LINEAR TECHNOLOGY LT3511 Technical data
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LT3511
Monolithic High Voltage
Isolated Flyback Converter
FEATURES
n
4.5V to 100V Input Voltage Range
n
Internal 240mA, 150V Power Switch
n
Boundary Mode Operation
n
No Transformer Third Winding or
Opto-Isolator Required for Regulation
n
Improved Primary-Side Winding Feedback
Load Regulation
n
V
Set with Two External Resistors
OUT
n
BIAS Pin for Internal Bias Supply and Power
Switch Driver
n
No External Start-Up Resistor
n
16-Lead MSOP Package
APPLICATIONS
n
Isolated Telecom Power Supplies
n
Isolated Auxiliary/Housekeeping Power Supplies
n
Isolated Industrial, Automotive and Medical Power
Supplies
DESCRIPTION
The LT3511 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.
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 R
property of their respective owners. Protected by U.S. Patents, including 5438499, 7471522.
is a trademark of Linear Technology Corporation. All other trademarks are the
SENSE
TYPICAL APPLICATION
48V to 5V Isolated Flyback Converter
V
IN
36V TO 72V
1
µF
1M
43.2k
69.8k
V
EN/UVLO
T
IN
LT3511
C
VC GND BIAS
16.9k
3.3nF
3511 TA01a
Output Load and Line Regulation
+
V
22
5V
0.3A
V
OUT
µF
OUT
–
4:1
•
19
µH300µH
169k
R
FB
R
REF
10k
SW
4.7µF
•
(V)
OUT
V
5.25
5.20
5.15
5.10
5.05
5.00
4.95
4.90
4.85
4.80
4.75
VIN = 48V
50
0
100
LOAD CURRENT (mA)
VIN = 36V
VIN = 72V
150 200
250
300
3511 TA01b
3511f
1
LT3511
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PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
(Note 1)
SW (Note 4) ............................................................150V
, EN/UVLO ..........................................................100V
V
IN
............................................................100V, VIN ±6V
R
FB
...................................................................VIN, 20V
BIAS
R
, VC .................................................................6V
REF,TC
Operating Junction Temperature Range (Note 2)
LT3511E, LT3511I ............................... –40°C to 125°C
LT3511H ............................................. –40°C to 150°C
Storage Temperature Range
.................. –65°C to 150°C
EN/UVLO
V
GND
BIAS
NC
GND
IN
TOP VIEW
1
3
5
6
7
8
MS PACKAGE
16(12)-LEAD PLASTIC MSOP
θJA = 90°C/W
SW
16
R
14
FB
12
R
REF
11
T
C
10
VC
9
GND
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
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 CONDITIONS MIN TYP MAX UNITS
l
Input Voltage Range
Quiescent Current Not Switching
EN/UVLO Pin Threshold EN/UVLO Pin Voltage Rising
EN/UVLO Pin Current V
Maximum Switching Frequency 650 kHz
Maximum Current Limit 240 330 430 mA
Minimum Current Limit 35 60 90 mA
Switch V
CESAT
Voltage
R
REF
Voltage Line Regulation 6V < V
R
REF
Pin Bias Current (Note 3)
R
REF
Error Amplifier Voltage Gain 150 V/V
Error Amplifier Transconductance
= BIAS
V
IN
V
= 0.2V
EN/UVLO
=1.1V
EN/UVLO
V
=1.4V
EN/UVLO
ISW = 100mA 0.3 V
< 100V 0.01 0.03 %/V
IN
∆I = 2µA
6
4.5
2.7
0
l
1.15 1.21 1.27 V
l
l
2.0 2.6
1.18
1.17
0
1.20 1.215
80 400 nA
140 μmhos
100
15
3.5 mA
μA
3.3 μA
μA
1.23
V
V
V
V
2
3511f
V
(V)
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ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
LT3511
temperature range, otherwise specifications are at TA = 25°C. VIN = 24V unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Switching Frequency 40 kHz
Current into R
T
C
REF
BIAS Pin Voltage Internally Regulated 3 3.1 3.2 V
RTC = 53.6k 9.5 μA
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
125°C operating junction temperature range. The LT3511H is guaranteed
over the full –40°C to 150°C operating junction temperature range. High
junction temperatures degrade operating lifetimes. Operating lifetime is
derated at junction temperatures greater than 125°C.
