LMR33630: Shortcircuit between VIN- and SW- pin

Florian Hieber

Part Number: LMR33630

Tool/software:

Hi everyone,

I am facing an issue with a LMR33630ADDAR (HSOIC) buck converter (24 V input to 5 V output): the IC is instantly destroyed (short circuit between SW and VIN) as soon as I apply the input voltage.
Interestingly, the same schematic works fine in another project/layout and is also similar to the reference design.

What I have checked so far:

Schematic and BOM carefully reviewed, multiple times
FB and EN pins correctly connected (confirmed both in schematic and layout)
PGND and thermal pad properly connected to ground
Input protected with a 33 V Zener diode (to clamp voltage spikes)
No output short circuit, no visible assembly issues

Question:

Is there any mistake I have overseen?

Attachments:
This is the schematics of the non working pcb

and this is the layout:

If you need more informations, feel free to ask!

Any feedback or experiences would be highly appreciated!

Thanks a lot,
Florian

2 months ago

0 Naresh Adepu 2 months ago

TI__Expert 5755 points

Thanks for your question Florian,

One of our engineers will get back to you

Thanks and Regards

Naresh

0 Naresh Adepu 2 months ago in reply to Naresh Adepu

TI__Expert 5755 points

Hi Florian,

Could you please confirm that if you remove the IC, you directly see short between SW and VIN?

at the top level, I don't see any mistakes in the schematic or layout. Can you confirm if there is any board level short?

Thanks and regards

naresh

0 Florian Hieber 2 months ago in reply to Naresh Adepu

Prodigy 10 points

Hi Naresh,

so we have 5 populated boards, tested 3 of them, all burned through on first power up. The other 2 new boards don't have a short between SW and VIN, only the ones, that were powered have a short between SW and VIN. I've desoldered the LMR33630 on one of the broken boards and I can't measure a short between the SW and VIN pad. So clearly the IC must be shorting.

Thanks!

Best regards,

Florian

0 Naresh Adepu 2 months ago in reply to Florian Hieber

TI__Expert 5755 points

Thanks for your question, Florian. I will get back to you

0 Florian Hieber 1 month ago in reply to Naresh Adepu

Prodigy 10 points

Hi Naresh,

do you have any updates on my problem?

Thanks!

Best regards,
Florian Hieber

0 Naresh Adepu 1 month ago in reply to Florian Hieber

TI__Expert 5755 points

Hello Florian,

I will get back to you before end of day today.

Thanks and Regards,

Naresh

0 Naresh Adepu 1 month ago in reply to Naresh Adepu

TI__Expert 5755 points

Hello Florian,

When you do this test, have you plotted how the VIN waveform looks closer to device at C11 and C15 cap? can you probe the waveform in high bandwidth mode?

We want to check how much VIN is spiking to (If it is exceeding the max limits of the device, it can damage the device).

Thanks and Regards,

Naresh

0 Florian Hieber 1 month ago in reply to Naresh Adepu

Prodigy 10 points

Hey Naresh,

we've just noticed, that the Termal Pad was covered in soldermask. This might be the root of our problems.
We will try to fix this issue and test again. So please don't mark this thread as resolved yet.

I will get back to you with our results.

Thank you so much for your help.

You're a legend!

0 Naresh Adepu 1 month ago in reply to Florian Hieber

TI__Expert 5755 points

Thanks Florian, please let us know when you have the results.

Best Regards,

Naresh

+1 Florian Hieber 1 month ago in reply to Naresh Adepu

Prodigy 10 points

Hi Naresh,

good news:
The missing pad for the AGND was indeed the problem.
We now just have to replace the faulty footprint.

Thank you so much for your time and help.

Have a great day!

0 Naresh Adepu 1 month ago in reply to Florian Hieber

TI__Expert 5755 points

Good to know that Florian. Thank you.

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LMR33630: Shortcircuit between VIN- and SW- pin