LT3511
Monolithic High Voltage
Isolated Flyback Converter
FEATURES
DESCRIPTION
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The LT®3511 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
240mA, 150V power switch, high voltage circuitry, and
control into a high voltage 16-lead MSOP package with
four leads removed.
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4.5V to 100V Input Voltage Range
Internal 240mA, 150V Power Switch
Boundary Mode Operation
No Transformer Third Winding or
Opto-Isolator Required for Regulation
Improved Primary-Side Winding Feedback
Load Regulation
VOUT Set with Two External Resistors
BIAS Pin for Internal Bias Supply and Power
Switch Driver
No External Start-Up Resistor
16-Lead MSOP Package
APPLICATIONS
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Isolated Telecom Power Supplies
Isolated Auxiliary/Housekeeping Power Supplies
Isolated Industrial, Automotive and Medical Power
Supplies
The LT3511 operates from an input voltage range of 4.5V
to 100V and delivers up to 2.5W 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.
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
Output Load and Line Regulation
5.25
1μF
VOUT+
5V
0.3A
4:1
1M
t
VIN
EN/UVLO
300μH
43.2k
LT3511
169k
RFB
RREF
10k
TC
SW
VC
69.8k
19μH
t
5.20
5.15
22μF
VOUT–
5.10
VOUT (V)
VIN
36V TO 72V
VIN = 48V
5.05
VIN = 36V
5.00
VIN = 72V
4.95
4.90
4.85
GND BIAS
4.80
16.9k
3.3nF
4.7μF
3511 TA01a
4.75
0
50
150
200
100
LOAD CURRENT (mA)
250
300
3511 TA01b
3511fc
1
LT3511
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
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)
LT3511E, LT3511I ............................... –40°C to 125°C
LT3511H ............................................. –40°C to 150°C
LT3511MP .......................................... –55°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
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
LT3511EMS#PBF
LT3511EMS#TRPBF
3511
16-Lead Plastic MSOP
–40°C to 125°C
LT3511IMS#PBF
LT3511IMS#TRPBF
3511
16-Lead Plastic MSOP
–40°C to 125°C
LT3511HMS#PBF
LT3511HMS#TRPBF
3511
16-Lead Plastic MSOP
–40°C to 150°C
LT3511MPMS#PBF
LT3511MPMS#TRPBF
3511
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.
Consult LTC Marketing for information on non-standard lead based finish parts.
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
CONDITIONS
MIN
MAX
UNITS
100
15
V
V
2.7
0
3.5
mA
μA
1.15
1.21
1.27
V
2.0
2.6
0
3.3
μA
μA
Maximum Current Limit
240
330
430
mA
Minimum Current Limit
35
60
90
mA
1.18
1.17
1.20
1.215
1.23
V
V
0.01
0.03
%/V
80
400
nA
l
Input Voltage Range
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
6
4.5
Maximum Switching Frequency
Switch VCESAT
650
ISW = 100mA
RREF Voltage
6V < VIN < 100V
RREF Pin Bias Current
(Note 3)
Error Amplifier Voltage Gain
ΔI = 2μA
kHz
0.3
l
RREF Voltage Line Regulation
Error Amplifier Transconductance
TYP
l
V
150
V/V
140
μmhos
3511fc
2
LT3511
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
MIN
TYP
Minimum Switching Frequency
TC Current into RREF
RTC = 53.6k
BIAS Pin Voltage
Internally Regulated
3
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 LT3511E 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
LT3511I is guaranteed to meet performance specifications from –40°C to
125°C operating junction temperature range. The LT3511H is guaranteed
Output Voltage
5.20
Quiescent Current
5.15
3.1
3.2
V
3.5
BIAS VOLTAGE (V)
IQ (mA)
VOUT (V)
4.95
μA
BIAS Pin Voltage
4
5.00
9.5
4.0
5.10
5.05
kHz
TA = 25°C, unless otherwise noted.
