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Original question:

LM51551: Flyback schematic check

LM51551: CS waveform question in LM51551

Gary Teng
Genius 5830 points
Part Number: LM51551

Tool/software:

Hi Sir,

 Our customer is testing their board and they can't get enough current at the output load:  There is 56V at output with no load, and the output is drop (protect?)  when the current load is more than 500mA. We see there's peak voltage shown at CS pin. Is that the cause for stopping output volgate? Any suggestion to increase its load to 1A or more current? Thanks for your kindly help.

Best regards,

Gary Teng

CH1:CS  CH2:Gate

  • 0
    TI__Intellectual 2426 points

    Hi Gary,


    The CS pin reaches over current protection -> 100mV and also the signal is noisy. The CS resistor you have is 8.3mohm which means the peak current limit for primary side is 12A. However, I am not sure of where is the high current of 12A coming from as the load current is small. 

    Possible reasons 1. inductor is saturating and if this is the case please check the inductor current or input current if it matches. 
    2. They have a lot of noise at the CS sense pin-  bigger filter or better layout review has to be done.

    Best regards,

    Mounika