LT3512
Monolithic High Voltage
Isolated Flyback Converter
Features
Description
n
n
The LT3512 is a high voltage monolithic switching regulator specifically designed for the isolated flyback topology.
No third winding or opto-isolator is required for regulation as the part senses output voltage directly from the
primary-side flyback waveform. The device integrates a
420mA, 150V power switch, high voltage circuitry, and
control into a high voltage 16-lead MSOP package with
four leads removed.
4.5V to 100V Input Voltage Range
Internal 420mA, 150V Power Switch
Boundary Mode Operation
No Transformer Third Winding or
Opto-Isolator Required for Regulation
n Improved Primary-Side Winding Feedback
Load Regulation
n V
OUT Set with Two External Resistors
n BIAS Pin for Internal Bias Supply and Power
Switch Driver
n No External Start-Up Resistor
n 16-Lead MSOP Package
n
n
The LT3512 operates from an input voltage range of 4.5V
to 100V and delivers up to 4.5W of isolated output power.
Two external resistors and the transformer turns ratio
easily set the output voltage. Off-the-shelf transformers
are available for several applications. The high level of
integration and the use of boundary mode operation results
in a simple, clean, tightly regulated application solution to
the traditionally tough problem of isolated power delivery.
Applications
Isolated Telecom Power Supplies
Isolated Auxiliary/Housekeeping Power Supplies
n Isolated Industrial, Automotive and Medical Power
Supplies
n
n
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
and No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 5438499, 7471522.
Typical Application
48V to 5V Isolated Flyback Converter
1µF
1M
EN/UVLO
43.2k
VIN
175µH
LT3512
RFB
RREF
169k
10k
TC
SW
VC
57.6k
•
GND BIAS
•
5.25
VOUT+
5V
0.5A
4:1
11µH
5.20
5.15
47µF
VOUT–
5.10
VOUT (V)
VIN
36V TO 72V
Output Load and Line Regulation
5.00
VIN = 72V
4.95
4.90
4.85
4.75
4.7µF
3512 TA01a
VIN = 36V
4.80
12.7k
4.7nF
VIN = 48V
5.05
0
100
200
300
400
LOAD CURRENT (mA)
500
3512 TA01b
3512fb
1
LT3512
Absolute Maximum Ratings
(Note 1)
Pin Configuration
SW (Note 4).............................................................150V
VIN, EN/UVLO, RFB...................................................100V
VIN to RFB...................................................................±6V
BIAS.................................................Lesser of 20V or VIN
RREF,TC, VC..................................................................6V
Operating Junction Temperature Range (Note 2)
LT3512E, LT3512I............................... –40°C to 125°C
LT3512H............................................. –40°C to 150°C
LT3512MP........................................... –55°C to 150°C
Storage Temperature Range................... –65°C to 150°C
TOP VIEW
EN/UVLO 1
16 SW
VIN 3
14 RFB
GND
BIAS
NC
GND
5
6
7
8
12
11
10
9
RREF
TC
VC
GND
MS PACKAGE
16(12)-LEAD PLASTIC MSOP
θJA = 90°C/W
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3512EMS#PBF
LT3512EMS#TRPBF
3512
16-Lead Plastic MSOP
–40°C to 125°C
LT3512IMS#PBF
LT3512IMS#TRPBF
3512
16-Lead Plastic MSOP
–40°C to 125°C
LT3512HMS#PBF
LT3512HMS#TRPBF
3512
16-Lead Plastic MSOP
–40°C to 150°C
LT3512MPMS#PBF
LT3512MPMS#TRPBF
3512
16-Lead Plastic MSOP
–55°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 24V unless otherwise noted.
PARAMETER
Input Voltage Range
CONDITIONS
MIN
l
VIN = BIAS
Quiescent Current
Not Switching
VEN/UVLO = 0.2V
EN/UVLO Pin Threshold
EN/UVLO Pin Voltage Rising
EN/UVLO Pin Current
VEN/UVLO =1.1V
VEN/UVLO =1.4V
l
TYP
MAX
UNITS
100
15
V
V
3.5
0
4.5
mA
μA
1.15
1.21
1.27
V
2.0
2.6
0
3.3
μA
μA
6
4.5
Maximum Switching Frequency
650
kHz
Minimum Switching Frequency
40
kHz
Maximum Current Limit
420
600
800
mA
Minimum Current Limit
80
120
150
mA
Switch VCESAT
ISW = 200mA
0.5
RREF Voltage
l
RREF Voltage Line Regulation
6V < VIN < 100V
RREF Pin Bias Current
(Note 3)
Error Amplifier Voltage Gain
Error Amplifier Transconductance
∆I = 2µA
l
1.18
1.17
V
1.20
1.215
1.23
V
V
0.01
0.03
%/V
80
400
nA
150
V/V
140
μmhos
3512fb
2
LT3512
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 24V unless otherwise noted.
