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TPS54050A - instability

Ondrej Palacky
Prodigy 70 points
Other Parts Discussed in Thread: TPS54040A

Hi guys,

I have stability issues with the TPS54040A in two power supplies (see attachment). I designed them using the SwitcherPro Desktop  software with the following inputs:

---------------------------------------

Supply 1: Vout=8.8V, Vin = 10V-40V, Iout=10mA-50mA, Fsw=300kHz, minimum gain margin=20dB, minimum phase margin=90d, output ceramic cap

The really used components: R9= 680k//1000k,

-------------------------------------------

Supply 2: Vout=18.4V, Vin = 20V-40V, Iout=10mA-150mA, Fsw=300kHz, minimum gain margin=20dB, minimum phase margin=90d, output tantalum cap

The really used components: R9= 680k//1000k, C2= 3x100uF tantalum + 10uF ceramic

-------------------------------------------

I have seen instability issues:
for supply 1 at 10mA load for Vin = cca 20V-30V, slightly increased output ripple (50mVpp-100mVpp)
for supply 2 at different loads for Vin = cca 20V-30V, significantly increased output ripple (200mVpp)


My question is: Are the calculated component values OK? I am worried mainly by supply 2, which is very unstable even when I forced  SwitcherPro Desktop to design for maximum gain and phase margins. Could the output caps (three parallel tantalum 100uF + parallel ceramic 10uF) be a problem? Any tips you can give me for my particular cases?

Thanks! BR

Ondrej

pack_supplies.zip
  • 0
    TI__Guru**** 191445 points

    First can you post some waveforms that show your issues?

    As to switcherpro, when you specify very low output currents, the software will pick a huge inductor value.  Typically, I over design in that case to force the inductor current lower, but it depends on whether or not you wnat to operate in DCM or CCM at your intended load.  Also, the output capacitor seems low.  I typically use 22uF minimum.  I can tell you more when I see the waveforms.

  • 0 in reply to JohnTucker
    Prodigy 70 points

    Thanks, John. I attach the output ripple and switch point waveforms for both supplies (18V4_detail1.tif comes from the central part of 18V4.tif).


    Are you saying the 330uH inductor is higher than needed but you would recommend to keep it + increase the output cap for supply 1? And what do you make of supply 2 ? - it is much more unstable (over a wider range of input voltages) with a high capacitace (needed to cover input voltage interruption).

    waveforms.zip
  • 0 in reply to Ondrej Palacky
    TI__Guru**** 191445 points

    You are just operating in DCM and sometimes entering eco-mode.  I don't see anything particularly wrong with those waveforms.