5
VIN = 48V
UNITS
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
MAX
40
3
2
4.90
3.0
2.5
4.85
1
VIN = 24V
VIN = 48V
VIN = 100V
4.80
4.75
–50 –25
0
0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
VIN = 24V, 10mA
VIN = 24V, NO LOAD
2.0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G01
0
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G03
3511 G02
Switch VCESAT
Switch Current Limit
1000
Quiescent Current vs VIN
4
400
800
600
400
200
3
300
250
IQ (mA)
CURRENT LIMIT (mA)
SWITCH VCESAT VOLTAGE (mV)
MAXIMUM CURRENT LIMIT
350
200
2
150
100
1
MINIMUM CURRENT LIMIT
50
0
0
50
100 150 200 250
SWITCH CURRENT (mA)
300
350
3511 G04
0
–50 –25
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G05
0
20
60
40
VOLTAGE (V)
80
100
3511 G06
3511fc
3
LT3511
TYPICAL PERFORMANCE CHARACTERISTICS
EN/UVLO Pin (Hysteresis) Current
vs Temperature
EN/UVLO PIN CURRENT (μA)
EN/UVLO PIN CURRENT (μA)
2
1
30
3.0
25
2.5
EN/UVLO THRESHOLD (V)
EN/UVLO = 1.2V
3
EN/UVLO Threshold
vs Temperature
EN/UVLO Pin Current
vs VEN/UVLO
5
4
TA = 25°C, unless otherwise noted.
20
15
10
0
1.0
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1.5
0.5
5
0
–50 –25
2.0
1
20
40
60
80
VEN/UVLO VOLTAGE (V)
3511 G07
100
0
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G09
3511 G08
Maximum Frequency
vs Temperature
Minimum Frequency
vs Temperature
1000
100
800
80
EN/UVLO Shutdown Threshold
vs Temperature
0.9
600
400
200
EN/UVLO THRESHOLD (V)
MINIMUM FREQUENCY (kHz)
MAXIMUM FREQUENCY (kHz)
0.8
60
40
20
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G10
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3511 G11
3511 G14
Light Load Discontinuous
Mode Waveform
Boundary Mode Waveform
20V/DIV
20V/DIV
1μs/DIV
3511 G12
2μs/DIV
3511 G13
3511fc
4
LT3511
PIN FUNCTIONS
EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/
UVLO pin is used to start up the LT3511. Pull the pin to 0V
to shut down the LT3511. 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 LT3511.
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 GND with a 1M resistor.
SW (Pin 16): Switch Pin. Collector of the internal power
switch. Minimize trace area at this pin to minimize EMI
and voltage spikes.
3511fc
5
LT3511
BLOCK DIAGRAM
D1
VIN
VOUT +
T1
C1
L1A
L1B
C2
R3
VOUT –
N:1
VIN
TC
CURRENT
Q3
SW
RFB
FLYBACK
ERROR
AMP
Q2
TC
R5
I2
1.2V
–g
m
+
–
+
ONE
SHOT
CURRENT
COMPARATOR
A2
–
A1
+
–
VIN
DRIVER
BIAS
RREF
S
R4
+
V1
120mV
R
Q1
Q
S
BIAS
MASTER
LATCH
C4
1.2V
R1
EN/UVLO
+
A5
–
R2
3μA
INTERNAL
REFERENCE
AND
REGULATORS
+
–
A4
RSENSE
0.02Ω
GND
OSCILLATOR
VC
Q4
R6
C3
3511 BD
3511fc
6
LT3511
OPERATION
The LT3511 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, opto-isolators or extra transformer windings
communicate this information across the transformer.
Opto-isolator circuits waste output power, and the extra
components increase the cost and physical size of the
power supply. Opto-isolators 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 LT3511, the primary-side flyback pulse provides
information about the isolated output voltage. In this manner, neither opto-isolator 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 LT3511 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 LT3511 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.
