TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
Synchronous Flyback Controller
The L T®3837 is an isolated switching regulator controller
designed for medium power fl yback topologies. A typical
application is 10W to 60W with the part powered from a
DC supply.
The L T3837 is a current mode controller that regulates an
output voltage based on sensing the secondary voltage
via a transformer winding during fl yback. This allows for
tight output regulation without the use of an optoisolator,
improving dynamic response and reliability. Synchronous
rectifi cation increases converter effi ciency and improves
output cross regulation in multiple output converters.
The L T3837 operates in forced continuous conduction
mode which improves cross regulation in multiple winding
applications. Switching frequency is user programmable
and can be externally synchronized. The part also has load
compensation, undervoltage lockout and soft-start circuitry.
The L T3837 is available in a thermally enhanced 16-pin
TSSOP package.
9V –18V to 3.3V at 10A Isolated Converter
n Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
n Synchronous Driver for High Effi ciency
n Supply Voltage Range 4.5V to 20V
n Accurate Regulation Without User T rims
n Programmable Switching Frequency from
50kHz to 250kHz
n Synchronizable
n Load Compensation
n Undervoltage Lockout
n Available in a Thermally Enhanced 16-Lead
TSSOP Package
n Isolated Medium Power (10W to 60W) Supplies
n Instrumentation Power Supplies
n Isolated Medical Supplies
LOAD CURRENT (A)
2
78
76
远大前程插曲E
F
F
I
C
I
E
N
C
Y
(
%
)
80
84
86
88
379
3837 TA01b
82
90
6
45810
3.00
O
U
T
P
U
T
(
V
)
3.10
3.30
3.40
3.50
3.60
3.20
2379
6
45810
Effi ciency vs Load Current
Regulation vs Load Current
Protected by U.S. Patents including 6498466, 5841643.
ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS
V CC  to GND ................................................–0.3V to 22V
UVLO, SYNC Pin Voltage ............................–0.3V to V CC SENSE –, SENSE + Pin Voltage ......................–0.5V , +0.5V FB Pin Current ........................................................±2mA
V C  Pin Current
........................................................±1mA Operating Junction Temperature Range
(Notes 2, 3)  ..........................................–40°C to 125°C Storage Temperature Range ...................–65°C to 150°C Lead Temperature (Soldering, 10 sec) ..................300°C
(Note 1)
The
l  denotes specifi cations which apply over the full operating temperature range, otherwise specifi cations are T A  = 25°C. V CC  = 14V; PG, SG Open; V C  = 1.4V ,  V SENSE  = 0, R CMP  = 1k, R TON  = 90k, R PGDL Y  = 27.4k, R ENDL Y  = 90k, unless otherwise specifi ed.
PARAMETER CONDITIONS
MIN TYP MAX UNITS
Power Supply V CC  Operating Range
l
4.520V V CC  Supply Current (I CC ) (Note 5)V C  = Open
l 4
6.410mA V CC  Shutdown Current V C  = Open, V UVLO  = OV
l
50
150μA Feedback Amplifi er
Feedback Regulation Voltage (V FB )l    1.220  1.237  1.251V Feedback Pin Input Bias Current R CMP  Open 200
nA Feedback Amplifi er T ransconductance ΔI C  = ±10μA
l 70010001400μmho Feedback Amplifi er Source or Sink Current l
25
5590
μA Feedback Amplifi er Clamp Voltage V FB  = 0.9
V FB  = 1.4  2.560.84V
V
Reference Voltage/Current Line Regulation
12V ≤ V IN  ≤ 18V
l 0.005
0.05%V
P IN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE L T3837EFE#PBF L T3837EFE#TRPBF 3837EFE 16-Lead Plastic TSSOP –40°C to 125°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LL T3837EFE
L T3837EFE#TR
3837EFE
16-Lead Plastic TSSOP
–40°C to 125°C
Consult L TC Marketing for parts specifi ed with wider operating temperature ranges.
For more information on lead free part marking, go to: www.linear /leadfree/ For more information on tape and reel specifi cations, go to: www.linear /tapeandreel/
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: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specifi ed maximum operating junction temperature may impair device reliability.
ELECTRICAL CHARACTERISTICS  The l  denotes specifi cations which apply over the full operating
temperature range, otherwise specifi cations are T A  = 25°C. V CC  = 14V; PG, SG Open; V C  = 1.4V ,  V SENSE  = 0, R CMP  = 1k, R TON  = 90k, R PGDL Y  = 27.4k, R ENDL Y  = 90k, unless otherwise specifi ed.
