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TPS40170: Power Good Signal Issue of TPS40170

Voyager Liao
Prodigy 170 points
Part Number: TPS40170
Other Parts Discussed in Thread: LM5145

HI TI Sirs

I have an issue on TPS40170 about the Power Good singnal.

Our products use TPS40170 whcih converting  24V into 5V for system,

And further more, we use the power good signal to enable the next stage power ( 5V to 3.3V)

Recently, we found quite a few boards'  Power Good Singal will drop to low unexpectedly!

And it seems related to the temperature. if we raise the case temperature of TPS40170 over about 60C, the PG signal will drop to low. 

However, the 5V output is still there without problem even if the PG signal drop to low. (??)

Do you have any idea about this issue.

Below is our schematic of TPS4017 , thank you!

  • 0
    TI__Guru 73752 points

    Hello,

    PGOOD is effectively a window comparator on the FB voltage plus a check on fault conditions such as undercurrent or undervoltage.

    Please check stability of the design using the compensation calculator in the TPS40170 product folder. You can also use the LM5145 quickstart calculator as it has exactly the same control architecture. Just ensure that the ceramic output capacitance (47uF/16V) is appropriately derated for applied voltage of 5V. Also verify that current limit is not engaging by decreasing the load current.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 170 points

    Hi Tim

    Thanks for your reply.

    We had check the design parameters, it seems no big problems there.

    And Below is the measurement waveform

    1)When we heating the Chip, PG and VSS go to low.

    2)5V Out do not goes to low

    Is there any possibility that the "Gate Drivers" block ok, but "Power Good Controller" block NG?

    Below picture shows that, the VSS goes to low when we heating TPS40170

    Once we remove the heat, then the VSS goes high

  • 0 in reply to Voyager Liao
    Prodigy 170 points

    Hi Tim

    One more thing.

    We have seen the power good is recommend pull high to VDD through 10K ohm in the datasheet.

    However, we pull up the power good signal to Vout.

    But I think it should not be an issue.

    What do you think?

    Thank you very much.

  • 0 in reply to Voyager Liao
    TI__Guru 73752 points

    Voyager,

    Check that the SS cap is not leaking current to GND - this could cause the SS voltage to decrease. It seems PGOOD goes low after a slight decrease in the SS voltage after startup - please verify there is no fault condition present.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 170 points

    Hi Tim

    Thanks for your advisement!

    Below are my experiments...

    I take off the SS cap (C206) on NG boards , let the SS pin floating.  And then the PG signal won’t drop low when I heating the chip! 
        1) Question1 : Does it mean that the  SS cap have current leaking issue?

         2) Question2: Are there any side effects that we just remove the SS cap as our short term solution?

    Thank you very much!

  • +1 in reply to Voyager Liao
    TI__Guru 73752 points

    Voyager,

    I recommend replacing the SS cap to see if it functions correctly. I suspect the old cap had a leakage issue at hot, maybe due to cracking. There will be no soft start if the SS cap is removed, so you may hit current limit at startup.

    Regards,

    Tim