3511fc
7
LT3511
APPLICATIONS INFORMATION
PSEUDO DC THEORY
In the Block Diagram, RREF (R4) and RFB (R3) are external
resistors used to program the output voltage. The LT3511
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:
⎛ VFLBK ⎞ VBG
⎛R ⎞
or VFLBK = VBG ⎜ FB ⎟
⎜
⎟=
⎝ RFB ⎠ RREF
⎝ RREF ⎠
VBG = Internal bandgap reference
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
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:
R
1 δVTC
δVF
= – FB •
•
or,
δT
RTC NPS δT
RTC =
δV
–RFB
1
R
•
• TC ≈ FB
NPS δVF / δT δT
NPS
(δVF/δT) = Diode’s forward voltage temperature coefficient
(δVTC/δT) = 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 ⎠
⎛V ⎞ R
– ⎜ TC ⎟ • FB – ISEC (ESR)
⎝ RTC ⎠ NPS
Output Power
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
3511fc
8
LT3511
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 1.6W at 72V but
lowers to 1W 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 offtime. 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 = ~85%
Duty Cycle = D =
Peak switch current = IPEAK = 0.26A
3.0
3.5
3.0
N = NPS(MAX)
2.0
N = 15 N = 12
N = 10
N=8
OUTPUT POWER (W)
2.5
OUTPUT POWER (W)
( VOUT + VF ) • NPS
( VOUT + VF ) • NPS + VIN
N=6
1.5
N=4
1.0
N=2
0.5
N=5
2.5
N=4
N = NPS(MAX)
N=3
2.0
N=2
1.5
N=1
1.0
0.5
0
0
20
40
60
INPUT VOLTAGE (V)
0
100
80
0
20
40
60
INPUT VOLTAGE (V)
80
3511 F01
3511 F03
Figure 1. Output Power for 3.3V Output
Figure 3. Output Power for 12V Output
3.0
3.0
N=8
N = NPS(MAX)
2.0
1.5
N=3
N=2
1.0
N=1
0.5
N = NPS(MAX)
2.5
N=7
N=6
N=5
N=4
OUTPUT POWER (W)
2.5
OUTPUT POWER (W)
100
N=2
2.0
N=1
1.5
1.0
0.5
0
0
0
20
40
60
INPUT VOLTAGE (V)
80
100
3511 F02
Figure 2. Output Power for 5V Output
0
20
40
60
INPUT VOLTAGE (V)
80
100
3511 F04
Figure 4. Output Power for 24V Output
3511fc
9
LT3511
APPLICATIONS INFORMATION
TRANSFORMER DESIGN CONSIDERATIONS
Successful application of the LT3511 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 LT3511. Table 1
shows the details of these transformers.
Table 1. Predesigned Transformers
TRANSFORMER
PART NUMBER
LPRI (μH)
LEAKAGE (μH)
NP:NS:NB
ISOLATION (V)
SATURATION
CURRENT (mA)
VENDOR
750311558
300
1.5
4:1:1
1500
500
Würth Elektronik
48V to 5V, 0.3A
24V to 5V, 0.2A
12V to 5V, 0.13A
48V to 3.3V, 0.33A
24V to 3.3V, 0.28A
12V to 3.3V, 0.18A
750311019
400
5
6:1:2
1500
750
Würth Elektronik
24V to 5V, 0.26A
12V to 5V, 0.17A
48V to 3.3V, 0.43A
24V to 3.3V, 0.35A
12V to 3.3V, 0.2A
750311659
300
2
1:1:0.2
1500
560
Würth Elektronik
48V to 24V, 0.07A
750311660
350
3
2:1:0.33
1500
520
Würth Elektronik
48V to 15V, 0.1A
48V to 12V, 0.12A
24V to 15V, 0.09A
12V to 15V, 0.045A
750311838
350
3
2:1:1
1500
520
Würth Elektronik
48V to ±15V, 0.05A
48V to ±12V, 0.06A
24V to ±15V, 0.045A
750311963
200
0.4
1:5:5
1500
650
Würth Elektronik
12V to ±70V, 0.004A
12V to ±100V, 0.003A
12V to ±150V, 0.002A
750311966
120
0.45
1:5:0.5
1500
900
Würth Elektronik
12V to +120V and
–12V, 0.002A
10396-T024
300
2.0
4:1:1
1500
500
Sumida
48V to 5V, 0.3A
24V to 5V, 0.2A
12V to 5V, 0.13A
48V to 3.3V, 0.33A
24V to 3.3V, 0.28A
12V to 3.3V, 0.18A
10396-T026
300
2.5
6:1:2
1500
500
Sumida
24V to 5V, 0.26A
12V to 5V, 0.17A
48V to 3.3V, 0.43A
24V to 3.3V, 0.35A
12V to 3.3V, 0.2A
01355-T057
250
2.0
1:1:0.2
1500
500
Sumida
48V to 24V, 0.07A
10396-T022
300
2.0
2:1:0.33
1500
500
Sumida
48V to 15V, 0.1A
48V to 12V, 0.12A
24V to 15V, 0.09A
12V to 15V, 0.045A
10396-T028
300
2.5
2:1:1
1500
500
Sumida
48V to ±15V, 0.05A
48V to ±12V, 0.06A
24V to ±15V, 0.045A
TARGET
APPLICATIONS
3511fc
10
LT3511
APPLICATIONS INFORMATION
Turns Ratio
Saturation Current
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.
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 LT3511.
