Document
FEATURES
LT8311 Synchronous Rectifier Controller with Opto-Coupler Driver for Forward Converters DESCRIPTION
n Wide Input Supply Range: 3.7V to 30V n Preactive Mode: n No Pulse Transformer Required n DCM Operation at Light Load n SYNC Mode: n FCM or DCM Operation at Light Load n Achieves Highest Efficiency n 1.5% Feedback Voltage Reference n 10mA Opto-Coupler Driver n Output Power Good Indicator n Integrated Soft-Start Function
APPLICATIONS
n Offline and HV Car Battery Isolated Power Supplies n 48V Isolated Power Supplies n Industrial, Automotive and Military Systems
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The LT®8311 is used on the secondary side of a forward converter to provide synchronous MOSFET control and output voltage feedback through an opto-coupler. The LT8311’s unique preactive mode allows control of the secondary-side MOSFETs without requiring a traditional pulse transformer for primary- to secondary-side communication. In preactive mode, the output inductor current operates in discontinuous conduction mode (DCM) at light load. If forced continuous mode (FCM) operation is desired at light load, the LT8311 can, alternatively, be used in SYNC mode, where a pulse transformer is required to send synchronous control signals from the primary-side IC to the LT8311. The LT8311 offers a full featured opto-coupler controller, incorporating a 1.5% reference, a transconductance error amplifier and a 10mA opto-driver. Power good monitoring and output soft-start/overshoot control are also included. The LT8311 is available in a 16-lead FE package with pins removed for high voltage spacing requirements.
TYPICAL APPLICATION
18V to 72V, 12V/8A Active Clamp Isolated Forward Converter
VIN 18V to
72V
4.7µF × 3 100k
UVLO_VSEC
5.9k
VIN 100nF
2.2µF 100V
4:4
••
100nF
6.8µH 2k
FSW
OVLO
AOUT
1.82k
LT3753
10k
2k 1.78k
FG
CSW LT8311 CSP
FB
71.5k 49.9k
240kHz
IVSEC RT
31.6k
SYNC SOUT
TAO
TAS TOS TBLNK GND SS1
OUT ISENSEP
OC ISENSEN
INTVCC COMP SS2 FB
34k 0.47µF 1µF
2k
100k 10pF
100k
6mΩ
4.7µF 100Ω 1µF
1.78k 1.5k
15nF 2.94k
CG VIN
CSN PGOOD
OPTO COMP
INTVCC
SYNC
PMODE
SS TIMER GND
124k 1µF
1k 2.2nF
For more information www.linear.com/LT8311
20k
2.2nF 100k
68pF 22µF
×2
VOUT
+ 12V
470µF
11.3k
100k
VOUT
4.7µF
2.2µF
8311 TA01
8311f
1
LT8311 TABLE OF CONTENTS
Features............................................................................................................................. 1 Applications........................................................................................................................ 1 Typical Application ................................................................................................................ 1 Description......................................................................................................................... 1 Absolute Maximum Ratings...................................................................................................... 3 Order Information.................................................................................................................. 3 Pin Configuration.................................................................................................................. 3 Electrical Characteristics......................................................................................................... 4 Typical Performance Characteristics........................................................................................... 7 Pin Functions......................................................................................................................11 Block Diagram.....................................................................................................................12 Operation.......................................................................................................................... 13
FUNDAMENTALS OF FORWARD CONVERTER OPERATION IN CCM...................................................................... 13 LT8311 SYNCHRONOUS CONTROL SCHEMES...................................................................................................... 17 PREACTIVE MODE SYNCHRONOUS CONTROL..................................................................................................... 17 SYNC MODE SYNCHRONOUS CONTROL.............................................................................................................. 19 OPTO-COUPLER CONTROL................................................................................................................................... 21 Applications Information........................................................................................................25 VIN BIAS SUPPLY..............................................................