LT3758
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APPLICATIONS
n Automotive
n Telecom
n Industrial
n Wide Input Voltage Range: 5.5V to 100V
n Positive or Negative Output Voltage Programming
with a Single Feedback Pin
n Current Mode Control Provides Excellent Transient
Response
n Programmable Operating Frequency (100kHz to
1MHz) with One External Resistor
n Synchronizeable to an External Clock
n Low Shutdown Current < 1μA
n Internal 7.2V Low Dropout Voltage Regulator
n Programmable Input Undervoltage Lockout with
Hysteresis
n Programmable Soft-Start
n Small 10-Lead DFN (3mm × 3mm) and
MSOPE Packages
TYPICAL APPLICATION
DESCRIPTION
High Input Voltage,
Boost, Flyback, SEPIC and
Inverting Controller
The LT
®
3758 is a wide input range, current mode, DC/DC
controller which is capable of generating either positive or
negative output voltages. It can be confi gured as either a
boost, fl yback, SEPIC or inverting converter. The LT3758
drives a low side external N-channel power MOSFET from
an internal regulated 7.2V supply. The fi xed frequency,
current-mode architecture results in stable operation over
a wide range of supply and output voltages.
The operating frequency of LT3758 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 5.5V, and a low
shutdown quiescent current of less than 1μA, make the
LT3758 ideally suited for battery-powered systems.
The LT3758 features soft-start and frequency foldback
functions to limit inductor current during start-up and
output short-circuit.
12V Output Nonisolated Flyback Power Supply
FEATURES
SENSE
LT3758
VIN
DSN
VIN
36V TO
72V CIN
2.2μF
100V
X7R
63.4k
200kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
0.022μF
100V T1
1,2,3
(SERIES)
4,5,6
(PARALLEL)
1M
44.2k
0.47μF
100pF 10k
10nF
0.030Ω 15.8k
1%
105k
1%
CVCC
4.7μF
10V
X5R
VOUT
12V
1.2A
3758 TA01
COUT
47μF
X5R
6.2k
D1
SW
M1
5.1Ω
1N4148
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
and No RSENSE and ThinSOT are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Patents pending.
LT3758
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PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
VIN, SHDN/UVLO (Note 7) .......................................100V
INTVCC .................................................... VIN + 0.3V, 20V
GATE .........................................................INTVCC + 0.3V
SYNC ..........................................................................8V
VC, SS .........................................................................3V
RT ...............................................................................................1.5V
SENSE ....................................................................±0.3V
FBX ................................................................. –6V to 6V
(Note 1)
TOP VIEW
DD PACKAGE
10-LEAD (3mm s 3mm) PLASTIC DFN
10
9
6
7
8
4
5
11
3
2
1VIN
SHDN/UVLO
INTVCC
GATE
SENSE
VC
FBX
SS
RT
SYNC
TJMAX = 125°C, θJA = 43°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
VC
FBX
SS
RT
SYNC
10
9
8
7
6
VIN
SHDN/UVLO
INTVCC
GATE
SENSE
TOP VIEW
MSE PACKAGE
10-LEAD PLASTIC MSOP
11
TJMAX = 150°C, θJA = 40°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3758EDD#PBF LT3758EDD#TRPBF LDNK 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C
LT3758IDD#PBF LT3758IDD#TRPBF LDNK 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C
LT3758EMSE#PBF LT3758EMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C
LT3758IMSE #PBF LT3758IMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C
LT3758HMSE#PBF LT3758HMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 150°C
LT3758MPMSE #PBF LT3758MPMSE#TRPBF LTDNM 10-Lead (3mm × 3mm) Plastic MSOP –55°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed 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 specifi cations, go to: http://www.linear.com/tapeandreel/
Operating Junction Temperature Range (Notes 2, 8)
LT3758E ............................................. –40°C to 125°C
LT3758I .............................................. –40°C to 125°C
LT3758H ............................................ –40°C to 150°C
LT3758MP.......................................... –55°C to 125°C
Storage Temperature Range
DFN .................................................... –65°C to 125°C
MSOP ................................................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP ............................................................... 300°C
LT3758
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ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating temp-
erature range, otherwise specifi cations are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Operating Range 5.5 100 V
VIN Shutdown IQSHDN/UVLO = 0V
SHDN/UVLO = 1.15V
0.1 1
6
μA
μA
VIN Operating IQVC = 0.3V, RT = 41.2k 1.75 2.2 mA
VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.2k, INTVCC = 7.5V 350 400 μA
SENSE Current Limit Threshold l100 110 120 mV
SENSE Input Bias Current Current Out of Pin –65 μA
Error Amplifi er
FBX Regulation Voltage (VFBX(REG)) FBX > 0V (Note 3)
FBX < 0V (Note 3)
l
l
1.569
–0.816
1.6
–0.800
1.631
–0.784
V
V
FBX Overvoltage Lockout FBX > 0V (Note 4)
FBX < 0V (Note 4)
6
7
8
11
10
14
%
%
FBX Pin Input Current FBX = 1.6V (Note 3)
FBX = –0.8V (Note 3) –10
70 100
10
nA
nA
Transconductance gm (ΔIVC /ΔFBX) (Note 3) 230 μS
VC Output Impedance (Note 3) 5 MΩ
VFBX Line Regulation (ΔVFBX/[ΔVIN • VFBX(REG)]) FBX > 0V, 5.5V < VIN < 100V (Notes 3, 6)
FBX < 0V, 5.5V < VIN < 100V (Notes 3, 6)
0.006
0.005
0.025
0.03
%/V
%/V
VC Current Mode Gain (ΔVVC /ΔVSENSE) 5.5 V/V
VC Source Current VC = 1.5V –15 μA
VC Sink Current FBX = 1.7V
FBX = –0.85V
12
11
μA
μA
Oscillator
Switching Frequency RT = 41.2k to GND, FBX = 1.6V
RT = 140k to GND, FBX = 1.6V
RT = 10.5k to GND, FBX = 1.6V
270 300
100
1000
330 kHz
kHz
kHz
RT Voltage FBX = 1.6V 1.2 V
Minimum Off-Time 220 ns
Minimum On-Time 220 ns
SYNC Input Low 0.4
SYNC Input High 1.5
SS Pull-Up Current SS = 0V, Current Out of Pin –10 μA
Low Dropout Regulator
INTVCC Regulation Voltage l7 7.2 7.4 V
INTVCC Undervoltage Lockout Threshold Falling INTVCC
UVLO Hysteresis
4.3 4.5
0.5
4.7 V
V
INTVCC Overvoltage Lockout Threshold 17.5 V
INTVCC Current Limit VIN = 100V
VIN = 20V
11 16
50
22 mA
mA
INTVCC Load Regulation (ΔVINTVCC /V
INTVCC) 0 < IINTVCC < 10mA, VIN = 8V –1 –0.4 %
INTVCC Line Regulation (ΔVINTVCC /[ΔV
IN • VINTVCC]) 8V < VIN < 100V 0.005 0.02 %/V
Dropout Voltage (VIN – VINTVCC)V
IN = 6V, IINTVCC = 10mA 500 mV
LT3758
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ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating temp-
erature range, otherwise specifi cations are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
INTVCC Current in Shutdown SHDN/UVLO = 0V, INTVCC = 8V 16 μA
INTVCC Voltage to Bypass Internal LDO 7.5 V
Logic Inputs
SHDN/UVLO Threshold Voltage Falling VIN = INTVCC = 8V l1.17 1.22 1.27 V
SHDN/UVLO Input Low Voltage IVIN Drops Below 1μA 0.4 V
SHDN/UVLO Pin Bias Current Low SHDN/UVLO = 1.15V 1.7 2 2.5 μA
SHDN/UVLO Pin Bias Current High SHDN/UVLO = 1.33V 10 100 nA
Gate Driver
tr Gate Driver Output Rise Time CL = 3300pF (Note 5), INTVCC = 7.5V 22 ns
tf Gate Driver Output Fall Time CL = 3300pF (Note 5), INTVCC = 7.5V 20 ns
Gate Output Low (VOL) 0.05 V
Gate Output High (VOH)INTVCC
–0.05
V
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 LT3758E is guaranteed to meet performance specifi cations
from the 0°C to 125°C junction temperature. Specifi cations over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3758I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3758H 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. The LT3758MP is 100% tested and
guaranteed over the full –55°C to 125°C operating junction temperature
range.
Note 3: The LT3758 is tested in a feedback loop which servos VFBX 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 VFBX(OVERVOLTAGE) relative
to regulated VFBX(REG).
Note 5: Rise and fall times are measured at 10% and 90% levels.
Note 6: SHDN/UVLO = 1.33V when VIN = 5.5V.
Note 7: For VIN below 6V, the SHDN/UVLO pin must not exceed VIN.
Note 8: The LT3758 includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specifi ed maximum operating junction temperature may impair device
reliability.
LT3758
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TEMPERATURE (°C)
–75 –50
1580
1585
REGULATED FEEDBACK VOLTAGE (V)
1590
1605
1600
05075
1595
–25 25 100 150125
3758 G01
VIN = 100V
VIN = 24V
VIN = 8V
VIN = INTVCC = 5.5V
SHDN/UVLO = 1.33V
TEMPERATURE (°C)
REGULATED FEEDBACK VOLTAGE (mV)
–802
–800
–798
–790
–792
–794
–804
–796
3758 G02
–75 –50 0 50 75–25 25 100 150125
VIN = 100V
VIN = 24V
VIN = 8V
VIN = INTVCC = 5.5V
SHDN/UVLO = 1.33V
TYPICAL PERFORMANCE CHARACTERISTICS
Positive Feedback Voltage
vs Temperature, VIN
Negative Feedback Voltage
vs Temperature, VIN
Quiescent Current
vs Temperature, VIN
TA = 25°C, unless otherwise noted.