PARAMETER
CONDITIONS
TC Current into RREF
RTC = 53.6k
BIAS Pin Voltage
Internally Regulated
Quiescent Current
VIN = 48V
5.15
5.05
IQ (mA)
VOUT (V)
5.10
5.00
4.95
4.90
4.0
6
3.5
4
0
0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
Switch VCESAT
Switch Current Limit
700
25 50 75 100 125 150
TEMPERATURE (°C)
Quiescent Current vs VIN
5
MAXIMUM CURRENT LIMIT
4
600
500
IQ (mA)
CURRENT LIMIT (mA)
1600
400
0
3512 G03
800
800
VIN = 24V, 10mA
VIN = 24V, NO LOAD
3512 G02
2000
SWITCH VCESAT VOLTAGE (mV)
3.0
2.0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
3512 G01
1200
V
2.5
VIN = 24V
VIN = 48V
VIN = 100V
4.80
4.75
–50 –25
μA
3.2
BIAS Pin Voltage
8
2
4.85
3.1
UNITS
TA = 25°C, unless otherwise noted.
BIAS VOLTAGE (V)
5.20
3
MAX
to meet performance specifications from –40°C to 150°C operating
junction temperature range. The LT3511MP is guaranteed over the full
–55°C to 150°C operating junction range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C.
Note 3: Current flows out of the RREF pin.
Note 4: The SW pin is rated to 150V for transients. Operating waveforms
of the SW pin should keep the pedestal of the flyback waveform below
100V as shown in Figure 5.
Typical Performance Characteristics
5.25
TYP
9.5
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 LT3512E 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
LT3512I is guaranteed to meet performance specifications from –40°C to
125°C operating junction temperature range. The LT3512H is guaranteed
Output Voltage
MIN
400
300
200
MINIMUM CURRENT LIMIT
3
2
1
100
0
0
300
100
200
400
SWITCH CURRENT (mA)
500
3512 G04
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3512 G05
0
0
20
60
40
VOLTAGE (V)
80
100
3512 G06
3512fb
3
LT3512
Typical Performance Characteristics
EN/UVLO Pin (Hysteresis) Current
vs Temperature
30
EN/UVLO = 1.2V
EN/UVLO PIN CURRENT (µA)
EN/UVLO PIN CURRENT (µA)
4
3
2
1
0
–50 –25
0
3.0
25
2.5
20
15
10
2.0
1.5
1.0
0.5
5
0
25 50 75 100 125 150
TEMPERATURE (°C)
EN/UVLO Threshold
vs Temperature
EN/UVLO Pin Current
vs VEN/UVLO
EN/UVLO THRESHOLD (V)
5
TA = 25°C, unless otherwise noted.
1
20
40
60
80
VEN/UVLO VOLTAGE (V)
0
–50 –25
100
Maximum Frequency
vs Temperature
3512 G09
Minimum Frequency
vs Temperature
EN/UVLO Shutdown Threshold
vs Temperature
900
90
0.8
800
80
500
400
300
200
70
60
40
40
30
20
0
25 50 75 100 125 150
TEMPERATURE (°C)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
10
100
0
–50 –25
EN/UVLO THRESHOLD (V)
0.9
MINIMUM FREQUENCY (kHz)
100
MAXIMUM FREQUENCY (kHz)
1000
600
25 50 75 100 125 150
TEMPERATURE (°C)
3512 G08
3512 G07
700
0
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3512 G11
3512 G10
3512 G14
Light Load Discontinuous
Mode Waveform
Boundary Mode Waveform
20V/DIV
20V/DIV
2µs/DIV
3512 G12
2µs/DIV
3512 G13
3512fb
4
LT3512
Pin Functions
EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/
UVLO pin is used to start up the LT3512. Pull the pin to 0V
to shut down the LT3512. This pin has an accurate 1.21V
threshold and can be used to program an undervoltage
lockout (UVLO) threshold using a resistor divider from
supply to ground. A 2.6μA pin current hysteresis allows
the programming of undervoltage lockout (UVLO) hysteresis. EN/UVLO can be directly connected to VIN. If left
open circuit the part will not power up.
VIN (Pin 3): Input Supply Pin. This pin supplies current to
the internal start-up circuitry, and serves as a reference
voltage for the DCM comparator and feedback circuitry.
Must be locally bypassed with a capacitor.
GND (Pin 5, 8, 9): Ground Pins. All three pins should be
tied directly to the local ground plane.
BIAS (Pin 6): Bias Voltage. This pin supplies current to
the switch driver and internal circuitry of the LT3512.
This pin may also be connected to VIN if a third winding
is not used and if VIN < 20V. The part can operate down
to 4.5V when BIAS and VIN are connected together. If a
third winding is used, the BIAS voltage should be lower
than the input voltage and greater than 3.3V for proper
operation. BIAS must be bypassed with a 4.7µF capacitor
placed close to the pin.
VC (Pin 10): Compensation Pin for Internal Error Amplifier.
Connect a series RC from this pin to ground to compensate
the switching regulator. An additional 100pF capacitor from
this pin to ground helps eliminate noise.
TC (Pin 11): Output Voltage Temperature Compensation. Connect a resistor to ground to produce a current
proportional to absolute temperature to be sourced into
the RREF node.
ITC = 0.55V/RTC.
RREF (Pin 12): Input Pin for External Ground-Referred
Reference Resistor. The resistor at this pin should be 10k.
For nonisolated applications, a traditional resistor voltage
divider from VOUT may be connected to this pin.
RFB (Pin 14): Input Pin for External Feedback Resistor.