Note 3: Current flows out of the R
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.
LT3511I is guaranteed to meet performance specifications from –40°C to
= 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
T
A
Output Voltage Quiescent Current BIAS Pin Voltage
5
4
3
(mA)
Q
I
2
1
0
–50 –25
0
Switch Current Limit
400
350
300
MAXIMUM CURRENT LIMIT
50
25
TEMPERATURE (°C)
75
VIN = 24V
= 48V
V
IN
= 100V
V
IN
100
3511 G02
4.0
3.5
3.0
BIAS VOLTAGE (V)
2.5
150125
2.0
–50
4
3
OUT
1000
5.25
5.20
5.15
5.10
5.05
5.00
4.95
4.90
4.85
4.80
4.75
800
VIN = 48V
–50
–25 25
Switch V
0
50
TEMPERATURE (°C)
CESAT
125
100
75
150
3511 G01
pin.
REF
VIN = 24V, 10mA
= 24V, NO LOAD
V
IN
–25 0 25 50
TEMPERATURE (°C)
75 100 150125
Quiescent Current vs V
3511 G03
IN
600
VOLTAGE (mV)
400
CESAT
200
SWITCH V
0
0 50
150
200
250
100
SWITCH CURRENT (mA)
300
3511 G04
350
250
200
150
CURRENT LIMIT (mA)
100
50
0
–50
MINIMUM CURRENT LIMIT
–25 0 50
25
TEMPERATURE (°C)
75 100 150125
3511 G05
(mA)
Q
I
2
1
0
20
0
40
VOLTAGE (V)
60
80
100
3511 G06
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TYPICAL PERFORMANCE CHARACTERISTICS
EN/UVLO Pin (Hysteresis) Current
vs Temperature
5
EN/UVLO = 1.2V
4
3
2
EN/UVLO PIN CURRENT (µA)
1
0
–50 –25
0
50
25
TEMPERATURE (°C)
75
100
Maximum Frequency
vs Temperature
1000
800
600
400
MAXIMUM FREQUENCY (kHz)
200
0
–50 –25
0
50
25
TEMPERATURE (°C)
75
100
150125
3511 G07
150125
3511 G10
EN/UVLO Pin Current
vs V
EN/UVLO
30
25
20
15
10
EN/UVLO PIN CURRENT (µA)
5
0
1
20 40 60 80
V
EN/UVLO
Minimum Frequency
vs Temperature
100
80
60
40
MINIMUM FREQUENCY (kHz)
20
0
–50 –25
0
25
TEMPERATURE (°C)
VOLTAGE (V)
50
75
100
TA = 25°C, unless otherwise noted.
EN/UVLO Threshold
vs Temperature
3.0
2.5
2.0
1.5
1.0
EN/UVLO THRESHOLD (V)
0.5
3511 G08
100
0
–50
–25 0
50 100 150125
25 75
TEMPERATURE (°C)
EN/UVLO Shutdown Threshold
vs Temperature
0.9
0.8
0.7
0.6
0.5
0.4
0.3
EN/UVLO THRESHOLD (V)
0.2
0.1
3511 G11
0
150125
–50
–25
0
25 150125
50
TEMPERATURE (°C)
3511 G09
75 100
3511 G14
4
Boundary Mode Waveform
20V/DIV
1µs/DIV
3511 G12
Light Load Discontinuous
Mode Waveform
20V/DIV
2µs/DIV
3511 G13
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PIN FUNCTIONS
LT3511
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) hys-
teresis. EN/UVLO can be directly connected to V
. If left
IN
open circuit the part will not power up.
(Pin 3): Input Supply Pin. This pin supplies current to
V
IN
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 V
is not used and if V
to 4.5V when BIAS and V
< 20V. The part can operate down
IN
are connected together. If a
IN
if a third winding
IN
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.
(Pin 11): Output Voltage Temperature Compensa-
T
C
tion. Connect a resistor to ground to produce a current
proportional to absolute temperature to be sourced into
REF
node.
the R
ITC = 0.55V/RTC.
(Pin 12): Input Pin for External Ground-Referred
R
REF
Reference Resistor. The resistor at this pin should be 10k.
