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WEBENCH® Tools/LM3478: Some issue about LM3478

user5005024
Intellectual 345 points
Part Number: LM3478
Other Parts Discussed in Thread: LM3481

Tool/software: WEBENCH® Design Tools

Hi All,

There are some issue about the LM3478 and need your help:

1)The circuit topology is flyback,we want to use the isolating method in the main feedback circuit,if this method is feasible or not?

(The FB connect to GND,the main feedback control the voltage of COMP on the optocoupler)

2)If the item # 1 is feasible,how can we calculate the max external pull-up resistance of COMP?

Please help to check the above issue and give your comment,thanks.



  • 0
    TI__Guru* 78190 points
    Its possible but the LM3481 may be a better alternative for flyback. Pull up COMP requires a resistor and a regulated votlage like VCC in the LM3481, or you need to create one with additional circuit for LM3478. The pull up resistor should be selected such that can provide adequate current to overcom the COMP sinking current such that the COMP pin votlage can be moved up freely for output regulation. With the LM3481 we usually recommend a ~5k resistor when pullup is implemented with VCC.