This pin is connected to the transformer primary (VSW).
The ratio of this resistor to the RREF resistor, times the
internal bandgap reference, determines the output voltage (plus the effect of any non-unity transformer turns
ratio). For nonisolated applications, this pin should be
connected to VIN.
SW (Pin 16): Switch Pin. Collector of the internal power
switch. Minimize trace area at this pin to minimize EMI
and voltage spikes.
3512fb
5
LT3512
Block Diagram
D1
VIN
VOUT +
T1
C1
L1A
L1B
C2
R3
VOUT –
N:1
TC
CURRENT
TC
VIN
Q3
SW
RFB
FLYBACK
ERROR
AMP
Q2
R5
I2
1.2V
–g
m
+
–
+
ONE
SHOT
CURRENT
COMPARATOR
A2
–
A1
+
S
S
BIAS
DRIVER
BIAS
R1
EN/UVLO
R
1.21V
R2
+
A5
–
3µA
Q4
INTERNAL
REFERENCE
AND
REGULATORS
Q1
Q
MASTER
LATCH
C4
–
VIN
RREF
R4
+
V1
120mV
+
–
A4
RSENSE
0.01Ω
GND
OSCILLATOR
VC
R6
C3
3512 BD
3512fb
6
LT3512
Operation
The LT3512 is a current mode switching regulator IC designed specifically for the isolated flyback topology. The
key problem in isolated topologies is how to communicate
information regarding the output voltage from the isolated
secondary side of the transformer to the primary side.
Historically, optoisolators or extra transformer windings
communicate this information across the transformer.
Optoisolator circuits waste output power, and the extra
components increase the cost and physical size of the
power supply. Optoisolators can also exhibit trouble due
to limited dynamic response, nonlinearity, unit-to-unit
variation and aging over life. Circuits employing an extra
transformer winding also exhibit deficiencies. Using an
extra winding adds to the transformer’s physical size and
cost, and dynamic response is often mediocre.
In the LT3512, the primary-side flyback pulse provides
information about the isolated output voltage. In this manner, neither optoisolator nor extra transformer winding is
required for regulation. Two resistors program the output
voltage. Since this IC operates in boundary mode, the part
calculates output voltage from the switch pin when the
secondary current is almost zero.
The Block Diagram shows an overall view of the system.
Many of the blocks are similar to those found in traditional
switching regulators including internal bias regulator, oscillator, logic, current amplifier, current comparator, driver,
and output switch. The novel sections include a special
flyback error amplifier and a temperature compensation
circuit. In addition, the logic system contains additional
logic for boundary mode operation.
The LT3512 features boundary mode control, where the part
operates at the boundary between continuous conduction
mode and discontinuous conduction mode. The VC pin
controls the current level just as it does in normal current
mode operation, but instead of turning the switch on at the
start of the oscillator period, the part turns on the switch
when the secondary-side winding current is zero.
Boundary Mode Operation
Boundary mode is a variable frequency, current mode
switching scheme. The switch turns on and the inductor
current increases until a VC pin controlled current limit.
After the switch turns off, the voltage on the SW pin rises
to the output voltage divided by the secondary-to-primary
transformer turns ratio plus the input voltage. When the
secondary current through the diode falls to zero, the SW
pin voltage falls below VIN. A discontinuous conduction
mode (DCM) comparator detects this event and turns the
switch back on.
Boundary mode returns the secondary current to zero every
cycle, so parasitic resistive voltage drops do not cause load
regulation errors. Boundary mode also allows the use of a
smaller transformer compared to continuous conduction
mode and does not exhibit subharmonic oscillation.
At low output currents, the LT3512 delays turning on the
switch, and thus operates in discontinuous mode. Unlike
traditional flyback converters, the switch has to turn on
to update the output voltage information. Below 0.6V on
the VC pin, the current comparator level decreases to
its minimum value, and the internal oscillator frequency
decreases. With the decrease of the internal oscillator,
the part starts to operate in DCM. The output current is
able to decrease while still allowing a minimum switch off
time for the flyback error amplifier. The typical minimum
internal oscillator frequency with VC equal to 0V is 40kHz.
3512fb
7
LT3512
Applications Information
PSEUDO DC THEORY
In the Block Diagram, RREF (R4) and RFB (R3) are external
resistors used to program the output voltage. The LT3512
operates similar to traditional current mode switchers,
except in the use of a unique error amplifier, which derives
its feedback information from the flyback pulse.
Operation is as follows: when the output switch, Q1, turns
off, its collector voltage rises above the VIN rail. The amplitude of this flyback pulse, i.e., the difference between
it and VIN, is given as:
VFLBK = (VOUT + VF + ISEC • ESR) • NPS
VF = D1 forward voltage
ISEC = Transformer secondary current
ESR = Total impedance of secondary circuit
NPS = Transformer effective primary-to-secondary turns
ratio
RFB and Q2 convert the flyback voltage into a current. Nearly
all of this current flows through RREF to form a groundreferred voltage. The resulting voltage forms the input
to the flyback error amplifier. The flyback error amplifier
samples the voltage information when the secondary side
winding current is zero. The bandgap voltage, 1.20V, acts
as the reference for the flyback error amplifier.
