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TPS51916: PGOOD indicator pulsing high then low

Pierre Lieutaud
Prodigy 230 points
Part Number: TPS51916

Hi,

I'm using TP51916 on a new design with DDR3.

At startup, PGOOD is pulsing high, 1.2ms, then goes low as captured below.

As PGOOD is controling the board reset, the complete board is held in reset.

The output voltage is measured to be 1,345V. DDR3L requires 1,35V. So it's fine for my application.

The reference voltage is 1,348V. This is less than 1% error.

The datasheet says that PGOOD unvalidates when output voltage deviates more than +/-16% of the ref voltage. That is not my case.

The part is configured in mode 3: DCAP2 / 500kHz.

I have also made a screenshoot of the pulse on the mode PIN at startup.

The voltage is 490 mV. According to the table p7, this is inline with mode 3.

Below is a screen capture of the output noise, when the device is loaded at 3 Amps.

I was expecting to see 500kHz pulses. That is not obvious on the screen shot.

Lastely, I'm sharing the schematics.

I have simulated the design with Webench.

Everything is fine there. Unfortunately, I cannot probe the PGOOD indicator in the simulator.

When I manually disable the device (forcing S3 and S5 low) then re-enabling the device,  PGOOD behaves the same.

I have asked my local TI support for a TPS51916 demo board to perform comparison.

Any ideas about the reason that makes PGOOD going low?

Regards,

Pierre

  • 0
    Prodigy 230 points

    Hi,

    One more question:

    What is the expected behaviour of PGOOD when the device is disabled (S3/S5 low) ?

    Regards,

    Pierre

  • 0 in reply to Pierre Lieutaud
    TI__Expert 5965 points

    Hi Pierre,

    Can you also provide the PGOOD connection as well? Do you have PGOOD from other device? To debug this issue, we recommend to disconnect all other PGOOD connection, and just check TPS51916 PGOOD during start up on your board to see if it still has the same issue. From the information you provided, PGOOD should not go low.

    If the device is disabled, PGOOD will be low.

    Regards,

    Weidong

  • 0 in reply to Weidong Zhu
    Prodigy 230 points

    Hi Weidong,

    You are right.

    Thanks to the demo board I received, I spent some time investiguating/comparing and came to the conclusion that another device might force the PGOOD to low.

    I finally found the root cause and fixed it.

    TPS51916 is doing fine in my design.

    Thanks for your support.

    Regards,

    Pierre