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TPS56121: Overshoot and undershoot issues - design review

Ariel Berdugo
Prodigy 20 points
Part Number: TPS56121

Hi,  

We are having the high overshoot and undershoot over the 3.3V output voltage.

TPS56121 on our board has the following properties :

·         Vinnom=12V , Vinmin=11V, Vinmax=13V , Vout=3.3V , Iout-peak(max)=12A for 10u seconds,  Iout-operation(max)=10A 

Below the schematics and feedback loop compensations values and bode plots.

Can you please provide design review regards the data below, and advise how the design can be improved in order to resolve the high overshoot and undershoot issues ?

The 3.3V output is connected on our board to 4x loads. Each load has a peak of 3A during 10u seconds on the 3.3V.

Each load is connected, on its input 3.3V pins, to the circuit below (Ferrite bead and the capacitors) as you can see below:

  • 0
    TI__Guru**** 191445 points

    Ariel,

    Can you post waveforms showing the transient response and the current load step?  Also, what is the load step slew rate?

  • 0 in reply to JohnTucker
    Prodigy 20 points

    Hi John,

    The picture below was measured on the load side(on one of the 4 loads).

    You can see +214mV peak overshoot on the 3.3V once we draw 4A to 12A during 1m seconds with a loader device, right after we forced 0A with 1m seconds - where you see the -214mV undershoot.

    The load step slew rate of our load is 1.2A/useconds. but in this picture below the load step slew rate of our loader was a little different(sorry not remembered the exact value).

    We'd like also to get lower overshoot/undershoot on the following case : 0A to 12A during 10useconds and then10A constant (for a long time) with load step slew rate of 1.2A/useconds.

    Thank you
  • 0 in reply to Ariel Berdugo
    TI__Guru**** 191445 points

    Ariel,

    Generally, I would first off change teh switching frequency to the maximum option of 1 MHz and recalculate a smaller inductor value, 470 nH.  Then trry adjusting your crossover frequency as close to 100 kHz as possible.  If that does not yeild enough improvement, then you will likely have to increase the output capacitance.  Let me know if this works.