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I have been struggling with an issue where the TPS25921x eFuse IC trips into fault mode when powering an LMZ12002 DC-DC converter. Both devices feature soft-start functionality, and I suspect an unintended interaction between them is causing the eFuse output voltage to stall at ~5V, before tripping current limit ~2ms later.

This is implemented on a custom PCB, and I’ve assembled four boards. Interestingly, one works fine, while the other three fail in the same way. Initially, I thought this was a soldering issue, but after assembling two additional boards with only the eFuse and DC-DC converter populated, they also fail in the same way—voltage rises to ~5V on V+ and then the eFuse fault pin asserts low (likely due to overcurrent). The board that works has the eFuse output voltage rise all the way to VRAW as it should. I can capture some oscilloscope traces later on if anyone would like to see that. I find it really odd that the eFuse output voltage stops rising at 5V.

Since one board works, I assume the circuit is close to working, but I've tried:

  1. Varying soft-start capacitor values for both the eFuse and the DC-DC converter.
  2. Removing capacitors on V+ to limit possible inrush current.

Despite these changes, the issue persists. I’m hoping someone has experienced a similar failure mode or has ideas on how to prevent the eFuse from tripping. Any suggestions?

Schematic of power supply with eFuse

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  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$ Commented Jan 31 at 0:43

2 Answers 2

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Sounds legit, switchers have the characteristics of a load with negative resistance.

That can bite you during start up.

Try increasing the startup threshold on LMZ12002 by using a voltage divider on the EN pin

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  • \$\begingroup\$ Ah, that's interesting. I will give it a go and report back, cheers! \$\endgroup\$ Commented Jan 31 at 4:20
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So it turned out to be an assembly problem (doh). I was using hot air reflow to mount the LMZ12002 which must have overheated the part. This resulted in a short to ground on the enable pin. When using a soldering iron the part seems to survive the soldering process.

Embarrassingly did not pick up on it sooner.

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  • \$\begingroup\$ It is an incredible coincidence though that on all of the boards where the 5V5 LMZ12002 was overheated and failed, the other 5V LMZ12002 works just fine. \$\endgroup\$ Commented Jan 31 at 10:23

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