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:
N<
Primary Inductance Requirements
The LT3511 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 ≥
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.
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.
The turns ratio is an important element in the isolated
feedback scheme. Make sure the transformer manufacturer
guarantees turns ratio accuracy within ±1%.
tOFF(MIN) • NPS • ( VOUT + VF )
IPEAK(MIN)
tOFF(MIN) = 400ns
IPEAK(MIN) = 55mA
In addition to the primary inductance requirement for
sampling time, the LT3511 has internal circuit constraints
that prevent the switch from staying on for less than 100ns.
If the inductor current exceeds the desired current limit
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 ≥
tON(MIN) • VIN(MAX)
IPEAK(MIN)
tON(MIN) = 100ns
IPEAK(MIN) = 55mA
3511fc
11
LT3511
APPLICATIONS INFORMATION
VSW
VSW
140V
The diode needs to handle the peak switch current of the
switch which was determined to be 0.24A. A 200V, 0.6A
diode from Diodes Inc. (BAV21W) is chosen.
1.2V
RREF = 10k
RTC =
RFB
NPS
Example:
RFB =
(15 + 0.5 + 0.55V ) • 2 • 10k = 267k
RTC =
267k
= 133k
2
VZENER(MAX) ≤ 78V
In addition, power loss in the clamp circuit is inversely
related to the clamp voltage as shown previously. Higher
clamp voltages lead to lower power loss.
( VOUT + VF + 0.55V ) • NPS • RREF
1.2V
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
16.8V
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.
3511fc
18
LT3511
APPLICATIONS INFORMATION
Calculate the temperature coefficient of VOUT:
ΔVOUT VOUT(HOT) – VOUT(COLD)
=
ΔTemp
THOT(°C) – TCOLD(°C)
VOUT measured at 100mA and 48VIN
ΔVOUT 15.70V – 15.37V
=
= 1.9mV/°C
ΔTemp 125°C – ( –50°C)
Step 11: Calculate new value for RTC.
R
1.85mV / °C
RTC(NEW) = FB •
ΔVOUT
NPS
ΔTemp
RC = 22.1k, CC = 4.7nF
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.
Example:
The minimum load at an input voltage of 72V is:
7mA
Step 16: EN/UVLO resistor values.
Determine amount of hysteresis required.
Example:
237k 1.85
RTC(NEW) =
•
= 118k
2
1.9
Step 12: Place new value for RTC, measure VOUT, and
readjust RFB due to RTC change.
VOUT
VOUT(MEAS)
Voltage hysteresis = 2.6μA • R1
Example:
Choose 2V of hysteresis.
R1=
2V
= 768k
2.6μA
• RFB(OLD)
Example:
RFB(NEW) =
The optimal compensation for the application is:
Step 15: Ensure minimum load.
Example:
RFB(NEW) =
Example:
Determine UVLO Threshold.
VIN(UVLO,FALLING) =
15V
• 237k = 237k
15V
Step 13: Verify new values of RFB and RTC over
temperature.
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.
R2 =
1.2V • (R1+ R2)
R2
1.2V • R1
VIN(UVLO,FALLING) – 1.2V
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
3511fc
19
LT3511
TYPICAL APPLICATIONS
48V to 5V Isolated Flyback Converter
VIN
36V TO 72V
4:1:1
C1
1μF
R1
1M
Z1
VIN
EN/UVLO
R2
43.2k
LT3511
TC
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER71C226KE18B
D1, D2: DIODES INC. SBR140S3
D3: DIODES INC. BAV21W
T1: WÜRTH 750311558
Z1: ON SEMI MMSZ5266BT1G
R4
10k
SW
VC
R5
69.8k
GND
BIAS
D2
R6
16.9k
C2
3.3nF
VOUT+
5V
0.3A
C4
22μF
19μH
D3
R3
169k
RFB
RREF
T1
300μH
D1
L1C