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
1.5
QUIESCENT CURRENT (mA)
1.6
1.9
1.8
1.7
3758 G03
VIN = 100V
VIN = 24V
VIN = INTVCC = 5.5V
Dynamic Quiescent Current
vs Switching Frequency
RT vs Switching Frequency
Normalized Switching
Frequency vs FBX
FBX VOLTAGE (V)
–0.8
0
NORMALIZED FREQUENCY (%)
20
40
60
80
120
–0.4 0 0.4 0.8
3758 G06
1.2 1.6
100
SWITCHING FREQUENCY (kHz)
0
0
IQ(mA)
15
20
35
300 500 600 700
10
5
25
30
100 200 400 900800 1000
3758 G04
CGATE = 3300pF
SWITCHING FREQUENCY (kHz)
0
10
RT(kΩ)
100
1000
300 500 600 700
100 200 400 900800 1000
3758 G05
LT3758
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Switching Frequency
vs Temperature
SENSE Current Limit Threshold
vs Temperature
SENSE Current Limit Threshold
vs Duty Cycle
SHDN/UVLO Threshold
vs Temperature
SHDN/UVLO Current vs Voltage
SHDN/UVLO Hysteresis Current
vs Temperature
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
100
SENSE THRESHOLD (mV)
105
110
115
120
3758 G08
DUTY CYCLE (%)
0
95
SENSE THRESHOLD (mV)
105
20 40 8060
115
100
110
100
3758 G09
SHDN/UVLO VOLTAGE (V)
0
0
SHDN/UVLO CURRENT (μA)
20
20 6040 80
40
50
10
30
100
3758 G11
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
1.6
ISHDN/ UVLO (μA)
1.8
2.0
2.2
2.4
3758 G12
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
270
SWITCHING FREQUENCY (kHz)
280
290
300
310
330
3758 G07
320
RT = 41.2K
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
1.18
SHDN/UVLO VOLTAGE (V)
1.22
1.24
1.26
1.28
1.20
3758 G10
SHDN/UVLO FALLING
SHDN/UVLO RISING
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
LT3758
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TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
INTVCC Line Regulation
INTVCC Dropout Voltage
vs Current, Temperature
INTVCC vs Temperature
INTVCC Minimum Output
Current vs VIN INTVCC Load Regulation
–75 –50 0 50 75–25 25 100 150125
TEMPERATURE (°C)
7.0
INTVCC (V)
7.1
7.2
7.3
7.4
3758 G13
VIN (V)
0
INTVCC VOLTAGE (V)
90
7.25
7.20
20 30 5010 40 60 70 80 100
7.15
7.10
7.30
3758 G16
1
0
5
10
20
30
10 100
45
40
15
25
35
TJ = 150°C
VIN (V)
3758 G14
INTVCC CURRENT (mA)
INTVCC = 6V
INTVCC = 4.7V
INTVCC LOAD (mA)
0
6.8
7
7.1
7.2
7.3
10 20 25
6.9
515
3758 G15
INTVCC VOLTAGE (V)
VIN = 8V
0426810
INTVCC LOAD (mA)
DROPOUT VOLTAGE (mV)
500
600
300
400
200
100
0
1000
900
800
700
3758 G17
150°C
25°C
0°C
–55°C
75°C
VIN = 6V
125°C
Gate Drive Rise
and Fall Time vs INTVCC Typical Start-Up Waveforms
FBX Frequency Foldback
Waveforms During Overcurrent
INTVCC (V)
3
TIME (ns)
20
25
15
10
9
612 15
5
0
30
3758 G19
CL = 3300pF
RISE TIME
FALL TIME
2ms/DIV
SEE TYPICAL APPLICATION: 18V TO 72V INPUT,
24V OUTPUT SEPIC CONVERTER
VOUT
10V/DIV
IL1A + IL1B
1A/DIV
3758 G20
VIN = 48V
50μs/DIV
VOUT
20V/DIV
VSW
50V/DIV
IL1A + IL1B
2A/DIV
3758 G21
VIN = 48V
SEE TYPICAL APPLICATION: 18V TO 72V INPUT,
24V OUTPUT SEPIC CONVERTER
Gate Drive Rise
and Fall Time vs CL
CL (nF)
0
TIME (ns)
60
70
80
50
40
51510 20 25 30
10
0
30
90
20
3758 G18
RISE TIME
INTVCC = 7.2V
FALL TIME
LT3758
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PIN FUNCTIONS
VC (Pin 1): Error Amplifi er Compensation Pin. Used to
stabilize the voltage loop with an external RC network.
FBX (Pin 2): Positive and Negative Feedback Pin. Receives
the feedback voltage from the external resistor divider
across the output. Also modulates the switching frequency
during start-up and fault conditions when FBX is close
to GND.
SS (Pin 3): Soft-Start Pin. This pin modulates compensation
pin voltage (VC) clamp. The soft-start interval is set with
an external capacitor. The pin has a 10μA (typical) pull-up
current source to an internal 2.5V rail. The soft-start pin is
reset to GND by an undervoltage condition at SHDN/UVLO,
an INTVCC undervoltage or overvoltage condition or an
internal thermal lockout.
RT (Pin 4): Switching Frequency Adjustment Pin. Set the
frequency using a resistor to GND. Do not leave this pin
open.
SYNC (Pin 5): Frequency Synchronization Pin. Used to
synchronize the switching frequency to an outside clock.
If this feature is used, an RT resistor should be chosen
to program a switching frequency 20% slower than the
SYNC pulse frequency. Tie the SYNC pin to GND if this
feature is not used. SYNC is bypassed when FBX is close
to GND.
SENSE (Pin 6): The Current Sense Input for the Control
Loop. Kelvin connect this pin to the positive terminal of
the switch current sense resistor in the source of the NFET.
The negative terminal of the current sense resistor should
be connected to GND plane close to the IC.
GATE (Pin 7): N-Channel MOSFET Gate Driver Output.
Switches between INTVCC and GND. Driven to GND when
IC is shut down, during thermal lockout or when INTVCC
is above or below the overvoltage or UV thresholds,
respectively.
INTVCC (Pin 8): Regulated Supply for Internal Loads and
Gate Driver. Supplied from VIN and regulated to 7.2V
(typical). INTVCC must be bypassed with a minimum
of 4.7μF capacitor placed close to pin. INTVCC can be
connected directly to VIN, if VIN is less than 17.5V. INTVCC
can also be connected to a power supply whose voltage is
higher than 7.5V, and lower than VIN, provided that supply
does not exceed 17.5V.
SHDN/UVLO (Pin 9): Shutdown and Undervoltage Detect
Pin. An accurate 1.22V (nominal) falling threshold with
externally programmable hysteresis detects when power
is okay to enable switching. Rising hysteresis is generated
by the external resistor divider and an accurate internal
2μA pull-down current. An undervoltage condition resets
sort-start. Tie to 0.4V, or less, to disable the device and
reduce VIN quiescent current below 1μA.
VIN (Pin 10): Input Supply Pin. Must be locally bypassed
with a 0.22μF, or larger, capacitor placed close to the
pin.
Exposed Pad (Pin 11): Ground. This pin also serves as the
negative terminal of the current sense resistor. The Exposed
Pad must be soldered directly to the local ground plane.
LT3758
9
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BLOCK DIAGRAM
Figure 1. LT3758 Block Diagram Working as a SEPIC Converter
L1
R1
R3R4
M1
R2
L2
FBX
1.22V
2.5V
D1
CDC
CIN
VOUT
COUT2
COUT1
CVCC
INTVCC
VIN
RSENSE
VISENSE
•+
+
VIN
IS1
2μA
10
8
7
1
9
SHDN/UVLO
INTERNAL
REGULATOR
AND UVLO
TSD
165˚C
A10
Q3
VC
VC
17.5V
5V UP
4.5V DOWN
A8
UVLO
IS2
10μA
IS3
CC1
CC2
RC
DRIVER
SLOPE
SENSE
GND
GATE
110mV
SR1
+
–
+
–
CURRENT
LIMIT
RAMP
GENERATOR
7.2V LDO
•
+
–
+
–
RO
S
2.5V
RT
RT
SS
CSS
SYNC
1.28V
1.2V
FBX
FBX
1.6V
–0.8V
+
–
+
–
2
3 5 4
+
–
+
–
6
11
RAMP
PWM
COMPARATOR
FREQUENCY
FOLDBACK
100kHz-1MHz
OSCILLATOR
FREQ
PROG
3758 F01
–
+
+Q1
A1
A2
1.72V
–0.88V
+
–
+
–
A11
A12
+
–
A3
A4
A5
A6
G2
G5
G6
A7
A9
Q2
G4 G3
G1
LT3758
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APPLICATIONS INFORMATION
Main Control Loop
The LT3758 uses a fi xed frequency, current mode control
scheme to provide excellent line and load regulation. Op-
eration can be best understood by referring to the Block
Diagram in Figure 1.
The start of each oscillator cycle sets the SR latch (SR1) and
turns on the external power MOSFET switch M1 through
driver G2. The switch current fl ows through the external
current sensing resistor RSENSE and generates a voltage
proportional to the switch current. This current sense
voltage VISENSE (amplifi ed by A5) is added to a stabilizing
slope compensation ramp and the resulting sum (SLOPE)
is fed into the positive terminal of the PWM comparator A7.
When SLOPE exceeds the level at the negative input of A7
(VC pin), SR1 is reset, turning off the power switch. The
level at the negative input of A7 is set by the error amplifi er
A1 (or A2) and is an amplifi ed version of the difference
between the feedback voltage (FBX pin) and the reference
voltage (1.6V or –0.8V, depending on the confi guration).
In this manner, the error amplifi er sets the correct peak
switch current level to keep the output in regulation.
The LT3758 has a switch current limit function. The current
sense voltage is input to the current limit comparator A6.
