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LMR33620: LM33620 AGND and PGND question

Part Number: LMR33620

Hi Team,

My customer using this part in his project and he found some issue need your clarify.

He mentioned that LM33620 block diagram show the AGEN and PGND is the some GND inside.

But they test PGND to AGND resistor value have M ohm. It seems not be the some like the datasheet said.

Because they find this issue from his soldering by hand the LMR33620 cannot startup , but he using the hot air gun to soldering again the LMR33620 will do.

The issue seems come from the AGND not really connect enough to happen.

Does the AGND not just the  stronger thermal capability pad for LMR33620 ? Or it has the other function in here so must to connect.

Eddie 

  • Eddie,

    The AGND and PGND pins must be connected close together on the PCB layout. AGND does not have as strong of a thermal capability to spread heat from the IC to the board compared to PGND but it still can spread some heat.

    -Sam

  • Hi Sam,

    Thanks your answer.

    But I still have some confuse, why the block diagram show the AGND and PGND seems connect internally in the chip(datasheet page10.).

    And you mentioned the layout still need to connect together. Could you help to comment this point ?

    If these GND connect internally why the layout still need to connect each other again ?

    Eddie

  • Eddie,

    Separating the GND planes on the PCB may result in notable voltage difference between AGND and PGND from the power stage current. The small internal connection would then need to pass current to try to balance these. The resistance of that connection may not be low enough to keep these voltages equal and we don't recommend passing lots of AC current through this internal connection.

    -Sam

  • Hi Sam,

    My customer question is the block diagram show the AGND and PGND is connect internally in IC , but he test the PGND to AGND resistor value is M ohm. The test value is not match with our block diagram.

    So the PGND and AGND is connected  IC inside ? or not be connect ? 

    if you have other detail picture can explain it that is good to let my customer to know  clearly.

    Eddie

  • Eddie,

    I will double check on this and get back to you today or tomorrow.

    -Sam

  • Eddie,

    AGND and PGND should be connected. Did you test this on a known good unit?

    -Sam

  • Hi Sam,

    This test result is come from my customer engineer.

    I don't know that they are checking good unit resistor value or not.

    But they said that this part just soldering by hand cannot work cause the PGND is not soldering enough on his pad. Need to using the hot air gun to soldering it again the PGND match on his pad then LMR33620 can work in safe.

    And between the soldering he test the PGND to AGND resistor value that they find this point. So I think this LMR33620 still is good unit because by his second time hot air gun soldering the LMR33620 is good to work.

    So please help to check this by your side and I will also check with customer both good unit and be soldering on board GND resistor value is the same or not. Thanks !

    Eddie

  • Eddie,

    I will check this when I am in the lab later this week. Please keep me posted on the customer's status.

    Thanks,

    -Sam

  • Eddie,

    I have checked in the lab on a known good unit and confirmed what you've seen. I have also aligned with our design team who confirmed. AGND and PGND should not be shorted on the block diagram in the datasheet. We will fix this on the next datasheet revision. Thank you for pointing this out.

    -Sam

  • Hi Sam,

    Thanks your clarify , if you update the datasheet's data please let me know.

    I will sent to customer the latest one again.

    Eddie