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LM3478: PWM frequency reduce to 700Hz when power on

chentsu chung
Mastermind 6180 points
Part Number: LM3478


Hi Sir,

When LM3478 power up, the PWM frequency reduce to around 700Hz, I suspect it result from OVP is triggered as the description in datasheet page12, am I right?

But accroding to datasheet,it should not happen if Vin is not crossing the 7.2V threshold anymore. The Vin(waveform V12) in my case is keep at 12V after start up.

Please help on this, thank you.

  • 0
    TI__Guru* 78645 points
    Is V12VO the output voltage? If does not look like you hit OVP, but more like the loop in action to stop switching in pulse skipping in order to regulate the output. Adding some load will move out of such condition and the circuit can return to normal switching.

    The datasheet section that you referred to on page 12 tells about a possible "false" OVP caused by crossing 7.2V by Vin. Since you Vin=12V, that false OVP will not happen, but the real OVP can stop switching. But the waveform that you show does not look like it.

    Thanks