Note 3: The L T3837E is guaranteed to meet performance specifi cations from 0°C to 85°C. Specifi cations over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls.
Note 4: T J  is calculated from the ambient temperature T A  and power dissipation PD according to the following formula:  T J  = T A  + (P D  • 40°C/W)
Note 5: Supply current does not include gate charge current to the MOSFETs. See Applications Information.
Note 6: Component value range guaranteed by design.
PARAMETER
CONDITIONS MIN
TYP MAX UNITS Feedback Amplifi er Voltage Gain V C  = 1V to 2V 1400V/V Soft-Start Charging Current V SFST  = 1.5V
162025
μA Soft-Start Discharge Current V SFST  = 1.5V , V UVLO  = 0V 0.7  1.3mA Control Pin Threshold (V C )Duty Cycle = Min
1.0V
Gate Outputs
PG, SG Output High Level PG, SG Output Low Level l l
6.6
7.4
0.018.00.05V V PG, SG Output Shutdown Strength V UVLO  = 0V; I PG , I SG  = 20mA l
1.6
2.3
V PG Rise Time C PG  = 1nF 11ns SG Rise Time C SG  = 1nF 15ns PG, SG Fall Time C PG , C SG  = 1nF 10
ns
Current Amplifi er
Switch Current Limit at Maximum V C V SENSE  +
l
8898110mV ΔV SENSE /ΔV C
0.07V/V Sense Voltage Overcurrent Fault Voltage V SENSE  +, V SFST  < 1V 206
230mV Timing
Switching Frequency (f OSC )C OSC  = 100pF l 84100110kHz Oscillator Capacitor Value (C OSC )(Note 6)
园游会33
200
pF Minimum Switch On Time (t ON(MIN))200ns Flyback Enable Delay Time (t ED )265ns PG Turn-On Delay Time (t PGDL Y )200
ns Maximum Switch Duty Cycle l 8588%
SYNC Pin Threshold l    1.53  2.1
V SYNC Pin Input Resistance 40
k ΩLoad Compensation
Load Comp to V SENSE  Offset Voltage V RCMP  with V SENSE+ = 00.8mV Feedback Pin Load Compensation Current V SENSE + = 20mV
20μA
UVLO Function
UVLO Pin Threshold (V UVLO )l    1.215  1.240  1.265V UVLO Pin Bias Current
V UVLO  = 1.2V
V UVLO  = 1.3V
–0.25–4.50
0.1–3.4
0.25–2.50
μA μA
TEMPERATURE (°C)
–50V C C  C U R R E N T  (μA )
60809025753837 G024*********–25
50100
125
100
TEMPERATURE (°C)
–50781025753837 G03
65
–25
50100
125
43
9
I V C C  (m A )
TEMPERATURE (°C)
–50
90S E N S E  V O L T A G E  (m V )92
9698100110
10405075
3837 G04
94106108102–25
25100
125
TEMPERATURE (°C)
–50V S E N S E + – V S E N S E – (m V )
21525
3837 G05
200190–25
50185
180
220
210
20519575100
125
TEMPERATURE (°C)
–5090f O S C  (k H z )
92
969810011010405075
3837 G0694106108102–25
25100
125
TEMPERATURE (°C)
–50
1.230
V F B  (V )1.231
1.2331.2341.2351.240
仲维军
1.23705075
3837 G07
1.2321.2381.2391.236–25
25100
125
TEMPERATURE (°C)
–50F E E D B A C K  P I N  I N P U T  B I A S  (n A )
20025030025753837 G0*******–25
50100
125
50
TEMPERATURE (°C)
–50V F B  R E S E T  (V )1.0325
3837 G09
1.000.98–25
500.97
0.96
1.041.021.010.9975100
125
V FB  (V)
0.9–70
I V C  (μA )
–50
–30–1070301  1.1
1.45010  1.2  1.3
1.5
3837G10
TYPICAL PERFORMANCE CHARACTERISTICS
V CC  Shutdown Current vs Temperature
V CC  Current vs Temperature
SENSE Voltage vs Temperature
SENSE Fault Voltage vs Temperature
Oscillator Frequency vs Temperature
V FB  vs Temperature
Feedback Pin Input Bias  vs Temperature
V FB  Reset vs Temperature
Feedback Amplifi er Output Current vs V FB
TEMPERATURE (°C)
–50V U V L O  (μA )
0.700.800.900.852******* G15a
0.600.650.750.550.50–25
50100125
0.45
0.40
TEMPERATURE (°C)
–50
I V C  (μA )
60657025753837 G11
5550
–25
50100125
4540
TEMPERATURE (°C)
–50
900g m  (μm h o )
950
1000
叶丽仪10501100
–25
02550
3837 G12
75100125
TEMPERATURE (°C)
–50U V L O  (V )1.240
1.2451.25025753837 G141.235
1.230–25
50100125
1.225
1.220
TEMPERATURE (°C)
–503.43.53.725753837 G153.33.2–250
50100
125
3.1
3.0
3.6I U V L O
(μA )
TEMPERATURE (°C)
–50S F S T  C H A R G E  C U R R E N T  (μA )
2325
3837 G162018–25
