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TPS82085: PG not behaving as expected

user4905413
Prodigy 20 points
Part Number: TPS82085

Hi all,

I have a board with the part number TPS82085 that I use for generating several voltages: 1v, 1v5, 1v8 and 3v3.

As these chips needs to power up in cascade, I have used its EN and PG signals to enable them in order.

I have followed the datasheet recommendations in term of components and layout. The power they need to deliver is over sized to not to have any power issues.

What could be the reason why the Power Good signals are not going into high-impedance mode?

Find attached sch I'm using plus how is routed in PCB:

At top&bottom, GND fill is present

Some oscilloscope signals I have taken:

PG at power up:

Steady State, the power good doens't arise to 1v, this means is drawing from pull-up resistor.

Any mistake you identify in design or causes could lead to this?

Thanks in advance,

imuguruza

  • 0
    TI__Expert 4155 points
    Hi Imuguruza,

    Looking at your schematic there are 2 changes I would suggest.

    First, you are pulling up to the Vout of the buck, however the high-level input voltage of the EN pin can be as high as 1V, meaning even a very small voltage drop across the pull-up resistor can result in the voltage being too low for the EN pin of other buck to register as a high voltage. I would suggest pulling up to the Vin pin instead, as is done on the EVM.

    The second change I would suggest, which I believe is the cause of the lower than expected voltage on your PG pin, is to decrease the size of your pull-up resistor. The typical pull-down resistance of the EN line is 400kohms, so when the PG pin goes high it is forming a resistor divider with the pull down of the EN pin of the other buck. You should choose a pull-up resistor that is small enough so that the high level input can be met.

    This app note can help you identify the possible range of your pull-up: www.ti.com/.../slva485.pdf

    I would try replacing your pull-up with a 10kohm to see if that fixes your issue, and then re-wire the pull-up voltage on your next board spin.

    Please let me know if this helps resolve your issue.

    Thank you,
    Nick
  • 0 in reply to Nick Jakse49
    Prodigy 20 points
    Hi Nick,

    Thanks for your answer. The pull-up value looks to be the problem here, I have placed 10k, and looks to be working fine.
    I'll follow the second suggestion in future.

    Regards,

    imuguruza