For nonisolated applications, a traditional resistor voltage
divider from V
(Pin 14): Input Pin for External Feedback Resistor.
R
FB
This pin is connected to the transformer primary (V
The ratio of this resistor to the R
may be connected to this pin.
OUT
resistor, times the
REF
SW
).
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 V
IN
.
SW (Pin 16): Switch Pin. Collector of the internal power
switch. Minimize trace area at this pin to minimize EMI
and voltage spikes.
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BLOCK DIAGRAM
Q1
T1
N:1
SW
R
SENSE
0.02Ω
D1
GND
V
C
3511 BD
V
IN
C1
R3
TC
CURRENT
T
C
R5
R
REF
R4
BIAS
C4
R1
R2
EN/UVLO
V
Q3
1.2V
3µA
Q4
R
IN
+
–
FB
Q2
I
2
A5
INTERNAL
REFERENCE
REGULATORS
AND
1.2V
FLYBACK
ERROR
AMP
–
g
m
+
CURRENT
COMPARATOR
–
A1
+
ONE
SHOT
RQS
S
MASTER
LATCH
OSCILLATOR
A2
120mV
DRIVER
BIAS
A4
L1A L1B
–
+
+
V1
–
V
IN
+
–
+
V
OUT
C2
–
V
OUT
R6
C3
6
3511f
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OPERATION
LT3511
The LT3511 is a current mode switching regulator IC de-
signed 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 LT3511, the primary-side flyback pulse provides
information about the isolated output voltage. In this man-
ner, 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, os-
cillator, 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 V
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.
. A discontinuous conduction
IN
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APPLICATIONS INFORMATION
PSUEDO DC THEORY
In the Block Diagram, R
(R4) and RFB (R3) are external
REF
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 V
rail. The am-
IN
plitude of this flyback pulse, i.e., the difference between
it and V
V
, is given as:
IN
= (V
FLBK
OUT
+ VF + I
• ESR) • N
SEC
PS
VF = D1 forward voltage
= Transformer secondary current
I
SEC
ESR = Total impedance of secondary circuit
= Transformer effective primary-to-secondary turns
N
PS
ratio
and Q2 convert the flyback voltage into a current. Nearly
R
FB
all of this current flows through R
to form a ground-
REF
referred 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 effect of nonzero secondary output impedance (ESR).
Boundary control mode minimizes the effect of this impedance term.
Temperature Compensation
The first term in the V
equation does not have tem-
OUT
perature dependence, but the diode forward drop has a
significant negative temperature coefficient. A positive
temperature coefficient current source connects to the
pin to compensate. A resistor to ground from the
R
REF
pin sets the compensation current.
T
C
The following equation explains the cancellation of the
temperature coefficient:
dV
dT
R
TC
R
F
= −
=
−R
N
R
FB
TC
FB
PS
1
dV
•
N
PS
1
•
dV
/ dT
F
TC
•
or,
dT
dV
•
dT
TC
R
FB
≈
N
PS
(dVF/dT) = Diode’s forward voltage temperature coefficient
/dT) = 2mV
(dV
TC
= 0.55V
V
TC
Experimentally verify the resulting value of R
and adjust as
TC
necessary to achieve optimal regulation over temperature.
The relatively high gain in the overall loop will then cause
the voltage at R
reference voltage V
and VBG approximately equals:
V
FLBK
V
FL BK
R
FB
=
to be nearly equal to the bandgap
REF
. The resulting relationship between
BG
V
BG
or V
R
RE F
FL BK
= V
BG
R
FB
R
RE F
VBG = Internal bandgap reference
Combination of the preceding expression with earlier
derivation of V
V
= V
OU T
The expression defines V
results in the following equation:
FLBK
1
N
PS
in terms of the internal ref-
OUT
− V
− I
F
BG
R
FB
R
RE F
SE C
(ESR)
erence, programming resistors, transformer turns ratio
and diode forward voltage drop. Additionally, it includes
8
The addition of a temperature coefficient current modifies
the expression of output voltage as follows:
FB
PS
1
− V
F
N
PS
– I
SE C
(ESR)
V
= V
OU T
BG
V
−
R
TC
TC
R
FB
R
RE F
R
•
N
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
3511f
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