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TPS54231: non-linear output step response

Mark Brinkman
Intellectual 260 points
Part Number: TPS54231

The output of my 12V-to-5V buck switcher is shown below. Light blue is the 12V power supply. Pink is the TPS54231 EN pin. Yellow is the 5V output.

Below is the schematic. There is no load on the output other than the two 22uF X7R capacitors. +5V_REF is from a low current LDO whose input is the 12V power supply as shown above.

Why isn't the 5V waveform a nice 8-10ms linear ramp?

thanks

  • 0
    TI__Guru**** 191445 points
    Typically you would see that due to pre-bias on the output. You claim there is no load attached, so there is no additional circuitry attached at all? If there is no possibility of a leakage oath from some other rail, you can look at the SS pin. IF it is not completely discharged to GND when your EN pin is asserted, your output voltage could ramp quickly up.
  • 0 in reply to JohnTucker
    Intellectual 260 points

    Thanks John, good ideas! I too thought something was getting pre-biased.

    No I've hacked up the board so that there is no VOUT (+5V) load except the Cout.

    I probed the SS pin while also probing VIN and VOUT and all signals look great. The SS pin goes from 0V to 0.8V (9ms) to 2V. VOUT has nice 9ms ramp to 5V. I repeated the numerous times and everything looks great. However as soon as I take probe (11.5pf/1Meg) off of SS the stepped VOUT returns. I verified the Css capacitor was connected between SS and GND and that it was .022uf. Strange. Any ideas?

    Mark

  • 0 in reply to Mark Brinkman
    TI__Guru**** 191445 points
    It sounds like the SS capacitor is not discharging completely. Looking at the TI EVM, there is a placeholder for a bleed resistor, R7. You might try placing 1 Meg resistor in parallel with the SS capacitor.
  • 0 in reply to JohnTucker
    Intellectual 260 points

    That bleed resistor did the trick.

    Thanks!!