The relatively high gain in the overall loop will then cause
the voltage at RREF to be nearly equal to the bandgap
reference voltage VBG. The resulting relationship between
VFLBK and VBG approximately equals:
the effect of nonzero secondary output impedance (ESR).
Boundary control mode minimizes the effect of this impedance term.
Temperature Compensation
The first term in the VOUT equation does not have temperature dependence, but the diode forward drop has a
significant negative temperature coefficient. A positive
temperature coefficient current source connects to the
RREF pin to compensate. A resistor to ground from the
TC pin sets the compensation current.
The following equation explains the cancellation of the
temperature coefficient:
dVF
R
1
= − FB •
•
NPS
dT
R TC
−R
1
R TC = FB •
NPS dVF / dT
(dVF /dT) = Diode’s forward voltage temperature coefficient
(dVTC /dT) = 2mV
VTC = 0.55V
Experimentally verify the resulting value of RTC and adjust as
necessary to achieve optimal regulation over temperature.
The addition of a temperature coefficient current modifies
the expression of output voltage as follows:
R 1
VOUT = VBG FB
− VF
RREF NPS
VFLBK VBG
R
or VFLBK = VBG FB
R = R
RREF
FB
REF
VBG = Internal bandgap reference
Output Power
Combination of the preceding expression with earlier
derivation of VFLBK results in the following equation:
R 1
VOUT = VBG FB
− VF − ISEC (ESR)
RREF NPS
The expression defines VOUT in terms of the internal reference, programming resistors, transformer turns ratio
and diode forward voltage drop. Additionally, it includes
dVTC
or,
dT
dV
R
• TC ≈ FB
dT
NPS
V R
− TC • FB – ISEC (ESR)
R TC NPS
A flyback converter has a complicated relationship between the input and output current compared to a buck
or a boost. A boost has a relatively constant maximum
input current regardless of input voltage and a buck has a
relatively constant maximum output current regardless of
input voltage. This is due to the continuous nonswitching
behavior of the two currents. A flyback converter has both
discontinuous input and output currents which makes it
3512fb
8
LT3512
Applications Information
similar to a nonisolated buck-boost. The duty cycle will
affect the input and output currents, making it hard to
predict output power. In addition, the winding ratio can
be changed to multiply the output current at the expense
of a higher switch voltage.
One design example would be a 5V output converter with
a minimum input voltage of 36V and a maximum input
voltage of 72V. A four-to-one winding ratio fits this design
example perfectly and outputs close to 3.0W at 72V but
lowers to 2.5W at 36V.
The graphs in Figures 1-4 show the typical maximum
output power possible for the output voltages 3.3V, 5V,
12V and 24V. The maximum power output curve is the
calculated output power if the switch voltage is 100V
during the off-time. 50V of margin is left for leakage voltage spike. To achieve this power level at a given input, a
winding ratio value must be calculated to stress the switch
to 100V, resulting in some odd ratio values. The following
curves are examples of common winding ratio values
and the amount of output power at given input voltages.
The equations below calculate output power:
Power = η • VIN • D • IPEAK • 0.5
Efficiency = η = ~83%
Duty cycle = D =
Peak switch current = IPEAK = 0.44A
5.0
5.0
OUTPUT POWER (W)
4.0
N = NPS(MAX)
N = 12
N = 10
N=8
3.0
N=6
2.0
N=4
1.0
20
40
60
INPUT VOLTAGE (V)
80
N=4
N=3
N=2
3.0
2.0
N=1
1.0
N=2
0
N=5
N = NPS(MAX)
4.0
OUTPUT POWER (W)
N = 15
0
( VOUT + VF ) •NPS
( VOUT + VF ) •NPS + VIN
0
100
0
20
40
60
INPUT VOLTAGE (V)
80
3512 F03
3512 F01
Figure 1. Output Power for 3.3V Output
Figure 3. Output Power for 12V Output
5.0
5.0
OUTPUT POWER (W)
4.0
N=4
3.0
N=3
2.0
N=2
N=1
1.0
0
0
20
40
60
INPUT VOLTAGE (V)
80
N = NPS(MAX)
N=2
4.0
OUTPUT POWER (W)
N = NPS(MAX)
N=8
N=7
N=6
N=5
100
3.0
N=1
2.0
1.0
100
3512 F02
Figure 2. Output Power for 5V Output
0
0
20
40
60
INPUT VOLTAGE (V)
80
100
3512 F04
Figure 4. Output Power for 24V Output
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9
LT3512
Applications Information
Table 1. Predesigned Transformers
TRANSFORMER
PART NUMBER
LPRI (µH)
LEAKAGE (µH)
NP:NS:NB
ISOLATION (V)
SATURATION
CURRENT (mA)
VENDOR
TARGET APPLICATIONS
750311559
175
1.5
4:1:1
1500
800
Würth
Elektronik
750311573
200
2
6:1:2
1500
800
Würth
Elektronik
750311662
151
2
1:1:0.2
1500
800
750311661
150
1.85
2:1:0.66
1500
1.1A
Würth
Elektronik
Würth
Elektronik
48V to 5V, 0.5A
24V to 5V, 0.38A
12V to 5V, 0.2A
48V to 3.3V, 0.59A
24V to 3.3V, 0.48A
12V to 3.3V, 0.29A
24V to 5V, 0.45A
12V to 5V, 0.23A
48V to 3.3V, 0.7A
24V to 3.3V, 0.59A
12V to 3.3V, 0.33A
48V to 24V, 0.11A
750311839
200
3
2:1:1
1500
800
