LMZ31710: PWRGD output is abnormal.

Yonghui zhu

Part Number: LMZ31710

Hi Expert,

We designed LMZ31710 on our board. We set the output 5V, the input is 12V. Overall, it's working well. But for some boards in this batch, the PWRGD is only 1.46V in case 5V output is normal. The PWRGD is pulled up to 5V with a 10K resistor externally and without any other connection. Since PWRGD is OD output, the output of PWRGD is supposed to 0-5V swing. We have no idea why the PWRGD is only about 1.46V. We never met such issue before.

Thanks

Peter

over 5 years ago

0 Anthony F over 5 years ago

TI__Mastermind 35185 points

Hi Peter,

Could you share the schematic and some oscilloscope waveforms showing the behavior of the device when this happens? At a starting point it would be good to see the output voltage and the PWRGD pin voltage.

Is the PWRGD voltage being measured with a DMM? If it is, the 1.46V measurement could just be an average voltage and PWRGD voltage might be toggling between low and high.

Anthony

0 Yonghui zhu over 5 years ago in reply to Anthony F

Intellectual 430 points

Hi Anthony:

The following is schematic. For troubleshooting, I disconnected all connections except pullup resistor. Yes, 1.46V is measured with DMM. You remind me of using a scope. 1.46V should be a average voltage. I will capture waveform next Monday.

Thank you.

Peter

0 Yonghui zhu over 5 years ago in reply to Yonghui zhu

Intellectual 430 points

Hi Anthony,

We captured the waveforms. 1. bad board PWRGD output, 2, good board PWRGD output, 3. bad board 5V output. The 5V output seems not to be so bad, I don't know why PWRGD is toggling?

Thanks

Peter

0 Gerold Dhanabalan - Online Design Tools over 5 years ago in reply to Yonghui zhu

TI__Genius 14455 points

Can you please check if the PWRGD toggling rate is aligned with the switching frequency - just checking if there is some coupling happening on the bad board? Also, PWRGD could toggle because of the below reasons as per the datasheet.

"Also, the PWRGD pin is pulled low if the input UVLO or thermal shutdown is asserted, the INH pin is pulled low, or the SS/TR pin is below 1.4 V"

can you please probe the SENSE+, INH and SS/TR pins?

Regards,

Gerold

0 Anthony F over 5 years ago in reply to Gerold Dhanabalan - Online Design Tools

TI__Mastermind 35185 points

Hi Peter,

Similar to Gerold suggested, can you please also measure a waveform of INH, PWRGD and SS/TR? Since the output appears to be regulated, it suggests maybe the part is getting restarted. If the SS/TR pin is also toggling, this suggests the LMZ31710 is restarting.

In this measurement, please use a timescale which only shows a single or a few cycles of the PWRGD pin toggling.

Anthony

0 Yonghui zhu over 5 years ago in reply to Anthony F

Intellectual 430 points

Hi Gerold, Anthony,

I measured these signals, INH is about 2.8V, SS/TR is about 1.8V, they are all stable. The PWRGD is as following, it's toggling with switching frequency(63.4KOHM).

Thanks

Peter

0 Gerold Dhanabalan - Online Design Tools over 5 years ago in reply to Yonghui zhu

TI__Genius 14455 points

Can you please check if there is any coupling between the SW node and Power good pin in the layout?

Regards,

Gerold

0 Anthony F over 5 years ago in reply to Gerold Dhanabalan - Online Design Tools

TI__Mastermind 35185 points

Hi Peter,

I suspect the PWRGD and SYNC_OUT pins might be shorted together.

Anthony

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LMZ31710: PWRGD output is abnormal.