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TPS650860EVM-116: Start-up BUCK3 over-current on TPS650860

David Ambry
Prodigy 125 points
Part Number: TPS650860EVM-116

We bring up BUCK3 @ 3V3 second last (BUCK1 @ 1V0 is last) and occasionally this rail fails to start up. What happens is that the 5V Point-of-Load supply that feeds BUCK3 drops out due to over-current.

The first thing we suspected was too much distributed capacitance on the 3V3 rail – in a detailed audit of local decoupling and reservoir capacitors we found in excess of 400uF. Removing most of this did not produce any change in behaviour.

On a hunch we decided to disable the BUCK3 discharge resistance and this caused the issue to vanish. The question is why? The default value is BUCK3_DISCHG[1:0] = 0b01 for a 100Ω load. This should equate to only 33mA maximum! The 5V PoL is designed for 5A – 25W and we should have ample headroom given the total dissipation of the unit is under half that figure.

Wonder if anyone could shed any more light on this.

Thanks,

Dave.

  • 0
    TI__Expert 4155 points
    Dave,

    I am glad you were able to fix the issue. It is strange that disabling the discharge resistor fixed it since it shouldn't be on when the BUCK is enabled.

    If you want to share any more information such as a register dump from the PMIC, schematic, layout, or scope shots of the issue I would be happy to take a look at them.

    If you try enabling the discharge resistor after the rails is already up does it work as expected?

    Thanks,
    Nick
  • 0 in reply to Nick Jakse49
    Prodigy 125 points
    Hi Nick,

    there is some bleed-thru from other rail(s) that are already up to the 3V3 rail - via parasitic / protection diodes and I'm wondering if this is upsetting things. The worst case was the 1V8 rail (- diode drop) from the centre-taps of the Ethernet switch transformer back through the switch silicon to the 3V3 rail. We used one of the spare switches in the TPS to bring up 1V8 to the CTs after 3V3 has been brought up, and this has functioned perfectly, but there is still a residual voltage - around 1V - from one of the other rails already powered-up.

    I've got a request from Russell Wetherley for more info and I'll organise that on (my) Monday. If you can get in touch with him or fling me your email I'll batch out the data then.

    Good idea on enabling the 3V3 discharge resistor after the rail is up - will give that a go Monday.

    Cheers,
    Dave.
  • 0 in reply to David Ambry
    TI__Expert 4155 points
    Dave,

    Since we have taken this discussion offline I am going to go ahead and close this thread.

    Thanks,
    Nick