If the SENSE pin voltage is higher than the sense current
limit threshold VSENSE(MAX) (110mV, typical), A6 will reset
SR1 and turn off M1 immediately.
The LT3758 is capable of generating either positive or
negative output voltage with a single FBX pin. It can
be confi gured as a boost, fl yback or SEPIC converter
to generate positive output voltage, or as an inverting
converter to generate negative output voltage. When
confi gured as a SEPIC converter, as shown in Figure 1,
the FBX pin is pulled up to the internal bias voltage of 1.6V
by a voltage divider (R1 and R2) connected from VOUT to
GND. Comparator A2 becomes inactive and comparator
A1 performs the inverting amplifi cation from FBX to VC.
When the LT3758 is in an inverting confi guration, the FBX
pin is pulled down to –0.8V by a voltage divider connected
from VOUT to GND. Comparator A1 becomes inactive and
comparator A2 performs the noninverting amplifi cation
from FBX to VC.
The LT3758 has overvoltage protection functions to
protect the converter from excessive output voltage
overshoot during start-up or recovery from a short-circuit
condition. An overvoltage comparator A11 (with 20mV
hysteresis) senses when the FBX pin voltage exceeds the
positive regulated voltage (1.6V) by 8% and provides a
reset pulse. Similarly, an overvoltage comparator A12
(with 10mV hysteresis) senses when the FBX pin voltage
exceeds the negative regulated voltage (–0.8V) by 11%
and provides a reset pulse. Both reset pulses are sent to
the main RS latch (SR1) through G6 and G5. The power
MOSFET switch M1 is actively held off for the duration of
an output overvoltage condition.
Programming Turn-On and Turn-Off Thresholds with
the SHDN/UVLO Pin
The SHDN/UVLO pin controls whether the LT3758 is
enabled or is in shutdown state. A micropower 1.22V
reference, a comparator A10 and a controllable current
source IS1 allow the user to accurately program the supply
voltage at which the IC turns on and off. The falling value
can be accurately set by the resistor dividers R3 and R4.
When SHDN/UVLO is above 0.4V, and below the 1.22V
threshold, the small pull-down current source IS1 (typical
2μA) is active.
The purpose of this current is to allow the user to program
the rising hysteresis. The Block Diagram of the comparator
and the external resistors is shown in Figure 1. The typical
falling threshold voltage and rising threshold voltage can
be calculated by the following equations:
VRR
R
VµA
VIN FALLING
VIN RISING
,
,
.•
()
•
=+
=
122 34
4
2RRV
IN FALLING
3+,
For applications where the SHDN/UVLO pin is only used
as a logic input, the SHDN/UVLO pin can be connected
directly to the input voltage VIN through a 1k resistor for
always-on operation.
LT3758
11
3758fb
APPLICATIONS INFORMATION
INTVCC Regulator Bypassing and Operation
An internal, low dropout (LDO) voltage regulator produces
the 7.2V INTVCC supply which powers the gate driver, as
shown in Figure 1. The LT3758 contains an undervoltage
lockout comparator A8 and an overvoltage lockout
comparator A9 for the INTVCC supply. The INTVCC
undervoltage (UV) threshold is 4.5V (typical), with 0.5V
hysteresis, to ensure that the MOSFETs have suffi cient gate
drive voltage before turning on. The logic circuitry within the
LT3758 is also powered from the internal INTVCC supply.
The INTVCC overvoltage threshold is set to be 17.5V
(typical) to protect the gate of the power MOSFET. When
INTVCC is below the UV threshold, or above the overvoltage
threshold, the GATE pin will be forced to GND and the
soft-start operation will be triggered.
The INTVCC regulator must be bypassed to ground
immediately adjacent to the IC pins with a minimum of
4.7μF ceramic capacitor. Good bypassing is necessary to
supply the high transient currents required by the MOSFET
gate driver.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
The on-chip power dissipation can be a signifi cant concern
when a large power MOSFET is being driven at a high
frequency and the VIN voltage is high. It is important to
limit the power dissipation through selection of MOSFET
and/or operating frequency so the LT3758 does not exceed
its maximum junction temperature rating. The junction
temperature TJ can be estimated using the following
equations:
T
J = TA + PIC • θJA
TA = ambient temperature
θJA = junction-to-ambient thermal resistance
PIC = IC power consumption
= VIN • (IQ + IDRIVE)
IQ = VIN operation IQ = 1.6mA
IDRIVE = average gate drive current = f • QG
f = switching frequency
QG = power MOSFET total gate charge
The LT3758 uses packages with an Exposed Pad for en-
hanced thermal conduction. With proper soldering to the
Exposed Pad on the underside of the package and a full
copper plane underneath the device, thermal resistance
(θJA) will be about 43°C/W for the DD package and 40°C/W
for the MSE package. For an ambient board temperature of
TA = 70°C and maximum junction temperature of 125°C,
the maximum IDRIVE (IDRIVE(MAX)) of the DD package can
be calculated as:
ITT
VIW
V
DRIVE MAX JA
JA IN QIN
()
()
(•)
..=−−= −
θ
128 166mA
The LT3758 has an internal INTVCC IDRIVE current limit
function to protect the IC from excessive on-chip power
dissipation. The IDRIVE current limit decreases as the VIN
increases (see the INTVCC Minimum Output Current vs VIN
graph in the Typical Performance Characteristics section).
If IDRIVE reaches the current limit, INTVCC voltage will fall
and may trigger the soft-start.
Based on the preceding equation and the INTVCC Minimum
Output Current vs VIN graph, the user can calculate the
maximum MOSFET gate charge the LT3758 can drive at
a given VIN and switch frequency. A plot of the maximum
QG vs VIN at different frequencies to guarantee a minimum
4.7V INTVCC is shown in Figure 2.
Figure 2. Recommended Maximum QG vs VIN at Different
Frequencies to Ensure INTVCC Higher Than 4.7V
VIN (V)
1
QG (nC)
10 100
3758 F02
0
20
40
80
120
140
60
100
300kHz
1MHz
LT3758
12
3758fb
APPLICATIONS INFORMATION
As illustrated in Figure 2, a trade-off between the operating
frequency and the size of the power MOSFET may be needed
in order to maintain a reliable IC junction temperature.
Prior to lowering the operating frequency, however, be
sure to check with power MOSFET manufacturers for their
most recent low QG, low RDS(ON) devices. Power MOSFET
manufacturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
An effective approach to reduce the power consumption
of the internal LDO for gate drive is to tie the INTVCC pin
to an external voltage source high enough to turn off the
internal LDO regulator.
If the input voltage VIN does not exceed the absolute
maximum rating of both the power MOSFET gate-source
voltage (VGS) and the INTVCC overvoltage lockout threshold
voltage (17.5V), the INTVCC pin can be shorted directly
to the VIN pin. In this condition, the internal LDO will be
turned off and the gate driver will be powered directly
from the input voltage VIN. With the INTVCC pin shorted to
VIN, however, a small current (around 16μA) will load the
INTVCC in shutdown mode. For applications that require
the lowest shutdown mode input supply current, do not
connect the INTVCC pin to VIN.
In SEPIC or fl yback applications, the INTVCC pin can be
connected to the output voltage VOUT through a blocking
diode, as shown in Figure 3, if VOUT meets the following
conditions:
1. VOUT < VIN (pin voltage)
2. VOUT < 17.5V
3. VOUT < maximum VGS rating of power MOSFET
A resistor RVCC can be connected, as shown in Figure 3, to
limit the inrush current from VOUT. Regardless of whether
or not the INTVCC pin is connected to an external voltage
source, it is always necessary to have the driver circuitry
bypassed with a 4.7μF low ESR ceramic capacitor to ground
immediately adjacent to the INTVCC and GND pins.
Operating Frequency and Synchronization
The choice of operating frequency may be determined
by on-chip power dissipation, otherwise it is a trade-off
between effi ciency and component size. Low frequency
operation improves effi ciency by reducing gate drive
current and MOSFET and diode switching losses. However,
lower frequency operation requires a physically larger
inductor. Switching frequency also has implications for
loop compensation. The LT3758 uses a constant-frequency
architecture that can be programmed over a 100kHz to
1000kHz range with a single external resistor from the
RT pin to ground, as shown in Figure 1. The RT pin must
have an external resistor to GND for proper operation of
the LT3758. A table for selecting the value of RT for a given
operating frequency is shown in Table 1.
Table 1. Timing Resistor (RT) Value
SWITCHING FREQUENCY (kHz) RT (kΩ)
100 140
200 63.4
300 41.2
400 30.9
500 24.3
600 19.6
700 16.5
800 14
900 12.1
1000 10.5
Figure 3. Connecting INTVCC to VOUT
CVCC
4.7μF
VOUT
3758 F03
INTVCC
GND
LT3758 RVCC
DVCC
LT3758
13
3758fb
APPLICATIONS INFORMATION
The operating frequency of the LT3758 can be synchronized
to an external clock source. By providing a digital clock
signal into the SYNC pin, the LT3758 will operate at the
SYNC clock frequency. If this feature is used, an RT resistor
should be chosen to program a switching frequency 20%
slower than SYNC pulse frequency. It is recommended the
SYNC pulse have a minimum pulse width of 200ns. Tie
the SYNC pin to GND if this feature is not used.
