LMZ31710: Pull down on INH pin of LMZ31710

Kumara Bhagavath

Part Number: LMZ31710
Other Parts Discussed in Thread: TPS54020

Tool/software:

Hi,

I have used couple of LMZ31710 in my design. Power Good of first device is connected to INH pin of 2^nd device. I have noticed one strange behaviour.

As per the datasheet, there is no pull down on INH pin but my circuit behaves as there is 10K pull down internally on INH pin.

I have connected 100K pull up to VCC_1V8 for the PWRGD pin which is connected to INH of 2^nd device. I am getting 0.164V on PWRGD pin but the regulator output voltage is proper. When I change the pull up to 10K, I am getting 0.9V & when I change it to 1K, I am getting 1.64V. As minimum voltage required on INH pin is 1.3V, I could enable the 2^nd regulator with 1K pull up on PWRGD. This behaviour exactly matches with 10K pull down on either PWRGD pin or INH pin. However, PWRGD pin seems OK as for the 2^nd device with 100K pull up to 3.3V, I am getting 3.3V at PWRGD pin.

Is there really 10K pull down on INH pin, which is not shown in the datasheet?

Best regards,

Kumara Bhagavath

over 1 year ago

0 Kumara Bhagavath over 1 year ago

Prodigy 90 points

LMZ31710RVQ Schematics.pdf

0 Savith Kumar Rayakanti over 1 year ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Kumara

Looking at datasheet section 7.4.1 Programmable Undervoltage Lockout (UVLO), and back calculating we see that there should not be a 10k pull down resistor on INH/UVLO pin

Thank you

Savith

0 Kumara Bhagavath over 1 year ago in reply to Savith Kumar Rayakanti

Prodigy 90 points

Thanks Savith for your response.

Is there any justification for the voltage I am getting at pin 8 of U52? The signal is not connected anywhere outside this page. My circuit response is as below.

R387 Voltage at pin 8

100K 0.164V

10K 0.9V

1K 1.64V

Best regards,

Bhagavath

0 Savith Kumar Rayakanti over 1 year ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Kumara

Do you have the voltages on the other end of the R387 resistor at the resistor pad with different pullup resistors?

so that we know the pull up voltage is same as your regulated output voltage as Pgood is pulled to output voltages on both devices

-Savith

0 Kumara Bhagavath over 1 year ago in reply to Savith Kumar Rayakanti

Prodigy 90 points

Dear Savith,

Other end was always 1.8V. We also tried connecting other end to 5V & the behavior is same. We are getting a resistor divider voltage on that pin as if there is a 10K pull down.

Best regards,

Bhagavath

0 Savith Kumar Rayakanti over 1 year ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Kumara,

can yo send the layout to further assist you, for you reference the module uses TPS54020 IC where the PWRGD pin open drain and EN is INH/UVLO.

Thank you

Savith

0 Kumara Bhagavath over 1 year ago in reply to Savith Kumar Rayakanti

Prodigy 90 points

Dear Savith,

My customer didn't agree to share the brd file. What is the module that you referred above.

best regards,

Bhagavath

0 Savith Kumar Rayakanti over 1 year ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Kumara,

Layout information can help us understand if the layout is good and not an issue, without knowing how the layout is, all we can do is keep guessing what might be issue.

LMZ31710 is a module and it uses TPS54020 IC,

I can think of some solder contamination on board or low impedance on the pins,

1. remove the pull up resistor and see the impedance from PWRGD pin to GND when part is mounted on the board

2. now remove the part from the board and measure the impedance from PWRGD pin to GND on the IC

This may help you if the PWRGD pin is damaged, you can do the same with INH/UVLO pin to make sure that is okay

and later you can swap the module with new LMZ31710 module and see if you are seeing the same issue

Thank you

Savith

0 Kumara Bhagavath over 1 year ago in reply to Savith Kumar Rayakanti

Prodigy 90 points

Dear Savith,

I checked with my customer again but didn't get permission to share layout file. As I am doing these type of designs from 25 years, I don't think any issue in layout. We have followed all the guidelines mentioned in datasheet.

Best regards,

Bhagavath

0 Savith Kumar Rayakanti over 1 year ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Kumara

Did you got a chance to check the impedance as suggested ? can you please share what you found

Thank you

Savith

0 Kumara Bhagavath 11 months ago in reply to Savith Kumar Rayakanti

Prodigy 90 points

Dear Savith,

The issue got resolved. There was a 10K pull down resistor in the board which was supposed to be DNP but that was mounted by mistake. Thanks for your support.

Best regards,

Bhagavath

0 Savith Kumar Rayakanti 11 months ago in reply to Kumara Bhagavath

TI__Expert 5990 points

Hi Bhagavath

Thank you for the update

-Savith

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LMZ31710: Pull down on INH pin of LMZ31710