LMR36520: is damaging, PoE PD with TPS2378 and LMR51450

Kewin Galuszka

Part Number: LMR36520
Other Parts Discussed in Thread: TPS2378, , LMR51450

Tool/software:

Hi. I have a problem with LMR36520 converters and TPS2378 interface IC getting damaged from time to time.

There is a design with PoE PD interface - TPS2378, LMR36520 for 24V and LMR51450 for 5V.

It happens most often during developing, for example when USB (on other board) is being disconnected/connected, or other device is being connected to this board (24V or 5V line). I think that probably the ESD is the reason but only LMR36520 is damaging fully, TPS2378 is damaging partly - PG pin is not working after damaging but interface is working good. and LMR51450 are still working fine. There are TVS on 5V, 24V, input power line (SMAJ58CA)

Problem occurs regardless of the power source. It occurred on:
- 48V power injector (Meanwell power supply + simple Passive PoE switch)
- Ubiquiti 48V PoE injector (U-POE-AF)
- Active PoE switch

Can you help me to debug this circuit? I cannot find the reason of damages. I attached schematic in PDF file and below in jpg and pdf. Thanks in advance.

poeproblem.pdf

11 months ago

0 Richard Fu22 11 months ago

TI__Mastermind 31315 points

Hi Kewin,

Sorry about the troubles!

Can you let me know the mode of failure on the LMR36520? Is there visible pin damage, pins are shorting, etc?

Regards,

Richard

0 Kewin Galuszka 11 months ago in reply to Richard Fu22

Prodigy 10 points

Hello,

pin1<-> pin 2 4ohm
1<->3 5ohm
1<->4 open line
1<->6 3 ohm
1<->7 5.8 ohm
1<->8 2.7 ohm

chip has no holes, looks normally.

0 Richard Fu22 11 months ago in reply to Kewin Galuszka

TI__Mastermind 31315 points

Hi Kewin,

It looks like the VIN pin got damaged. The main cause that could damage the pin is if the VIN pin exceeds the abs max of 70V or below -0.3V. Are you able to replace the damaged unit with a known good unit and then keep a probe on the VIN+ to see if the input voltage spikes during developing?

Thanks,

Richard

0 Kewin Galuszka 11 months ago in reply to Richard Fu22

Prodigy 10 points

Thanks for your answer. I will check this, but i think that will be hard to record.

If an ESD discharge hits ground, the VIN pin will definitely have a lower potential than ground. And this can be reason. But I have 2xSMAJ58CA (1st on input, 2nd on VIN of LMR36520) so is it other better way to protect this chip?

0 Richard Fu22 11 months ago in reply to Kewin Galuszka

TI__Mastermind 31315 points

Hi Kewin,

On the PCB, it could be with having large ground planes to dissipate the charges, which is layout related. But before we dive into too many different causes, let's examine this one first. You may provide me the layout just so I can take a look.

Thanks,

Richard

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LMR36520: is damaging, PoE PD with TPS2378 and LMR51450