Duty Cycle Consideration
Switching duty cycle is a key variable defi ning converter
operation. As such, its limits must be considered. Minimum
on-time is the smallest time duration that the LT3758 is
capable of turning on the power MOSFET. This time is
generally about 220ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LT3758 keeps the power switch off for at least
220ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time and minimum off-time and the
switching frequency defi ne the minimum and maximum
switching duty cycles a converter is able to generate:
Minimum duty cycle = minimum on-time • frequency
Maximum duty cycle = 1 – (minimum off-time • frequency)
Programming the Output Voltage
The output voltage VOUT is set by a resistor divider, as
shown in Figure 1. The positive and negative VOUT are set
by the following equations:
VV
R
R
V
OUT POSITIVE
OUT NEGATIV
,
,
.•=+
⎛
⎝
⎜⎞
⎠
⎟
16 1 2
1
EE VR
R
=+
⎛
⎝
⎜⎞
⎠
⎟
–. •08 1 2
1
The resistors R1 and R2 are typically chosen so that
the error caused by the current fl owing into the FBX pin
during normal operation is less than 1% (this translates
to a maximum value of R1 at about 158k).
Soft-Start
The LT3758 contains several features to limit peak switch
currents and output voltage (VOUT) overshoot during
start-up or recovery from a fault condition. The primary
purpose of these features is to prevent damage to external
components or the load.
High peak switch currents during start-up may occur in
switching regulators. Since VOUT is far from its fi nal value,
the feedback loop is saturated and the regulator tries to
charge the output capacitor as quickly as possible, resulting
in large peak currents. A large surge current may cause
inductor saturation or power switch failure.
The LT3758 addresses this mechanism with the SS pin.
As shown in Figure 1, the SS pin reduces the power
MOSFET current by pulling down the VC pin through
Q2. In this way the SS allows the output capacitor to
charge gradually toward its fi nal value while limiting the
start-up peak currents. The typical start-up waveforms
are shown in the Typical Performance Characteristics
section. The inductor current IL slewing rate is limited by
the soft-start function.
Besides start-up (with SHDN/UVLO), soft-start can also
be triggered by the following faults:
1. INTVCC > 17.5V
2. INTVCC < 4.5V
3. Thermal lockout
Any of these three faults will cause the LT3758 to stop
switching immediately. The SS pin will be discharged by
Q3. When all faults are cleared and the SS pin has been
discharged below 0.2V, a 10μA current source IS2 starts
charging the SS pin, initiating a soft-start operation.
The soft-start interval is set by the soft-start capacitor
selection according to the equation:
TC V
µA
SS SS
=•.125
10
LT3758
14
3758fb
APPLICATIONS INFORMATION
FBX Frequency Foldback
When VOUT is very low during start-up or a GND fault on
the output, the switching regulator must operate at low
duty cycles to maintain the power switch current within
the current limit range, since the inductor current decay
rate is very low during switch off time. The minimum on-
time limitation may prevent the switcher from attaining a
suffi ciently low duty cycle at the programmed switching
frequency. So, the switch current will keep increasing
through each switch cycle, exceeding the programmed
current limit. To prevent the switch peak currents from
exceeding the programmed value, the LT3758 contains
a frequency foldback function to reduce the switching
frequency when the FBX voltage is low (see the Normalized
Switching Frequency vs FBX graph in the Typical
Performance Characteristics section).
During frequency foldback, external clock synchronization
is disabled to prevent interference with frequency reducing
operation.
Thermal Lockout
If LT3758 die temperature reaches 165°C (typical), the
part will go into thermal lockout. The power switch will
be turned off. A soft-start operation will be triggered. The
part will be enabled again when the die temperature has
dropped by 5°C (nominal).
Loop Compensation
Loop compensation determines the stability and transient
performance. The LT3758 uses current mode control to
regulate the output which simplifi es loop compensation.
The optimum values depend on the converter topology, the
component values and the operating conditions (including
the input voltage, load current, etc.). To compensate the
feedback loop of the LT3758, a series resistor-capacitor
network is usually connected from the VC pin to GND.
Figure 1 shows the typical VC compensation network. For
most applications, the capacitor should be in the range of
470pF to 22nF, and the resistor should be in the range of
5k to 50k. A small capacitor is often connected in parallel
with the RC compensation network to attenuate the VC
voltage ripple induced from the output voltage ripple
through the internal error amplifi er. The parallel capacitor
usually ranges in value from 10pF to 100pF. A practical
approach to design the compensation network is to start
with one of the circuits in this data sheet that is similar
to your application, and tune the compensation network
to optimize the performance. Stability should then be
checked across all operating conditions, including load
current, input voltage and temperature.
SENSE Pin Programming
For control and protection, the LT3758 measures the
power MOSFET current by using a sense resistor (RSENSE)
between GND and the MOSFET source. Figure 4 shows a
typical waveform of the sense voltage (VSENSE) across the
sense resistor. It is important to use Kelvin traces between
the SENSE pin and RSENSE, and to place the IC GND as
close as possible to the GND terminal of the RSENSE for
proper operation.
Figure 4. The Sense Voltage During a Switching Cycle
3758 F04
VSENSE(PEAK)
$VSENSE = CvVSENSE(MAX)
VSENSE
t
DTS
VSENSE(MAX)
TS
Due to the current limit function of the SENSE pin, RSENSE
should be selected to guarantee that the peak current sense
voltage VSENSE(PEAK) during steady state normal operation
is lower than the SENSE current limit threshold (see the
Electrical Characteristics table). Given a 20% margin,
VSENSE(PEAK) is set to be 80mV. Then, the maximum
LT3758
15
3758fb
APPLICATIONS INFORMATION
switch ripple current percentage can be calculated using
the following equation:
χ= −
Δ
Δ
V
mV V
SENSE
SENSE
80 0 5.•
χ is used in subsequent design examples to calculate
inductor value. ΔVSENSE is the ripple voltage across
RSENSE.
The LT3758 switching controller incorporates 100ns timing
interval to blank the ringing on the current sense signal
immediately after M1 is turned on. This ringing is caused
by the parasitic inductance and capacitance of the PCB
trace, the sense resistor, the diode, and the MOSFET. The
100ns timing interval is adequate for most of the LT3758
applications. In the applications that have very large and
long ringing on the current sense signal, a small RC
fi lter can be added to fi lter out the excess ringing. Figure
5 shows the RC fi lter on the SENSE pin. It is usually
suffi cient to choose 22Ω for RFLT and 2.2nF to 10nF for
CFLT. Keep RFLT’s resistance low. Remember that there is
65μA (typical) fl owing out of the SENSE pin. Adding RFLT
will affect the SENSE current limit threshold:
V
SENSE_ILIM = 110mV – 65μA • RFLT
APPLICATION CIRCUITS
The LT3758 can be confi gured as different topologies. The
fi rst topology to be analyzed will be the boost converter,
followed by the fl yback, SEPIC and inverting converters.
Boost Converter: Switch Duty Cycle and Frequency
The LT3758 can be confi gured as a boost converter for
the applications where the converter output voltage is
higher than the input voltage. Remember that boost con-
verters are not short-circuit protected. Under a shorted
output condition, the inductor current is limited only by
the input supply capability. For applications requiring a
step-up converter that is short-circuit protected, please
refer to the Applications Information section covering
SEPIC converters.
The conversion ratio as a function of duty cycle is:
V
VD
OUT
IN
=−
1
1
in continuous conduction mode (CCM).
For a boost converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (VOUT) and the input voltage (VIN). The maximum
duty cycle (DMAX) occurs when the converter has the
minimum input voltage:
DVV
V
MAX
OUT IN MIN
OUT
=−()
Discontinuous conduction mode (DCM) provides higher
conversion ratios at a given frequency at the cost of reduced
effi ciencies and higher switching currents.
Boost Converter: Inductor and Sense Resistor Selection
For the boost topology, the maximum average inductor
current is:
II D
L MAX O MAX MAX
()()
•=−
1
1
Then, the ripple current can be calculated by:
ΔII I D
L L MAX O MAX MAX
== −
χχ•••
() ()
1
1
Figure 5. The RC Filter on the SENSE Pin
CFLT
3758 F05
LT3758
RFLT
RSENSE
M1
SENSE
GATE
GND
LT3758
16
3758fb
APPLICATIONS INFORMATION
The constant χ in the preceding equation represents the
percentage peak-to-peak ripple current in the inductor,
relative to IL(MAX).
The inductor ripple current has a direct effect on the choice
of the inductor value. Choosing smaller values of ΔIL
requires large inductances and reduces the current loop
gain (the converter will approach voltage mode). Accepting
larger values of ΔIL provides fast transient response and
allows the use of low inductances, but results in higher input
current ripple and greater core losses. It is recommended
that χ fall within the range of 0.2 to 0.6.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value of the boost converter can be determined
using the following equation:
LV
IfD
IN MIN
LMAX
=()
••
Δ
The peak and RMS inductor current are:
II
II
LPEAK LMAX
LRMS LMAX
()()
()()
•
•
=+
⎛
⎝
⎜⎞
⎠
⎟
=
12
χ
11 12
2
+χ
Based on these equations, the user should choose the
inductors having suffi cient saturation and RMS current
ratings.
Set the sense voltage at IL(PEAK) to be the minimum of the
SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
RmV
I
SENSE LPEAK
=80
()
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-source voltage rating (VDS), the threshold voltage
(VGS(TH)), the on-resistance (RDS(ON)), the gate to source
and gate to drain charges (QGS and QGD), the maximum
drain current (ID(MAX)) and the MOSFET’s thermal
resistances (RθJC and RθJA).
The power MOSFET will see full output voltage, plus a
diode forward voltage, and any additional ringing across
its drain-to-source during its off-time. It is recommended
to choose a MOSFET whose BVDSS is higher than VOUT by
a safety margin (a 10V safety margin is usually suffi cient).
The power dissipated by the MOSFET in a boost converter is:
P
FET = I2L(MAX) • RDS(ON) • DMAX + 2 • V2OUT • IL(MAX)
• CRSS • f/1A
The fi rst term in the preceding equation represents the
conduction losses in the device, and the second term, the
switching loss. CRSS is the reverse transfer capacitance,
which is usually specifi ed in the MOSFET characteristics.
