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LM25011 from 32V to 3.3V, problem to calculate Vout

Georges Palauqui
Prodigy 20 points
Other Parts Discussed in Thread: LM25011

Hi,

I have a design with Vin = 32V and 3 LM25011 that regulate to 16V, 5V and 3,3V.

Calculating Rfb1 / Rfb2 give me :

- 16V -> 1k69 / 9k09
- 5V -> 4k99 / 4k99
- 3,3V -> 4k99 / 1k58

The 16V and 5V are OK, but 3,3V setup give me 2,58V... I have to change Rfb2 to 15k to have 3,3V approx... so 10 time bigger value than theorical !!!

For my 3,3V setup, I have put RT = 88k7 so FS = 800kHz, and Ton min = 126ns (>90ns) , Toff min = 1120 ns (> 15ns). L = 22uH. Cout = 47uF.

What possibly goes wrong for my 3,3V regulation ? Specially because 5V and 16V are all right, so my calculation should be good...

  • 0
    TI__Intellectual 1435 points

    Hello Georges,

    Have you checked the value and solder connections carefully for Rfb1 in the 3.3V design?   The measured 2.58V is not far from the value you would get with Rfb1 open. 

    The RT and and FS settings should not cause gross errors in Vout.  If the required on-time is too short for high step-down ratio, the controller will should skip cycles to achieve the desired average duty cycle.

  • 0
    TI__Genius 11490 points

    it is also possible that you have 49.9k installed for Rfb1 by some error. That would explain why Rfb2 needs to be 10x bigger as well.

    regards,

  • 0 in reply to Vijay86929
    Prodigy 20 points

    Dear Vijay,

    you were right, on the prototype we made, we soldered 49k9 instead of 4k99, for both 5V and 3,3V, but as for 5V setup both Rfb1 and Rfb2 were 10 time bigger, the result give a good 5V regulation...

    Sorry for my mistake, and thanks you for your support to give us the good hint to find error.