Würth
Elektronik
750311964
100
0.7
1:5:5
1500
900
Würth
Elektronik
750311966
120
0.45
1:5:0.5
1500
900
750311692
80
2
1:5:5
1500
1.0A
10396-T025
200
2.0
4:1:1.2
1500
800
Würth
Elektronik
Würth
Elektronik
Sumida
10396-T027
200
2.0
6:1:2
1500
800
Sumida
01355-T058
10396-T023
125
200
2.0
2.0
1:1:0.2
2:1:0.33
1500
1500
800
800
Sumida
Sumida
10396-T029
200
2.5
2:1:1
1500
800
Sumida
01355-T061
100
2
1:5:5
1500
800
Sumida
48V to 15V, 0.2A
48V to 12V, 0.22A
24V to 15V, 0.15A
12V to 15V, 0.075A
48V to ±15V, 0.1A
48V to ±12V, 0.11A
24V to ±15V, 0.075A
12V to ± 70V, 0.007A
12V to ± 100V, 0.005A
12V to ± 150V, 0.004A
12V to +120V& -12V, 0.005A
12V ± 70V, 0.007A
48V to 5V, 0.5A
24V to 5V, 0.38A
12V to 5V, 0.2A
48V to 3.3V, 0.59A
24V to 3.3V, 0.48A
12V to 3.3V, 0.29A
24V to 5V, 0.45A
12V to 5V, 0.23A
48V to 3.3V, 0.7A
24V to 3.3V, 0.59A
12V to 3.3V, 0.33A
48V to 24V, 0.11A
48V to 15V, 0.2A
48V to 12V, 0.22A
24V to 15V, 0.15A
12V to 15V, 0.075A
48V to ±15V, 0.1A
48V to ±12V, 0.11A
24V to ±15V, 0.075A
12V to ± 70V, 0.007A
12V to ± 100V, 0.005A
12V to ± 150V, 0.004A
3512fb
10
LT3512
Applications Information
TRANSFORMER DESIGN CONSIDERATIONS
Successful application of the LT3512 relies on proper
transformer specification and design. Carefully consider
the following information in addition to the traditional
guidelines associated with high frequency isolated power
supply transformer design.
Linear Technology has worked with several leading magnetic component manufacturers to produce pre-designed
flyback transformers for use with the LT3512. Table 1
shows the details of these transformers.
Note that when using an RFB/RREF resistor ratio to set
output voltage, the user has relative freedom in selecting
a transformer turns ratio to suit a given application. In
contrast, the use of simple ratios of small integers, e.g.,
1:1, 2:1, 3:2, provides more freedom in setting total turns
and mutual inductance.
Typically, choose the transformer turns to maximize available output power. For low output voltages (3.3V or 5V), a
N:1 turns ratio can be used with multiple primary windings
relative to the secondary to maximize the transformer’s
current gain (and output power). However, remember that
the SW pin sees a voltage that is equal to the maximum
input supply voltage plus the output voltage multiplied by
the turns ratio. In addition, leakage inductance will cause
a voltage spike (VLEAKAGE) on top of this reflected voltage.
This total quantity needs to remain below the absolute
maximum rating of the SW pin to prevent breakdown of
the internal power switch. Together these conditions place
an upper limit on the turns ratio, N, for a given application.
Choose a turns ratio low enough to ensure:
The turns ratio is an important element in the isolated
feedback scheme. Make sure the transformer manufacturer
guarantees turns ratio accuracy within ±1%.
Saturation Current
Turns Ratio
N<
For lower output power levels, choose a 1:1 or 1:N
transformer for the absolute smallest transformer size. A
1:N transformer will minimize the magnetizing inductance
(and minimize size), but will also limit the available output
power. A higher 1:N turns ratio makes it possible to have
very high output voltages without exceeding the breakdown
voltage of the internal power switch.
150V – VIN(MAX) – VLEAKAGE
VOUT + VF
For larger N:1 values, choose a transformer with a larger
physical size to deliver additional current. In addition,
choose a large enough inductance value to ensure that
the off-time is long enough to measure the output voltage.
The current in the transformer windings should not exceed its rated saturation current. Energy injected once the
core is saturated will not be transferred to the secondary
and will instead be dissipated in the core. Information on
saturation current should be provided by the transformer
manufacturers. Table 1 lists the saturation current of the
transformers designed for use with the LT3512.
Primary Inductance Requirements
The LT3512 obtains output voltage information from the
reflected output voltage on the switch pin. The conduction
of secondary winding current reflects the output voltage
on the primary. The sampling circuitry needs a minimum
of 400ns to settle and sample the reflected output voltage.
In order to ensure proper sampling, the secondary winding
needs to conduct current for a minimum of 400ns. The
following equation gives the minimum value for primaryside magnetizing inductance:
LPRI ≥
tOFF(MIN) •NPS • ( VOUT + VF )
IPEAK(MIN)
tOFF(MIN) = 400ns
IPEAK(MIN) = 100mA
In addition to the primary inductance requirement for
sampling time, the LT3512 has internal circuit constraints
that prevent the switch from staying on for less than 100ns.