For maximum effi ciency, RDS(ON) and CRSS should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
J = TA + PFET • θJA = TA + PFET • (θJC + θCA)
TJ must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
Boost Converter: Output Diode Selection
To maximize effi ciency, a fast switching diode with low
forward drop and low reverse leakage is desirable. The
peak reverse voltage that the diode must withstand is
equal to the regulator output voltage plus any additional
ringing across its anode-to-cathode during the on-time.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to:
II I
D PEAK L PEAK L MAX()() ()
•==+
⎛
⎝
⎜⎞
⎠
⎟
12
χ
It is recommended that the peak repetitive reverse voltage
rating VRRM is higher than VOUT by a safety margin (a 10V
safety margin is usually suffi cient).
The power dissipated by the diode is:
P
D = IO(MAX) • VD
and the diode junction temperature is:
T
J = TA + PD • RθJA
LT3758
17
3758fb
APPLICATIONS INFORMATION
Figure 6. The Output Ripple Waveform of a Boost Converter
VOUT
(AC)
tON
$VESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
$VCOUT
3758 F06
tOFF
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 6. The RMS ripple
current rating of the output capacitor can be determined
using the following equation:
IID
D
RMS COUT O MAX MAX
MAX
()()
•≥−1
Multiple capacitors are often paralleled to meet ESR require-
ments. Typically, once the ESR requirement is satisfi ed, the
capacitance is adequate for fi ltering and has the required
RMS current rating. Additional ceramic capacitors in par-
allel are commonly used to reduce the effect of parasitic
inductance in the output capacitor, which reduces high
frequency switching noise on the converter output.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical than
the output capacitor, due to the fact that the inductor is
in series with the input, and the input current waveform
is continuous. The input voltage source impedance
determines the size of the input capacitor, which is typically
in the range of 10μF to 100μF. A low ESR capacitor is
recommended, although it is not as critical as for the
output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I
RMS(CIN) = 0.3 • ΔIL
FLYBACK CONVERTER APPLICATIONS
The LT3758 can be confi gured as a fl yback converter for the
applications where the converters have multiple outputs,
high output voltages or isolated outputs. Figure 7 shows
a simplifi ed fl yback converter.
The fl yback converter has a very low parts count for multiple
outputs, and with prudent selection of turns ratio, can
have high output/input voltage conversion ratios with a
desirable duty cycle. However, it has low effi ciency due to
the high peak currents, high peak voltages and consequent
power loss. The fl yback converter is commonly used for
an output power of less than 50W.
The RθJA to be used in this equation normally includes the
RθJC for the device plus the thermal resistance from the board
to the ambient temperature in the enclosure. TJ must not
exceed the diode maximum junction temperature rating.
Boost Converter: Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct output
capacitors for a given output ripple voltage. The effect of
these three parameters (ESR, ESL and bulk C) on the output
voltage ripple waveform for a typical boost converter is
illustrated in Figure 6.
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step ΔVESR and the charging/discharging
ΔVCOUT. For the purpose of simplicity, we will choose
2% for the maximum output ripple, to be divided equally
between ΔVESR and ΔVCOUT. This percentage ripple will
change, depending on the requirements of the application,
and the following equations can easily be modifi ed. For a
1% contribution to the total ripple voltage, the ESR of the
output capacitor can be determined using the following
equation:
ESR V
I
COUT OUT
DPEAK
≤001.•
()
For the bulk C component, which also contributes 1% to
the total ripple:
CI
Vf
OUT
OMAX
OUT
≥()
.• •001
LT3758
18
3758fb
APPLICATIONS INFORMATION
The fl yback converter can be designed to operate either
in continuous or discontinuous mode. Compared to
continuous mode, discontinuous mode has the advantage
of smaller transformer inductances and easy loop
compensation, and the disadvantage of higher peak-to-
average current and lower effi ciency.
due to the number of variables involved. The user can
choose either a duty cycle or a turns ratio as the start
point. The following trade-offs should be considered when
selecting the switch duty cycle or turns ratio, to optimize
the converter performance. A higher duty cycle affects the
fl yback converter in the following aspects:
• Lower MOSFET RMS current ISW(RMS), but higher
MOSFET VDS peak voltage
• Lower diode peak reverse voltage, but higher diode
RMS current ID(RMS)
• Higher transformer turns ratio (NP/NS)
The choice,
D
DD+=
2
1
3
(for discontinuous mode operation with a given D3) gives
the power MOSFET the lowest power stress (the product
of RMS current and peak voltage). The choice,
D
DD+=
2
2
3
(for discontinuous mode operation with a given D3) gives
the diode the lowest power stress (the product of RMS
current and peak voltage). An extreme high or low duty
cycle results in high power stress on the MOSFET or diode,
and reduces effi ciency. It is recommended to choose a
duty cycle between 20% and 80%.
Figure 7. A Simplifi ed Flyback Converter
RSENSE
NP:NS
VIN
CIN CSN
VSN
LP
D
SUGGESTED
RCD SNUBBER
ID
VDS
ISW
3758 F07
GATE
GND
LT3758
SENSE
LS
M
+
–
+
–
RSN
DSN
+
–
+
COUT
+
Figure 8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
3758 F08
ISW
VDS
ID
t
DTSD2TSD3TS
ISW(MAX)
ID(MAX)
TS
Flyback Converter: Switch Duty Cycle and Turns Ratio
The fl yback converter conversion ratio in the continuous
mode operation is:
V
V
N
N
D
D
OUT
IN
S
P
=−
•1
Where NS/NP is the second to primary turns ratio.
Figure 8 shows the waveforms of the fl yback converter
in discontinuous mode operation. During each switching
period TS, three subintervals occur: DTS, D2TS, D3TS.
During DTS, M is on, and D is reverse-biased. During
D2TS, M is off, and LS is conducting current. Both LP and
LS currents are zero during D3TS.
The fl yback converter conversion ratio in the discontinuous
mode operation is:
V
V
N
N
D
D
OUT
IN
S
P
=•2
According to the preceding equations, the user has relative
freedom in selecting the switch duty cycle or turns ratio to
suit a given application. The selections of the duty cycle
and the turns ratio are somewhat iterative processes,
LT3758
19
3758fb
APPLICATIONS INFORMATION
Flyback Converter: Transformer Design for
Discontinuous Mode Operation
The transformer design for discontinuous mode of opera-
tion is chosen as presented here. According to Figure 8,
the minimum D3 (D3MIN) occurs when the the converter
has the minimum VIN and the maximum output power
(POUT). Choose D3MIN to be equal to or higher than 10%
to guarantee the converter is always in discontinuous
mode operation. Choosing higher D3 allows the use of
low inductances but results in higher switch peak current.
The user can choose a DMAX as the start point. Then, the
maximum average primary currents can be calculated by
the following equation:
II P
DV
LP MAX SW MAX
OUT MAX
MAX IN MIN
() ()
()
()
••
== η
where η is the converter effi ciency.
If the fl yback converter has multiple outputs, POUT(MAX)
is the sum of all the output power.
The maximum average secondary current is:
II
I
D
LS MAX D MAX
OUT MAX
()()
()
==
2
where
D2 = 1 – DMAX – D3
the primary and secondary RMS currents are:
II
D
II
LP RMS LP MAX MAX
LS RMS LS MAX
() ()
() (
••
•
=
=
23
2)) •D2
3
According to Figure 8, the primary and secondary peak
currents are:
I
LP(PEAK) = ISW(PEAK) = 2 • ILP(MAX)
I
LS(PEAK) = ID(PEAK) = 2 • ILS(MAX)
The primary and second inductor values of the fl yback
converter transformer can be determined using the fol-
lowing equations:
LDV
Pf
LDV
P
MAX IN MAX
OUT MAX
SO
=
=
22
2
2
2
••
••
•(
()
()
η
UUT D
OUT MAX
V
If
+)
••
()
2
The primary to second turns ratio is:
N
N
L
L
P
S
P
S
=
Flyback Converter: Snubber Design
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the MOS-
FET turn-off. This is increasingly prominent at higher load
currents, where more stored energy must be dissipated.
In some cases a snubber circuit will be required to avoid
overvoltage breakdown at the MOSFET’s drain node. There
are different snubber circuits, and Application Note 19 is
a good reference on snubber design. An RCD snubber is
shown in Figure 7.
The snubber resistor value (RSN) can be calculated by the
following equation:
R
VVV
N
N
ILf
SN
SN SN OUT P
S
SW PEAK LK
=
−
2
2
2
•
••
••
()
where VSN is the snubber capacitor voltage. A smaller
VSN results in a larger snubber loss. A reasonable VSN is
2 to 2.5 times of:
VN
N
OUT P
S
•
LT3758
20
3758fb
APPLICATIONS INFORMATION
LLK is the leakage inductance of the primary winding,
which is usually specifi ed in the transformer character-
istics. LLK can be obtained by measuring the primary
inductance with the secondary windings shorted. The
snubber capacitor value (CCN) can be determined using
the following equation:
CV
VR f
CN SN
SN CN
=Δ••
where ΔVSN is the voltage ripple across CCN. A reasonable
ΔVSN is 5% to 10% of VSN. The reverse voltage rating of
DSN should be higher than the sum of VSN and VIN(MAX).