19μH
C3
4.7μF
3511 TA02
OPTIONAL THIRD
WINDING FOR
HV OPERATION
48V to 15V Isolated Flyback Converter
VIN
36V TO 72V
C1
1μF
D1
2:1
R1
1M
Z1
VIN
T1
350μH
EN/UVLO
R2
43.2k
R3
237k
RFB
RREF
LT3511
TC
SW
VC
R5
97.6k
R4
10k
GND
BIAS
R6
13k
C2
6.8nF
D2
88μH
VOUT+
15V
0.1A
C4
10μF
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM31CR71E106KA12
D1: DIODES INC. SBR0560S1
D2: DIODES INC. BAV21W
T1: WÜRTH 750311660
Z1: ON SEMI MMSZ5266BT1G
C3
4.7μF
3511 TA03
3511fc
20
LT3511
TYPICAL APPLICATIONS
48V to 24V Isolated Flyback Converter
VIN
36V TO 72V
C1
1μF
Z1
VIN
T1
300μH
EN/UVLO
R2
43.2k
R3
187k
RFB
RREF
LT3511
TC
R5
200k
GND
C4
4.7μF
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER71H475KA88B
D1: DIODES INC. SBR1U150SA
D2: DIODES INC. BAV21W
T1: WÜRTH 750311659
Z1: ON SEMI MMSZ5266BT1G
R4
10k
BIAS
R6
33.2k
C2
3.3nF
300μH
D2
SW
VC
VOUT+
24V
65mA
D1
1:1
R1
1M
C3
4.7μF
3511 TA04
24V to 5V Isolated Flyback Converter
VIN
20V TO 30V
D1
6:1
C1
4.7μF
R1
1M
Z1
VIN
T1
300μH
EN/UVLO
R2
80.6k
R3
249k
LT3511
RFB
RREF
SW
TC
VC
R5
73.2k
GND
BIAS
R6
9.31k
C2
15nF
R4
10k
D2
8μH
VOUT+
5V
0.25A
C4
22μF
VOUT–
C1: MURATA GRM31CR71H475KA12B
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM32ER71C226KE18B
D1: DIODES INC. SBR2A30P1
D2: DIODES INC. BAV20W
T1: SUMIDA 10396-T026
Z1: ON SEMI MMSZ5270BT1G
C3
4.7μF
3511 TA05
3511fc
21
LT3511
TYPICAL APPLICATIONS
24V to 15V Isolated Flyback Converter
VIN
20V TO 30V
C1
4.7μF
R1
1M
Z1
VIN
EN/UVLO
R2
80.6k
R3
237k
RFB
RREF
LT3511
T1
350μH
VC
R5
133k
GND
D2
VOUT–
C1: MURATA GRM31CR71H475KA12B
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM31CR71E106KA12B
D1: DIODES INC. SBR140S3
D2: DIODES INC. BAV20W
T1: WÜRTH 750311660
Z1: ON SEMI MMSZ5270BT1G
R4
10k
BIAS
R6
20k
C2
4.7nF
C4
10μF
88μH
SW
TC
VOUT+
15V
0.09A
D1
2:1
C3
4.7μF
3511 TA06
12V to 15V Isolated Flyback Converter
VIN
8V TO 20V
2:1
C1
4.7μF
R1
1M
Z1
VIN
T1
350μH
EN/UVLO
R2
562k
R3
237k
LT3511
RFB
RREF
SW
TC
VC
R5
133k
GND
BIAS
R6
26.1k
C2
4.7nF
C3
4.7μF
R4
10k
D2
VOUT+
15V
40mA
D1
88μH
C4
4.7μF
Z2
VOUT–
OPTIONAL
MINIMUM LOAD
C1: MURATA GRM31CR71H475KA12B
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM31CR71H475KA12
D1: DIODES INC. SBR130S3
D2: DIODES INC. BAV20W
T1: WÜRTH 750311660
Z1: ON SEMI MMSZ5270BT1G
3511 TA08
3511fc
22
LT3511
TYPICAL APPLICATIONS
12V to ±70V Isolated Flyback Converter
VIN
8V TO 20V
1:5:5
C1
2.2μF
R1
1M
VIN
Z1
EN/UVLO
R2
562k
RFB
RREF
T1
80μH
D2
R4
10k
VC
R5
191k
GND
BIAS
R6
90.9k
C2
6.8nF
VOUT1–
VOUT2+
4mA
C5
0.47μF
VOUT2–
–70V
SW
TC
VOUT1+
70V
4mA
C4
0.47μF
D3
R3
105k
LT3511
D1
C1: MURATA GRM21BR71E225KA73B
C3: TAIYO YUDEN EMK212B7475KG
C4, C5: NIPPON CHEMI-CON KTS251B474M43N0T00
D1, D2: CENTRAL SEMI CRM1U-06M
D3: DIODES INC. BAV20W
T1: WÜRTH 750311692
Z1: NXP BZX100A
C3
4.7μF
3511 TA07
48V to 3.3V Non-Isolated Flyback Converter
VIN
36V TO 72V
D1
6:1
C1
1μF
R1
1M
Z1
VIN
RFB
EN/UVLO
R3
1M
R2
43.2k
LT3511
11μH
D2
VC
GND
C4
47μF
VOUT–
8.66k
VOUT
RREF
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: TAIYO YUDEN LMK325B7476MM-TR
D1: DIODES INC. SBR2A30P1
D2: DIODES INC. BAV21W
T1: WÜRTH 750311019
Z1: ON SEMI MMSZ5266BT1G
R4
5.11k
SW
TC
R5
1M
T1
400μH
