LMZ21700: Timing of Enable and Vin

Andrew Hood

Part Number: LMZ21700

We have a design that is using a LMZ21700 to step down an input voltage.

We have observed that if we apply +5V to EN and then some seconds later apply +5V to Vin, the part fails and is not recoverable.

The circuit only works if +5V is applied to both Vin and EN simultaneously or if +5V is applied EN once +5V is up on Vin.

Is this the expected behavior of this part? I don't see anything mentioned in the datasheet.

over 6 years ago

0 Andrew Hood over 6 years ago

Prodigy 30 points

After submitting my original question I realized it’s all spelled out in the absolute maximum ratings. EN cannot exceed VIN by more than 0.3V. Still, I’m surprised a logic signal coming up before the supply can fry a chip like this.

0 Denislav Petkov over 6 years ago

TI__Mastermind 18940 points

Hi Andrew,

Bringing EN up before VIN can forward bias ESD structure from EN to VIN. This is why EN has to come up after VIN.

Cheers,
Denislav

0 Andrew Hood over 6 years ago in reply to Denislav Petkov

Prodigy 30 points

Right. But I can't tell you how many parts of all sorts that I've applied logic signals to before bringing up the power rails and have not had an issue.

I understand it's documented in the datasheet but this seems like a limitation that isn't present in other parts.

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LMZ21700: Timing of Enable and Vin