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TPS546C20A: Low value of R1 feedback resistor causes instability at Vout

Peter Woo
Prodigy 20 points
Part Number: TPS546C20A

Hi,

I am having trouble of using a low value of feedback resistors to set Vout (See picture below).

- I want the output to be around 0.82V, but when I used 665Ohm for R1655 and 2.32kOhm for R1674, the Vout doesn't stay at 0.8V at all and Vout go down almost zero.

- When I replaced R1655 with 10.2kOhm and R1674 with 53.6kOhm, the Vout became stable with 37A of load.

Note that after R1674, there is 5Kohm (default value of Digipot). Also I opted to use digipot over PMBUS method (datasheet mentioned) for controlling voltage.

My question is, why does low value of feedback resistor causes instable Vout? I tried to find the recommended range of feedback resistor values from datasheet, but I couldn't find the info. What is the recommended range of feedback resistors? It looks like it is related to poles/zeros of feedback loop, but more explanation would be appreciated.

Thanks,

Peter

  • 0
    TI__Expert 5910 points

    Peter,

    R1655 is directly part of the feedback loop and the stability will get affected. Please check the phase and gain margin with the low value you used and it could be seen that the phase margin is very low, Also check the switching waveforms.

    Regards,

    Mathew

  • 0 in reply to Mathew Jacob1
    Prodigy 20 points

    Hi Mathew,

    Thanks for your reply. I guessed it is the part of feedback room that is affecting here. But do you have the range of recommendation for the values of this feedback resistors for customer convenience? I saw some other TI power supplies had that info, but not this particular part.

    If there is no such info, I probably need to setup the sim, and do the simulation, but I was hoping to get quick answer from TI.

    Thanks,

    Peter

  • 0 in reply to Peter Woo
    TI__Expert 5910 points
    Peter,

    The range is determined by 2 factors. Highest value of resistor should be such that it can support at least 20 times the feedback pin bias current which is +/-75nA in this case. Lets say 400k in this case. Lowest value would be set by loop compensation. Rcomp/RFB(top) sets the mid band gain so too low a value in relation to Rcomp would give very high gain and high cross over frequency and lower phase margin. I would recommend setting the cross over frequency to 1/10th the switching frequency.

    Regards,
    Mathew