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LM5175: Loop gain and phase problems - poor stability

Carlos Acosta1
Prodigy 70 points
Part Number: LM5175

I have designed a test circuit for the LM5175 for a VIN 10-35 / VOUT 16.5V-10A application. I referred to the eval design, Webbench, and of course the datasheet to get started. The circuit runs, but is unstable. I can operate it at about 5A steady state but it loses control as I go higher current. Changes to VIN can also disturb the stability. 

I examined the loop gain/phase and see that the gain passes 0db at about 700hz and the phase margin is very low - around 15-20 degrees. I started with the datasheet calculated slope and comp values and adjusted/measured to see if I could experimentally push it in a better direction. 

I measured with VIN=14v and VIN=22v with Iout=1 to 3A. 

Schematically, it is nearly identical to the EVM board and the layout is also very similar. I have reviewed the schematic and layout for a couple of days looking for simple mistakes, but have not yet found anything that seems like it would be a problem for loop gain/phase/stability. Since I have not seen a considerable change after adjusting SLOPE and COMP - I am guessing I may have some other contributing factor. 


Not sure where to look next. 

  • 0
    TI__Mastermind 21720 points

    Hi Carlos,

    Please place your design parameters in the LM5175 quick-start calculator. This will help to check the cross over frequency and phase margin.

    If you can post the schematic that will allow me to give more suggestions.

    -Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 70 points

    That calculator is very useful! Thank you. 

    I made some adjustments based on the calculator and it appeared to going in a good direction. It became unstable at 8A loads - and then the circuit stopped working at all. It appears to be the controller itself, but I do not have any more to try. The Vcc pin is no longer developing 7.5v. Up until it stopped working, the crossover frequency was getting closer to the expected value, the phase margin was still not quite enough - did not match the calculated Bode plot. 

    I ordered more LM5175's from DigiKey, but obviously, have to wait a few days to try again.