLO RISING
1.24
EN/UVLO CURRENT (μA)
EN/UVLO VOLTAGE (V)
30
I
EN/UVLO
(μA)
0
10
20
30
EN/UVLO VOLTAGE (V)
40
3957 G11
2.2
20
2.0
1.22
EN/UVLO FALLING
10
1.20
1.8
1.18
–50
0
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1.6
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3957 G10
3957 G12
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