If the inductor current exceeds the desired current limit
3512fb
11
LT3512
Applications Information
during that time, oscillation may occur at the output as
the current control loop will lose its ability to regulate. The
following equation based on maximum input voltage must
also be followed in selecting primary-side magnetizing
inductance:
LPRI ≥
VSW
140V
The diode needs to handle the peak switch current of the
switch which was determined to be 0.45A. A 200V, 1.0A
diode from Diodes Inc. (DFLS1200) is chosen.
Step 7: Compensation.
Compensation will be optimized towards the end of the
design procedure. Connect a resistor and capacitor from
the VC node to ground. Use a 15k resistor and a 4.7nF
capacitor.
( VOUT + VF + 0.55V ) •NPS •RREF
1.2V
RREF = 10k
R
R TC = FB
NPS
Example:
RFB =
R TC =
(15 + 0.5 + 0.55V ) • 2 • 10k = 267k
1.2V
267k
= 133k
2
Step 9: Adjust RFB based on output voltage.
Power up the application with application components
connected and measure the regulated output voltage.
Readjust RFB based on the measured output voltage.
RFB(NEW) =
VOUT
VOUT(MEAS)
•RFB(OLD)
Example:
RFB(NEW) =
15V
• 267k = 237k
16.7V
Step 10: Remove RTC and measure output voltage
over temperature.
Measure output voltage in a controlled temperature environment like an oven to determine the output temperature
coefficient. Measure output voltage at a consistent load
current and input voltage, across the temperature range
of operation. This procedure will optimize line and load
regulation over temperature.
Calculate the temperature coefficient of VOUT :
∆VOUT VOUT(HOT) – VOUT(COLD)
=
∆Temp
THOT(°C) – TCOLD(°C)
3512fb
18
LT3512
Applications Information
Example:
Step 15: Ensure minimum load.
VOUT measured at 200mA and 48VIN
Check minimum load requirement at maximum input
voltage. The minimum load occurs at the point where the
output voltage begins to climb up as the converter delivers
more energy than what is consumed at the output.
∆VOUT 15.42V – 15.02V
=
= 2.26mV °C
∆Temp 125°C – ( −50°C)
Step 11: Calculate new value for RTC.
1.85mV °C
R
R TC(NEW) = FB •
∆VOUT
NPS
∆Temp
Example:
R TC(NEW) =
The minimum load at an input voltage of 72V is:
11mA
Step 16: EN/UVLO resistor values.
Determine amount of hysteresis required.
237k 1.85
•
= 97.6k
2
2.26
Step 12: Place new value for RTC, measure VOUT , and
readjust RFB due to RTC change.
RFB(NEW) =
Example:
VOUT
VOUT(MEAS)
•RFB(OLD)
Voltage hysteresis = 2.6µA • R1
Example:
Choose 2V of hysteresis.
Measure output voltage over temperature with RTC
connected.
Step 14: Optimize compensation.
Now that values for RFB and RTC are fixed, optimize the
compensation. Compensation should be optimized for
transient response to load steps on the output. Check
transient response across the load range.
Example:
1.2V • (R1+R2)
R2
1.2V •R1
R2 =
VIN(UVLO,FALLING) – 1.2V
VIN(UVLO,FALLING) =
15V
• 237k = 243k
14.7V
Step 13: Verify new values of RFB and RTC over
temperature.
2V
= 768k
2.6µA
Determine UVLO Threshold.
Example:
RFB(NEW) =
R1=
Set UVLO falling threshold to 30V.
1.2V • 768k
= 32.4k
30V – 1.2V
1.2V • (R1+R2)
VIN(UVLO,FALLING) =
R2
1.2V • ( 768k + 32.4k )
=
= 30V
32.4k
R2 =
VIN(UVLO,RISING) = VIN(UVLO,FALLING) + 2.6µA • R1 = 30V
+ 2.6µA • 768k = 32V
The optimal compensation for the application is:
RC = 18.7k, CC = 4.7nF
3512fb
19
LT3512
Typical Applications
48V to 5V Isolated Flyback Converter
VIN
36V TO 72V
4:1:1
C1
1µF
R1
1M
Z1
VIN
R2
43.2k
EN/UVLO
RFB
RREF
LT3512
TC
D3
R3
169k
T1
175µH
R5
57.6k
GND
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN LMK325B7476MM-TR
D1: DIODES INC. SBR2A40P1
D2: CENTRAL SEMI CMDSH-3
D3: DIODES INC. DFLS1200
T1: WÜRTH 750311559
Z1: ON SEMI MMSZ5266BT1G
R4
10k
