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[FAQ] Ripple lm3478 no load.

Ewgeny Kolomyicev
Prodigy 50 points
Other Parts Discussed in Thread: LM3478

Hi!

I made a converter 21-30V -> 48V 1A. The chip LM3478. There was a problem. No load ripple 1,5V 4Hz. Under load 50mA or more do not. The scheme was developed in 

WEBENCH. All denominations except the dividers are met. In what could be wrong?

  • 0
    TI__Intellectual 2930 points

    Hi Ewgeny,


    Have you looked at load currents between 0 and 50mA? Whats the value of your inductor?

    It looks like you're hitting Over Voltage Protection at no load.
    The LM3478 has a minimum on time of 325ns, so at no load this means the the device is still dumping a small current onto the output cap, which will cause the voltage to increase. It will continue to increase until it hits OVP at which point the part will stop switching until the output voltage drops a little below regulation. OVP is reached when the FB pin is 50mV over regulation, which transfers to around the voltage swing you are seeing at the output.
    You may need to put a small dummy load at your output to consume this minimum current if your application cannot handle these No-load voltage swings.

    Best regards,
    Tommy