501716
2422
21
1975100
125
CAPACITANCE (nF)
0T I M E  (n s )
80706050403020100
83837 G172
4610
71359
TEMPERATURE (°C)
–50
–25
19.0
V C C  (V )
20.0
05075
3837 G18
19.5
21.0
20.5
25100
125
TEMPERATURE (°C)
–50A V  (V /V )
155025
3837 G13
14001300–25
50125012001150
1100
1600
1650170015001450135075100125
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Amplifi er Source and Sink Current vs Temperature
Feedback Amplifi er g m vs Temperature
Feedback Amplifi er Voltage Gain vs Temperature
UVLO vs Temperature
I UVLO  Hysteresis vs Temperature
Soft-Start Charge Current vs Temperature
PG, SG Rise and Fall Times vs Load Capacitance
Minimum On-Time vs Temperature
UVLO Shutdown Threshold vs Temperature
TEMPERATURE (°C)
–50M I N I M U M  E N A B L E  T I M E  (n s )
22024026025753837 G21
200180–25
50100125
160
140
TEMPERATURE (°C)
–500
t P G  (n s )50
150200250
–1030503837 G20
100–301070
90110
300李准郑素敏分手
PIN FUNCTIONS
TYPICAL PERFORMANCE CHARACTERISTICS
SG (Pin 1): Synchronous gate drive output. This pin pro-vides an output signal for a secondary-side synchronous switch. Large dynamic currents may fl ow during voltage transitions. See the Applications Information for details.V CC  (Pin 2): Supply voltage pin. Bypass this pin to ground with a 4.7μF capacitor or more.
t ON  (Pin 3): Pin for external programming resistor to set the minimum time that the primary switch is on for each cycle. Minimum turn-on facilitates the isolated feedback method. See Applications Information for details.ENDL Y (Pin 4): Pin for external programming resistor to set enable delay time. The enable delay time disables the feedback amplifi er for a fi xed time after the turn-off of the primary-side MOSFET . This allows the leakage inductance voltage spike to be ignored for fl yback voltage sensing. See Applications Information for details.
SYNC (Pin 5): Pin for synchronizing the internal oscilla-tor with an external clock. The positive edge on a pulse causes the oscillator to discharge causing PG to go low (off) and SG high (on). The sync threshold is typically 1.4V . See Applications Information for details. Tie to ground if unused.
SFST (Pin 6): This pin, in conjunction with a capacitor to ground, controls the ramp-up of peak primary current as sensed through the sense resistor . This is used to control
converter inrush current at start-up. The V C  pin voltage cannot exceed the SFST pin voltage, so as SFST increases, the maximum voltage on V C  increases commensurately, allowing higher peak currents. Total V C  ramp time is ap-proximately 70ms per μF of capacitance. Leave pin open if not using the soft-start function.
OSC (Pin 7): This pin in conjunction with an external capacitor defi nes the controller oscillator frequency. The frequency is approximately 100kHz • 100/C OSC (pF).FB (Pin 8): Pin for the feedback node for the power sup-ply feedback amplifi er . Feedback is sensed via a trans-former winding and enabled during the fl yback period. This pin also sinks additional current to compensate for load current variation as set by the R CMP  pin. Keep the Thevenin equivalent resistance of the feedback divider at roughly 3k.
V C  (Pin 9): Pin used for frequency compensation for the switcher control loop. It is the output of the feedback amplifi er and the input to the current comparator . Switcher frequency compensation components are normally placed on this pin to ground. The voltage on this pin is propor-tional to the peak primary switch current. The feedback amplifi er output is enabled during the synchronous switch on time.
PG Delay Time vs Temperature
Enable Delay Time vs Temperature