BIAS
D2
R6
12.7k
C2
4.7nF
VOUT+
5V
0.5A
C4
47µF
11µH
SW
VC
D1
L1C
11µH
C3
4.7µF
3512 TA02
OPTIONAL THIRD
WINDING FOR
HV OPERATION
48V to 15V Isolated Flyback Converter
VIN
36V TO 72V
C1
1µF
Z1
VIN
R2
43.2k
T1
200µH
EN/UVLO
R3
243k
RFB
RREF
LT3512
TC
50µH
D2
R5
97.6k
GND
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER71E226KE15B
D1: DIODES INC. DFLS160
D2: DIODES INC. DFLS1200
T1: SUMIDA 10396-T023
Z1: ON SEMI MMSZ5266BT1G
R4
10k
BIAS
R6
18.7k
C2
4.7nF
C4
22µF
VOUT–
SW
VC
VOUT+
15V
0.2A
D1
2:1
R1
1M
C3
4.7µF
3512 TA03
48V to 24V Isolated Flyback Converter
VIN
36V TO 72V
C1
1µF
1:1
R1
1M
Z1
VIN
R2
43.2k
EN/UVLO
RFB
RREF
LT3512
TC
R4
10k
SW
VC
R5
162k
R3
187k
GND
BIAS
R6
24.3k
C2
2.2nF
D2
T1
151µH
D1
151µH
VOUT+
24V
110mA
C4
10µF
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN UMK316AB7475KL-T
D1: DIODES INC. SBR1U150SA
D2: DIODES INC. DFLS1200
T1: WÜRTH 750311662
Z1: ON SEMI MMSZ5266BT1G
C3
4.7µF
3512 TA04
3512fb
20
LT3512
Typical Applications
24V to 5V Isolated Flyback Converter
VIN
20V TO 30V
C1
4.7µF
D1
6:1
R1
1M
Z1
VIN
R2
80.6k
EN/UVLO
LT3512
R3
249k
RFB
RREF
D2
T1
200µH
VC
R5
69.8k
GND
C1: TAIYO YUDEN UMK316AB7475KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN LMK32587476MM-TR
D1: DIODES INC. SBR2A30P1
D2: DIODES INC. DFLS1150
T1: SUMIDA 10396-T027
Z1: ON SEMI MMSZ5270BT1G
R4
10k
BIAS
R6
6.49k
C2
4.7nF
C4
47µF
VOUT–
SW
TC
5.5µH
VOUT+
5V
0.45A
C3
4.7µF
3512 TA05
24V to 15V Isolated Flyback Converter
VIN
20V TO 30V
C1
4.7µF
D1
2:1
R1
1M
Z1
VIN
R2
80.6k
EN/UVLO
LT3512
RFB
RREF
R4
10k
SW
TC
VC
R5
150k
R3
237k
GND
BIAS
R6
20k
C2
4.7nF
D2
T1
200µH
50µH
VOUT+
15V
0.15A
C4
22µF
VOUT–
C1: TAIYO YUDEN UMK316AB7475KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER71E226KE158
D1: DIODES INC. SBR140S3
D2: DIODES INC. DFLS1150
T1: SUMIDA 10396-T023
Z1: ON SEMI MMSZ5270BT1G
C3
4.7µF
3512 TA06
3512fb
21
LT3512
Typical Applications
12V to 15V Isolated Flyback Converter
VIN
8V TO 20V
C1
4.7µF
2:1
R1
1M
R2
562k
Z1
VIN
EN/UVLO
RFB
RREF
LT3512
D2
R3
237k
T1
150µH
VC
R5
107k
GND
Z2
OPTIONAL
MINIMUM LOAD
R4
10k
C1: TAIYO YUDEN UMK316AB7475KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER7IE226K
D1: DIODES INC. SBR2A40P1
D2: DIODES INC. DFLS1150
T1: WÜRTH 750311661
Z1: ON SEMI MMSZ5270BT1G
BIAS
R6
21.5k
C2
6.8nF
C4
10µF
38µH
VOUT–
SW
TC
VOUT+
15V
70mA
D1
C3
4.7µF
3512 TA08
12V to ±70V Isolated Flyback Converter
C6
R7 10pF
3k
VIN
10V TO 20V
1:5:5
C1
2.2µF
R1
1M
VIN
R2
562k
Z1
EN/UVLO
D3
LT3512
RFB
RREF
SW
TC
VC
R5
1M
GND
BIAS
R6
24.9k
C2
6.8nF
C3
4.7µF
3512 TA07
R3
100k
R4
10k
D1
T1
100µH
VOUT1+
70V
7mA
C4
0.47µF
C7
R8 10pF
3k
D2
VOUT1–
VOUT2+
7mA
C5
0.47µF
VOUT2–
–70V
C1: TAIYO YUDEN UMK316AB7475KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4, C5: NIPPON CHEMI-CON KTS251B474M43N0T00
D1, D2: DIODES INC. ES1G
D3: DIODES INC. DFLS1150
T1: WÜRTH 750311692
Z1: ON SEMI MMS2527OBT1G
3512fb
22
LT3512
Typical Applications
48V to 3.3V Non-Isolated Flyback Converter
VIN
36V TO 72V
C1
1µF
6:1
R1
1M
Z1
VIN
RFB
EN/UVLO
R2
43.2k
LT3512
VC
R5
1M
R3
1M
D2
T1
200µH
8.66k
RREF
VOUT
R4
5.11k
SW
TC
GND
D1
BIAS
R6
9.53k
C2
4.7nF
VOUT+
3.3V
0.7A
C4
47µF
×2
VOUT–
5.5µH
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN LMK325B7476MM-TR ×2
D1: DIODES INC. SBR3U30P1
D2: DIODES INC. DFLS1200
T1: WÜRTH 750311573
Z1: ON SEMI MMSZ5266BT1G
C3
4.7µF
3512 TA09
48V to 12V Isolated Flyback Converter
VIN
36V TO 72V
C1
1µF
D1
2:1
R1
1M
Z1
VIN
R2
43.2k
EN/UVLO
LT3512
RFB
RREF
GND
BIAS
TC
R4
10k
SW
VC
R5
75k
R3
191k
R6
5.23k
C2
4.7nF
D2
T1
200µH
50µH
VOUT+
12V
0.2A
C4
10µF
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN TMK316AB7106KL-T
D1: DIODES INC. DFLS160
D2: DIODES INC. DFLS1200
T1: SUMIDA 10396-T023