Flyback Converter: Sense Resistor Selection
In a fl yback converter, when the power switch is turned on,
the current fl owing through the sense resistor (ISENSE) is:
I
SENSE = ILP
Set the sense voltage at ILP(PEAK) to be the minimum of
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
RmV
I
SENSE LP PEAK
=80
()
Flyback Converter: Power MOSFET Selection
For the fl yback confi guration, the MOSFET is selected with
a VDC rating high enough to handle the maximum VIN, the
refl ected secondary voltage and the voltage spike due to
the leakage inductance. Approximate the required MOSFET
VDC rating using:
BVDSS > VDS(PEAK)
where
V
DS(PEAK) = VIN(MAX) + VSN
The power dissipated by the MOSFET in a fl yback con-
verter is:
P
FET = I2M(RMS) • RDS(ON) + 2 • V2DS(PEAK) • IL(MAX) •
C
RSS • f/1A
The fi rst term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. CRSS is the reverse transfer capacitance, which is
usually specifi ed in the MOSFET characteristics.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
equation:
T
J = TA + PFET • θJA = TA + PFET • (θJC + θCA)
TJ must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
Flyback Converter: Output Diode Selection
The output diode in a fl yback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating VRRM using:
VN
NVV
RRM S
PIN MAX OUT
>+•()
The power dissipated by the diode is:
P
D = IO(MAX) • VD
and the diode junction temperature is:
T
J = TA + PD • RθJA
The RθJA to be used in this equation normally includes the
RθJC for the device, plus the thermal resistance from the board
to the ambient temperature in the enclosure. TJ must not
exceed the diode maximum junction temperature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the fl yback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of COUT and ESRCOUT.
The RMS ripple current rating of the output capacitors
in discontinuous operation can be determined using the
following equation:
II
D
RMS COUT DISCONTINUOUS O MAX(), ()
•(• )
•
≥−432
3DD2
LT3758
21
3758fb
APPLICATIONS INFORMATION
Flyback Converter: Input Capacitor Selection
The input capacitor in a fl yback converter is subject to
a large RMS current due to the discontinuous primary
current. To prevent large voltage transients, use a low
ESR input capacitor sized for the maximum RMS current.
The RMS ripple current rating of the input capacitors in
discontinuous operation can be determined using the
following equation:
IP
V
RMS CIN DISCONTINUOUS
OUT MAX
IN MIN
(),
()
()
•
≥η•• (• )
•
43
3
−D
D
MAX
MAX
SEPIC CONVERTER APPLICATIONS
The LT3758 can be confi gured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 1. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
VV
V
D
D
OUT D
IN
+=−1
in continuous conduction mode (CCM).
In a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.
Compared to the fl yback converter, the SEPIC converter
has the advantage that both the power MOSFET and the
output diode voltages are clamped by the capacitors (CIN,
CDC and COUT), therefore, there is less voltage ringing
across the power MOSFET and the output diodes. The
SEPIC converter requires much smaller input capacitors
than those of the fl yback converter. This is due to the fact
that, in the SEPIC converter, the inductor L1 is in series
with the input, and the ripple current fl owing through the
input capacitor is continuous.
SEPIC Converter: Switch Duty Cycle and Frequency
For a SEPIC converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (VOUT), the input voltage (VIN) and the diode
forward voltage (VD).
The maximum duty cycle (DMAX) occurs when the converter
has the minimum input voltage:
DVV
VVV
MAX OUT D
IN MIN OUT D
=+
++
()
SEPIC Converter: Inductor and Sense Resistor Selection
As shown in Figure 1, the SEPIC converter contains two
inductors: L1 and L2. L1 and L2 can be independent, but
can also be wound on the same core, since identical volt-
ages are applied to L1 and L2 throughout the switching
cycle.
For the SEPIC topology, the current through L1 is the
converter input current. Based on the fact that, ideally, the
output power is equal to the input power, the maximum
average inductor currents of L1 and L2 are:
III D
D
I
L MAX IN MAX O MAX MAX
MAX
LMAX
1
2
1
() ()()
(
•== −
))()
=IOMAX
In a SEPIC converter, the switch current is equal to IL1 +
IL2 when the power switch is on, therefore, the maximum
average switch current is defi ned as:
IIII D
SW MAX L MAX L MAX O MAX MAX
() () ()()
•=+= −
12
1
1
and the peak switch current is:
II
D
SW PEAK O MAX MAX
() ()
••=+
⎛
⎝
⎜⎞
⎠
⎟−
12
1
1
χ
The constant χ in the preceding equations represents
the percentage peak-to-peak ripple current in the switch,
relative to ISW(MAX), as shown in Figure 9. Then, the switch
ripple current ΔISW can be calculated by:
ΔISW = χ • ISW(MAX)
The inductor ripple currents ΔIL1 and ΔIL2 are identical:
ΔIL1 = ΔIL2 = 0.5 • ΔISW
The inductor ripple current has a direct effect on the
choice of the inductor value. Choosing smaller values of
LT3758
22
3758fb
APPLICATIONS INFORMATION
ΔIL requires large inductances and reduces the current
loop gain (the converter will approach voltage mode).
Accepting larger values of ΔIL allows the use of low
inductances, but results in higher input current ripple and
greater core losses. It is recommended that χ falls in the
range of 0.2 to 0.6.
Figure 9. The Switch Current Waveform of the SEPIC Converter
3758 F09
$ISW = CvISW(MAX)
ISW
t
DTS
ISW(MAX)
TS
where
χ
χ
LL
LMAX
LRMS LMAX L
I
I
II
11
1
22
2
2
112
=
=+
Δ
()
() ()
•
where
χLL
LMAX
I
I
22
2
=Δ
()
Based on the preceding equations, the user should choose
the inductors having suffi cient saturation and RMS cur-
rent ratings.
In a SEPIC converter, when the power switch is turned on,
the current fl owing through the sense resistor (ISENSE) is
the switch current.
Set the sense voltage at ISENSE(PEAK) to be the minimum
of the SENSE current limit threshold with a 20% margin.
The sense resistor value can then be calculated to be:
RmV
I
SENSE SW PEAK
=80
()
SEPIC Converter: Power MOSFET Selection
For the SEPIC confi guration, choose a MOSFET with a
VDC rating higher than the sum of the output voltage and
input voltage by a safety margin (a 10V safety margin is
usually suffi cient).
The power dissipated by the MOSFET in a SEPIC con-
verter is:
P
FET = I2SW(MAX) • RDS(ON) • DMAX
+ 2 • (VIN(MIN) + VOUT)2 • IL(MAX) • CRSS • f/1A
The fi rst term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. CRSS is the reverse transfer capacitance, which is
usually specifi ed in the MOSFET characteristics.
For maximum effi ciency, RDS(ON) and CRSS should be
minimized. From a known power dissipated in the power
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value (L1 and L2 are independent) of the
SEPIC converter can be determined using the following
equation:
LL V
If
D
IN MIN
SW MAX
12
05
== ()
.• ••
Δ
For most SEPIC applications, the equal inductor values
will fall in the range of 1μH to 100μH.
By making L1 = L2, and winding them on the same core, the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
LV
If
D
IN MIN
SW MAX
=()
••
Δ
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
I
L1(PEAK) = IL1(MAX) + 0.5 • ΔIL1
I
L2(PEAK) = IL2(MAX) + 0.5 • ΔIL2
The RMS inductor currents are:
II
LRMS LMAX L
11
2
1
112
() ()
•=+
χ
LT3758
23
3758fb
APPLICATIONS INFORMATION
MOSFET, its junction temperature can be obtained using
the following equation:
T
J = TA + PFET • θJA = TA + PFET • (θJC + θCA)
TJ must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
SEPIC Converter: Output Diode Selection
To maximize effi ciency, a fast switching diode with a low
forward drop and low reverse leakage is desirable. The
average forward current in normal operation is equal to
the output current, and the peak current is equal to:
II
D
DPEAK OMAX MAX
() ()
••=+
⎛
⎝
⎜⎞
⎠
⎟−
12
1
1
χ
It is recommended that the peak repetitive reverse voltage
rating VRRM is higher than VOUT + VIN(MAX) by a safety
margin (a 10V safety margin is usually suffi cient).
The power dissipated by the diode is:
P
D = IO(MAX) • VD
and the diode junction temperature is:
T
J = TA + PD • RθJA
The RθJA used in this equation normally includes the RθJC
for the device, plus the thermal resistance from the board,
to the ambient temperature in the enclosure. TJ must not
exceed the diode maximum junction temperature rating.
SEPIC Converter: Output and Input Capacitor Selection
The selections of the output and input capacitors of the
SEPIC converter are similar to those of the boost converter.
Please refer to the Boost Converter: Output Capacitor
Selection and Boost Converter: Input Capacitor Selection
sections.
SEPIC Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (CDC,
as shown in Figure 1) should be larger than the maximum
input voltage:
V
CDC > VIN(MAX)
CDC has nearly a rectangular current waveform. During
the switch off-time, the current through CDC is IIN, while
approximately –IO fl ows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
IIVV
V
RMS CDC O MAX OUT D
IN MIN
() ( ) ()
•>+
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for CDC.
INVERTING CONVERTER APPLICATIONS
The LT3758 can be confi gured as a dual-inductor inverting
topology, as shown in Figure 10. The VOUT to VIN ratio
is:
VV
V
D
D
OUT D
IN
−=− −1
in continuous conduction mode (CCM).
Figure 10. A Simplifi ed Inverting Converter
RSENSE
CDC
VIN
CIN
L1
D1
COUT VOUT
3758 F10
+
GATE
GND
LT3758
SENSE
L2
M1
+
–
+–
+
Inverting Converter: Switch Duty Cycle and Frequency
For an inverting converter operating in CCM, the duty cycle
of the main switch can be calculated based on the negative
output voltage (VOUT) and the input voltage (VIN).
The maximum duty cycle (DMAX) occurs when the converter
has the minimum input voltage:
DVV
VVV
MAX OUT D
OUT D IN MIN
=−
−−()
LT3758
24
3758fb
Inverting Converter: Inductor, Sense Resistor, Power
MOSFET, Output Diode and Input Capacitor Selections
The selections of the inductor, sense resistor, power
MOSFET, output diode and input capacitor of an inverting
converter are similar to those of the SEPIC converter. Please
refer to the corresponding SEPIC converter sections.
