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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
delayon
T
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- Vres pin ................................................................................................. 20
2.4.3 Advanced Turn off delay
delayoff
T
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- 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
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