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LM25085 switching frequency

Ran Aloni
Expert 2365 points
Other Parts Discussed in Thread: LM25085

Dear all,

we are implementing an LM25085 DC/DC system.

powered by a battery (12V-16.8V mostly 14.8V) and output is 11.8V7A.

when testing the frequency we see 100 Khz instead of the designed (confirmed by Webench) 300 Khz.

we measure the right resistance on between Vin an RT but not the right frequency.

what should we look into?


many thanks,

  • 0
    TI__Guru**** 191445 points
    LM25085 is a constant on time controller. The RT resistor sets the on time and the frequency will be what it is. Check that the on time matches the value you have programmed. Also make sure you are operating in CCM. The on time and Fsw equations are only valid for CCM.
  • 0 in reply to JohnTucker
    Expert 2365 points
    Dear John,

    my RT resistor value was offered from webench and seem to match the equations in the datasheet.
    if by CCM you mean that I have a load connected then yes.
    I noticed the difference when the DC/DC voltage is floating.
    other than the Fsw issue I get the designed voltage and current, and ripple is acceptable.

    can I assume that something in my circuit is effecting the voltage "seen" by the RT pin to reduce the Fsw?
    or is there anything else effecting the frequency? should I check something in my layout?

    regards,
  • 0 in reply to Ran Aloni
    TI__Genius 11490 points
    I think we need to look at the schematic, the test conditions, and possibly SW node and inductor current waveforms.
    regards,Vijay