Inverting Converter: Output Capacitor Selection
The inverting converter requires much smaller output
capacitors than those of the boost, fl yback and SEPIC
converters for similar output ripples. This is due to the fact
that, in the inverting converter, the inductor L2 is in series
with the output, and the ripple current fl owing through the
output capacitors are continuous. The output ripple voltage
is produced by the ripple current of L2 fl owing through the
ESR and bulk capacitance of the output capacitor:
ΔΔV I ESR fC
OUT P P L COUT OUT
(–) •••
=+
⎛
⎝
⎜⎞
⎠
⎟
2
1
8
After specifying the maximum output ripple, the user can
select the output capacitors according to the preceding
equation.
The ESR can be minimized by using high quality X5R or
X7R dielectric ceramic capacitors. In many applications,
ceramic capacitors are suffi cient to limit the output volt-
age ripple.
The RMS ripple current rating of the output capacitor
needs to be greater than:
I
RMS(COUT) > 0.3 • ΔIL2
Inverting Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor
(CDC, as shown in Figure 10) should be larger than the
maximum input voltage minus the output voltage (nega-
tive voltage):
V
CDC > VIN(MAX) – VOUT
CDC has nearly a rectangular current waveform. During
the switch off-time, the current through CDC is IIN, while
approximately –IO fl ows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
II D
D
RMS CDC O MAX MAX
MAX
() ( )
•>−1
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for CDC.
Board Layout
The high speed operation of the LT3758 demands careful
attention to board layout and component placement. The
Exposed Pad of the package is the only GND terminal of
the IC, and is important for thermal management of the
IC. Therefore, it is crucial to achieve a good electrical and
thermal contact between the Exposed Pad and the ground
plane of the board. For the LT3758 to deliver its full output
power, it is imperative that a good thermal path be pro-
vided to dissipate the heat generated within the package.
It is recommended that multiple vias in the printed circuit
board be used to conduct heat away from the IC and into
a copper plane with as much area as possible.
To prevent radiation and high frequency resonance
problems, proper layout of the components connected
to the IC is essential, especially the power paths with
higher di/dt. The following high di/dt loops of different
topologies should be kept as tight as possible to reduce
inductive ringing:
• In boost confi guration, the high di/dt loop contains
the output capacitor, the sensing resistor, the power
MOSFET and the Schottky diode.
• In fl yback confi guration, the high di/dt primary loop
contains the input capacitor, the primary winding, the
power MOSFET and the sensing resistor. The high
di/dt secondary loop contains the output capacitor,
the secondary winding and the output diode.
• In SEPIC confi guration, the high di/dt loop contains
the power MOSFET, sense resistor, output capacitor,
Schottky diode and the coupling capacitor.
• In inverting confi guration, the high di/dt loop con-
tains power MOSFET, sense resistor, Schottky diode
and the coupling capacitor.
APPLICATIONS INFORMATION
LT3758
25
3758fb
APPLICATIONS INFORMATION
Check the stress on the power MOSFET by measuring its
drain-to-source voltage directly across the device terminals
(reference the ground of a single scope probe directly to
the source pad on the PC board). Beware of inductive
ringing, which can exceed the maximum specifi ed voltage
rating of the MOSFET. If this ringing cannot be avoided,
and exceeds the maximum rating of the device, either
choose a higher voltage device or specify an avalanche-
rated power MOSFET.
The small-signal components should be placed away
from high frequency switching nodes. For optimum load
regulation and true remote sensing, the top of the output
voltage sensing resistor divider should connect indepen-
dently to the top of the output capacitor (Kelvin connec-
tion), staying away from any high dV/dt traces. Place the
divider resistors near the LT3758 in order to keep the high
impedance FBX node short.
Figure 11 shows the suggested layout of the 10V to 40V
input, 48V output boost converter in the Typical Applica-
tions section.
Figure 11. Suggested Layout of the 10V to 40V Input, 48V Output
Boost Converter in the Typical Applications Section
VIN
3758 F11
VOUT
L1
VIAS TO GROUND
PLANE
D1
COUT1
COUT2
1
2
8
7
3
4
6
5
M1
CIN
RC
R1
R2
CSS
RT
R3
R4
CVCC
CC1
CC2
LT3758
1
2
3
4
5
9
10
6
7
8
RS
LT3758
26
3758fb
APPLICATIONS INFORMATION
Table 2. Recommended Component Manufacturers
VENDOR COMPONENTS WEB ADDRESS
AVX Capacitors avx.com
BH Electronics Inductors,
Transformers
bhelectronics.com
Coilcraft Inductors coilcraft.com
Cooper Bussmann Inductors bussmann.com
Diodes, Inc Diodes diodes.com
Fairchild MOSFETs fairchildsemi.com
General Semiconductor Diodes generalsemiconductor.
com
International Rectifi er MOSFETs, Diodes irf.com
IRC Sense Resistors irctt.com
Kemet Tantalum Capacitors kemet.com
Magnetics Inc Toroid Cores mag-inc.com
Microsemi Diodes microsemi.com
Murata-Erie Inductors, Capacitors murata.co.jp
Nichicon Capacitors nichicon.com
On Semiconductor Diodes onsemi.com
Panasonic Capacitors panasonic.com
Pulse Inductors pulseeng.com
Sanyo Capacitors sanyo.co.jp
Sumida Inductors sumida.com
Taiyo Yuden Capacitors t-yuden.com
TDK Capacitors, Inductors component.tdk.com
Thermalloy Heat Sinks aavidthermalloy.com
Tokin Capacitors nec-tokinamerica.com
Toko Inductors tokoam.com
United Chemi-Con Capacitors chemi-com.com
Vishay/Dale Resistors vishay.com
Vishay/Siliconix MOSFETs vishay.com
Würth Elektronik Inductors we-online.com
Vishay/Sprague Capacitors vishay.com
Zetex Small-Signal Discretes zetex.com
Recommended Component Manufacturers
Some of the recommended component manufacturers
are listed in Table 2.
LT3758
27
3758fb
TYPICAL APPLICATIONS
10V to 40V Input, 48V Output Boost Converter
Effi ciency vs Output Current
SENSE
LT3758
VIN
VIN
10V TO 40V CIN
4.7μF
50V
X7R
s2VOUT
48V
1A
RS
0.012Ω
RT
41.2k
300kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
R3
200k
R4
32.4k
CSS
0.68μF
CC2
100pF
RC
10k
CC1
10nF
L1
18.7μH
3758 TA02a
R2
464k
D1
M1
R1
15.8k
CVCC
4.7μF
10V
X5R
COUT2
4.7μF
50V
X7R
s4
COUT1
100μF
63V
+
CIN, COUT2: MURATA GRM32ER71H475KA88L
COUT1: PANASONIC ECG EEV-TG1J101UP
D1: VISHAY SILICONIX 30BQ060
L1: PULSE PB2020.223
M1: VISHAY SILICONIX SI7460DP
OUTPUT CURRENT (A)
0.001
EFFICIENCY (%)
10
50
40
30
20
60
70
80
90
100
0.01 0.1
3758 TA02b
1
VIN = 40V
VIN = 24V
VIN = 10V
Start-Up Waveforms
5ms/DIV
VOUT
20V/DIV
IL1
2A/DIV
3758 TA02c
VIN = 24V
LT3758
28
3758fb
TYPICAL APPLICATIONS
12V Output Nonisolated Flyback Power Supply
Effi ciency vs Output Current Start-Up Waveform
SENSE
LT3758
VIN
DSN
VIN
36V TO 72V
CIN
2.2μF
100V
X7R
63.4k
200kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
0.022μF
100V T1
1,2,3
(SERIES)
4,5,6
(PARALLEL)
1M
44.2k
0.47μF
100pF 10k
10nF
0.030Ω
5.1Ω
1N4148
15.8k
1%
105k
1%
CVCC
4.7μF
10V
X5R
VOUT
12V
1.2A
3758 TA03a
COUT
47μF
X5R
6.2k
D1
SW
M1
CIN: MURATA GRM32ER72A225KA35L
T1: COILTRONICS VP2-0066
M1: VISHAY SILICONIX SI4848DY
D1: ON SEMICONDUCTOR MBRS360T3G
DSN: VISHAY SILICONIX ES1D
COUT: MURATA GRM32ER61C476ME15L
OUTPUT CURRENT (A)
0.01
EFFICIENCY (%)
20
50
40
30
60
70
80
90
100
0.1 1
3758 TA03b
10
VIN = 48V
5ms/DIV
VOUT
5V/DIV
3758 TA03c
VIN = 48V
Frequency Foldback Waveforms
When Output Short-Circuit
20μs/DIV
VOUT
5V/DIV
VSW
50V/DIV
3758 TA03d