Z1: ON SEMI MMSZ5266BT1G
C3
4.7µF
3512 TA10
3512fb
23
LT3512
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MS Package
Variation: MS16 (12)
16-Lead Plastic MSOP with 4 Pins Removed
(Reference LTC DWG # 05-08-1847 Rev A)
1.0
(.0394)
BSC
5.23
(.206)
MIN
0.889 ± 0.127
(.035 ± .005)
3.20 – 3.45
(.126 – .136)
4.039 ± 0.102
(.159 ± .004)
(NOTE 3)
16 14 121110 9
0.50
(.0197)
BSC
0.305 ± 0.038
(.0120 ± .0015)
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.280 ± 0.076
(.011 ± .003)
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0° – 6° TYP
1
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
0.18
(.007)
SEATING
PLANE
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
3 567 8
1.0
(.0394)
BSC
0.86
(.034)
REF
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MS12) 0510 REV A
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
3512fb
24
LT3512
Revision History
REV
DATE
DESCRIPTION
A
9/11
Added MP-Grade part. Changes reflected throughout the data sheet.
PAGE NUMBER
B
11/11
Revised Absolute Maximum Ratings.
Minor corrections to the Typical Applications drawings, TA07 and TA08.
1 - 26
2
22, 23
3512fb
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
25
LT3512
Typical Application
48V to ±15V Isolated Flyback Converter
VIN
36V TO 72V
2:1:1
C1
1µF
R1
1M
Z1
VIN
R2
43.2k
EN/UVLO
RREF
TC
VC
D3
R3
237k
RFB
LT3512
R5
287k
D1
GND
R4
10k
C4
10µF
VOUT1–
50µH
VOUT2+
100mA
C5
10µF
VOUT2–
–15V
BIAS
C3
4.7µF
3512 TA11
50µH
D2
SW
R6
8.66k
C2
6.8nF
T1
200µH
VOUT1+
15V
100mA
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4, C5: TAIYO YUDEN TMK316AB7106KL-T
D1, D2: DIODES INC. SBR0560S1
D3: DIODES INC. DFLS1200
T1: WÜRTH 750311839
Z1: ON SEMI MMSZ5266BT16
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LT3511
Monolithic High Voltage Isolated Flyback Converter
4.5V ≤ VIN ≤ 100V, 240mA/150V Onboard Power Switch, MSOP-16 with
High Voltage Spacing
LT3748
100V Isolated Flyback Controller
5V ≤ VIN ≤ 100V, No Opto-Isolator or “Third Winding” Required, Onboard
Gate Driver, MSOP-16 with High Voltage Pin Spacing
LT3958
High Input Voltage Boost, Flyback, SEPIC and
Inverting Converter
5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, 5mm × 6mm QFN-36
with High Voltage Pin Spacing
LT3957
Boost, Flyback, SEPIC and Inverting Converter
3V ≤ VIN ≤ 40V, 5A/40V Onboard Power Switch, 5mm × 6mm QFN-36 with
High Voltage Pin Spacing
LT3956
Constant-Current, Constant-Voltage Boost, Buck,
Buck-Boost, SEPIC or Flyback Converter
4.5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, True PWM Dimming,
5mm × 6mm QFN-36 with High Voltage Pin Spacing
LT3575
Isolated Flyback Switching Regulator with 60V/2.5A
Integrated Switch
3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required,
Up to 14W, TSSOP-16E
LT3573
Isolated Flyback Switching Regulator with 60V/1.25A
Integrated Switch
3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 7W,
MSOP-16E
LT3574
Isolated Flyback Switching Regulator with 60V/0.65A
Integrated Switch
3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 3W,
MSOP-16
LT3757
Boost, Flyback, SEPIC and Inverting Controller
2.9V ≤ VIN ≤ 40V, 100kHz to 1MHz Programmable Operating Frequency,
3mm × 3mm DFN-10 and MSOP-10E Package
LT3758
Boost, Flyback, SEPIC and Inverting Controller
5.5V ≤ VIN ≤ 100V, 100kHz to 1MHz Programmable Operating Frequency,
3mm × 3mm DFN-10 and MSOP-10E Package
LTC1871/LTC1871-1/ No RSENSE™ Low Quiescent Current Flyback, Boost
and SEPIC Controller
LTC1871-7
2.5V ≤ VIN ≤ 36V, Burst Mode® Operation at Light Loads, MSOP-10
3512fb
26 Linear Technology Corporation
LT 1111 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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