VOUT
3.3V
0.4A
BIAS
R6
8.06k
C2
4.7nF
C3
4.7μF
3511 TA09
48V to 12V Isolated Flyback Converter
VIN
36V TO 72V
C1
1μF
D1
2:1
R1
1M
Z1
VIN
T1
300μH
EN/UVLO
R2
43.2k
R3
191k
LT3511
TC
R4
10k
SW
VC
R5
143k
RFB
RREF
GND
BIAS
R6
15k
C2
6.8nF
D2
75μH
VOUT+
12V
0.1A
C4
4.7μF
VOUT–
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4: MURATA GRM31CR71H475KA12
D1: DIODES INC. SBR0560S1
D2: DIODES INC. BAV21W
T1: SUMIDA 10396-T022
Z1: ON SEMI MMSZ5266BT1G
C3
4.7μF
3511 TA10
3511fc
23
LT3511
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 p 0.127
(.035 p .005)
3.20 – 3.45
(.126 – .136)
4.039 p 0.102
(.159 p .004)
(NOTE 3)
16 14 121110 9
0.305 p 0.038
(.0120 p .0015)
TYP
0.50
(.0197)
BSC
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.280 p 0.076
(.011 p .003)
REF
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
DETAIL “A”
0o – 6o TYP
1
GAUGE PLANE
0.53 p 0.152
(.021 p .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 p 0.0508
(.004 p .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
3511fc
24
LT3511
REVISION HISTORY
REV
DATE
DESCRIPTION
A
4/11
Added MP-grade.
PAGE NUMBER
2, 3
Revised RFB pin description in the Pin Functions section.
B
6/11
9, 10
Revised the Typical Applications drawings.
20, 21
Deleted text from Turns Ratio section and added text to Primary Inductance Requirements of Applications Information
11
Minor edit to text and revision to Table 3 in Leakage Inductance and Clamp Circuits section of Applications Information
12-13
Replaced Step 3 in Design Procedure/Design Example section of Applications Information
16
Revised equation and made minor text edit to Step 6 in Design Procedure/Design Example section of Applications
Information
Updated “D2: Diodes” part numbers in all Typical Applications
18
Added LT3512 to Related Parts section
C
12/11
5
Updated efficiency equation and Table 1 in the Applications Information section.
20-23, 26
26
Revised Absolute Maximum Ratings and H-grade Temperature Range
2
Updated resistor values on Typical Applications drawings TA07 and TA08
23
3511fc
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
LT3511
TYPICAL APPLICATION
48V to ±15V Isolated Flyback Converter
VIN
36V TO 72V
2:1:1
C1
1μF
R1
1M
Z1
VIN
T1
350μH
EN/UVLO
R2
43.2k
TC
VC
R5
154k
C4
4.7μF
VOUT1–
D2
RFB
RREF
88μH
D3
R3
237k
LT3511
VOUT1+
15V
50mA
D1
R4
10k
88μH
VOUT2+
50mA
C5
4.7μF
SW
GND
BIAS
R6
20k
C2
6.8nF
C3
4.7μF
3511 TA11
C1: TAIYO YUDEN HMK316B7105KL-T
C3: TAIYO YUDEN EMK212B7475KG
C4, C5: MURATA GRM31CR71H475KA12
D1, D2: DIODES INC. SBR0560S1
D3: DIODES INC. BAV21W
T1: WÜRTH 750311838
Z1: CENTRAL SEMI CMHZ5266B
VOUT2–
–15V
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3512
Monolithic High Voltage, Isolated Flyback Converter,
No Opto-Coupler Required
4.5V ≤ VIN ≤ 100V, 420mA/150V Onboard Power Switch, MSOP-16(12)
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
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
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
3511fc
26 Linear Technology Corporation
LT 1211 REV C • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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