Page
Application Note
4
2011-07-06
Table of Content
1
Abstract ............................................................................................. 5
2
Design Procedure ............................................................................ 5
2.1
Target Specifications .................................................................................................................... 5
2.2
Design of Power Stage ................................................................................................................. 6
2.2.1
System specifications ...................................................................................................................... 6
2.2.2
Selection of resonant factor m ........................................................................................................ 6
2.2.3
Voltage gain .................................................................................................................................... 7
2.2.4
Transformer turns ratio .................................................................................................................... 7
2.2.5
Effective load resistance ................................................................................................................. 7
2.2.6
Resonant network ........................................................................................................................... 7
2.2.7
Transformer design ......................................................................................................................... 9
2.2.8
SR MOSFET ................................................................................................................................. 10
2.3
Design of Control Parameters and Protections ....................................................................... 10
2.3.1
Frequency setting: ......................................................................................................................... 10
2.3.2
Minimum/Maximum frequency setting: ......................................................................................... 10
2.3.3
Frequency setting for OCP: ........................................................................................................... 11
2.3.4
Dead time ...................................................................................................................................... 12
2.3.5
Softstart time, OLP blanking time and auto-restart time ............................................................... 13
2.3.6
Load pin setting ............................................................................................................................. 13
2.3.7
Current sense ................................................................................................................................ 13
2.3.8
VINS pin setting ............................................................................................................................ 15
2.3.9
Latch off function and burst mode selection ................................................................................. 15
2.4
Design of Synchronous Rectification (SR) control ................................................................. 16
2.4.1
On-time control - SRD pin and CL pin .......................................................................................... 18
2.4.2
Turn-on delay
delay
on
T
_
- Vres pin ................................................................................................. 20
2.4.3
Advanced Turn off delay
delay
off
T
_
- Delay pin .............................................................................. 21
2.4.4
A review of the control scheme ..................................................................................................... 21
2.4.5
SR Protections .............................................................................................................................. 22
2.5
Design summary ......................................................................................................................... 22
3
Tips on PCB layout ........................................................................ 24
3.1
Star connection for Power stage ............................................................................................... 24
3.2
Star connection for IC ................................................................................................................. 25