VIN = 48V
LT3758
29
3758fb
10ms/DIV
VOUT1,
VOUT2
20V/DIV
3758 TA04b
VIN = 12V VOUT1
VOUT2
2μs/DIV
VSW
50V/DIV
VOUT2
1V/DIV
(AC)
VOUT1
1V/DIV
(AC)
3758 TA04c
TYPICAL APPLICATIONS
VFD (Vacuum Fluorescent Display) Flyback Power Supply
Start-Up Waveforms Switching Waveforms
SENSE
LT3758
VIN
VIN
9V TO 16V CIN
22μF
25V
63.4k
200kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
COUT2
2.2μF
100V
X7R
T1
1, 2, 3
4
5
6
178k
32.4k
0.47μF
47pF 10k
10nF
0.019Ω
0.5W 1.62k
95.3k
D1
D2
CVCC
4.7μF
10V
X5R
VOUT
96V
80mA
VOUT2
64V
40mA
3758 TA04a
COUT1
1μF
100V
X7R
SWM1
CIN: MURATA GRM32ER61E226KE15L
COUT1: MURATA GRM31CR72A105K01L
COUT2: MURATA GRM32ER72A225KA35L
D1: VISHAY SILICONIX ES1D
D2: VISHAY SILICONIX ES1C
M1: VISHAY SILICONIX Si4100DY
T1: COILTRONICS VP1-0102
(*PRIMARY = 3 WINDINGS IN PARALLEL)
220pF
22Ω
LT3758
30
3758fb
TYPICAL APPLICATIONS
36V to 72V Input, 3.3V Output Isolated Telecom Power Supply
SENSE
LT3758
VIN
INTVCC
VC
BAV21W
FDC2512
0.03Ω
VIN
36V TO 72V CIN
2.2μF
100V
X7R
63.4k
200kHz
GATE
FBX
GND
SHDN/UVLO
SYNC
RT
SS
0.022μF
100V
4.7μF
25V
X5R
1M
44.2k
0.47μF
VOUT+
3.3V
3A
VOUT-
3758 TA05a
5.6k
16k
4.7μF
25V
X5R
10Ω
274Ω
BAS516
PS2801-1
BAS516
2
4
3
8
7
6
UPS840
5
1
0.47μF
LT4430
47nF
1μF
VIN
GND
OPTO
COMP
OC 0.5V FB
47pF
2k
22.1k
100k
BAT54CWTIG
2200pF
250VAC
100pF
COUT
100μF
6.3V
s3
PA1277NL
OUTPUT CURRENT (A)
0.01
EFFICIENCY (%)
20
50
40
30
60
70
80
90
100
0.1 1
3758 TA05b
10
VIN = 36V VIN = 72V
VIN = 48V
Effi ciency vs Output Current
LT3758
31
3758fb
TYPICAL APPLICATIONS
18V to 72V Input, 24V Output SEPIC Converter
SENSE
LT3758
VIN
VIN
18V TO 72V
CIN
2.2μF
100V
X7R
CDC
2.2μF
100V
X7R, s2VOUT
24V
1A
0.025Ω
M1
41.2k
300kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
•
•
232k
20k
0.47μF
10nF
10k
L1A
L1B
D1
CIN, CDC: TAIYO YUDEN HMK325B7225KN-T
COUT: MURATA GRM31CR61E106KA12L
3757 TA06a
280k
1%
20k
1% COUT
10μF
25V
X5R
s4
CVCC
4.7μF
10V
X5R
L1A, L1B: COILTRONICS DRQ127-470
M1: FAIRCHILD SEMICONDUCTOR FDMS2572
D1: ON SEMICONDUCTOR MBRS3100T3G
Effi ciency vs Output Current
OUTPUT CURRENT (A)
0.001
10
EFFICIENCY (%)
30
20
40
50
60
70
80
90
100
0.01 0.1
3757 TA06b
1
VIN = 72V
VIN = 48V
VIN = 18V
Load Step Waveform
500μs/DIV
VOUT
1V/DIV
(AC)
0.8A
0.2A
IOUT
0.5A/DIV
3757 TA06c
VIN = 48V
Start-Up Waveform
2ms/DIV
VOUT
10V/DIV
IL1A + IL1B
1A/DIV
3757 TA06d
VIN = 48V
50μs/DIV
VSW
50V/DIV
VOUT
20V/DIV
IL1A + IL1B
2A/DIV
3757 TA06e
VIN = 48V
Frequency Foldback Waveforms
When Output Short-Circuit
LT3758
32
3758fb
TYPICAL APPLICATIONS
10V to 40V Input, –12V Output Inverting Converter
SENSE
LT3758
VIN
VIN
10V TO 40V CIN
4.7μF
50V
X7R
s2
CDC
4.7μF
50V
X7R, s2VOUT
–12V
2A
0.015Ω
M1
41.2k
300kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
•
•
R1
200k
R2
32.4k
0.47μF 6.8nF
10k
L1A
L1B
D1
CIN, CDC: MURATA GRM32ER71H475KA88L
COUT: MURATA GRM32ER61C226KE20
D1: VISHAY SILICONIX 30BQ060
3757 TA07a
105k
7.5k COUT
22μF
16V
X5R
s4
CVCC
4.7μF
10V
X5R
L1A, L1B: COILTRONICS DRQ127-150
M1: VISHAY SILICONIX SI7850DP
Effi ciency vs Output Current
OUTPUT CURRENT (A)
0.001
10
EFFICIENCY (%)
30
20
40
50
60
70
80
90
100
0.01 0.1 1
3757 TA07b
10
VIN = 40V
VIN = 24V
VIN = 10V
Load Step Waveforms
500μs/DIV
VOUT
500mV/DIV
(AC)
1.6A
0.4A
IOUT
1A/DIV
3757 TA07c
VIN = 24V
Start-Up Waveforms
5ms/DIV
VOUT
5V/DIV
IL1A + IL1B
2A/DIV
3757 TA07d
VIN = 24V
50μs/DIV
VSW
20V/DIV
VOUT
10V/DIV
IL1A + IL1B
5A/DIV 3757 TA07e
VIN = 24V
Frequency Foldback Waveforms
When Output Short-Circuit
LT3758
33
3758fb
PACKAGE DESCRIPTION
3.00 p0.10
(4 SIDES)
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
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. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
0.40 p 0.10
BOTTOM VIEW—EXPOSED PAD
1.65 p 0.10
(2 SIDES)
0.75 p0.05
R = 0.125
TYP
2.38 p0.10
(2 SIDES)
15
106
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DD) DFN REV B 0309
0.25 p 0.05
2.38 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.65 p0.05
(2 SIDES)
2.15 p0.05
0.50
BSC
0.70 p0.05
3.55 p0.05
PACKAGE
OUTLINE
0.25 p 0.05
0.50 BSC
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev B)
LT3758
34
3758fb
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev C)
MSOP (MSE) 0908 REV C
0.53 p 0.152
(.021 p .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
12345
4.90 p 0.152
(.193 p .006)
0.497 p 0.076
(.0196 p .003)
REF
8910
10
1
76
3.00 p 0.102
(.118p .004)
(NOTE 3)
3.00 p 0.102
(.118p .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
0.254
(.010) 0o – 6o TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
0.889p 0.127
(.035 p .005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 p 0.038
(.0120 p .0015)
TYP
2.083p 0.102
(.082p .004)
2.794 p 0.102
(.110 p .004)
0.50
(.0197)
BSC
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.83p 0.102
(.072 p .004)
2.06 p 0.102
(.081p .004)
0.1016 p 0.0508
(.004 p .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
LT3758
35
3758fb
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 3/10 Deleted Bullet from Features and Last Line of Description
Updated All Sections to Include H-Grade and Military Grade
Deleted Vendor Telephone Information from Table 2 in Applications Information Section
Revised TA04 and TA04c in Typical Applications
Replaced Related Parts List
1
2 to 7
26
29
36
B 5/10 Revised last sentence of SYNC Pin description
Updated Block Diagram
Revised value in last sentence of Programming Turn-on and Turn-off Thresholds in the SHDN/UVLO Pin Section
Revised penultimate sentence of Operating Frequency and Synchronization section
8
9
10
13
LT3758
36
3758fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
LT 0510 REV B • PRINTED IN USA
TYPICAL APPLICATIONS
PART NUMBER DESCRIPTION COMMENTS
LT3757 Boost, Flyback, SEPIC and Inverting Controller 2.9V ≤ VIN ≤ 40V, Current Mode Control, 100kHz to 1MHz Programmable
Operation Frequency, 3mm × 3mm 10-Lead DFN and 10-Lead MSOP-E
Packages
LT3573 Isolated Flyback Switching Regulator with 60V
Integrated Switch
3V ≤ VIN ≤ 40V, No Opto-Isolator or Third Winding Required, Up to 7W,
16-Lead MSOP-E Package
LTC1871/LTC1871-1/
LTC1871-7
Boost, Flyback and SEPIC Controller, No RSENSE™,
Low Quiescent Current
Adjustable Switching Frequency, 2.5V ≤ VIN ≤ 36V, Burst Mode
®
Operation at
Light Loads
LTC3872 Boost, Flyback, SEPIC Controller 2.75V ≤ VIN ≤ 9.8V, 23-Lead ThinSot™ and 2mm × 3mm 8-Lead DFN
Packages
LT3837 Isolated No-Opto Synchronous Flyback Controller Ideal for VIN from 4.5V to 36V Limited by External Components, Up to 60W,
Current Mode Control
LT3825 Isolated No-Opto Synchronous Flyback Controller VIN 16V to 75V Limited by External Components, Up to 60W, Current Mode
Control
LTC3803/LTC3803-3/
LTC3803-5
200kHz Flyback DC/DC Controller VIN and VOUT Limited Only by External Components, 6-Lead ThinSot Package
LTC3805/LTC3805-5 Adjustable Fixed 70kHz to 700kHz Operating
Frequency Flyback Controller
VIN and VOUT Limited Only by External Components, 3mm × 3mm 10-Lead
DFN, 10-Lead MSOP-E Packages
RELATED PARTS
8V to 72V Input, 12V Output SEPIC Converter
SENSE
LT3758
VIN
VIN
8V TO 72V CIN
2.2μF
100V
X7R
s2
CDC
2.2μF
100V
X7R, s2VOUT
12V
2A
0.012Ω
M1
Si7456DP
41.2k
300kHz
GATE
FBX
GND INTVCC
SHDN/UVLO
SYNC
RT
SS
VC
•
•
154k
32.4k
0.47μF
10nF
10k
L1A
L1B
D1
MBRS3100T3G
3757 TA08a
105k
1%
15.8k
1% COUT2
10μF
16V
X5R
s4
COUT1
47μF
20V
s2
CVCC
4.7μF
10V
X5R
L1A, L1B: COILTRONICS DRQ127-220
+
Effi ciency vs Output Current
VIN = 8V
OUTPUT CURRENT (A)
10
EFFICIENCY (%)
30
20
40
50
60
70
80
90
100
3757 TA08b
0.001 0.01 0.1 1 10
VIN = 72V
VIN = 42V