LM5017 over current behavior

Yong Chen72

Other Parts Discussed in Thread: LM5017

For the LM5017, if an over current event (>1.03A) occurs, the cycle is terminated for a fixed off time. However, instead of turning off both upper and lower FETs, it appears that only the upper FET is turned off and the lower FET remained on for the entire off time duration. This is causing circulating current due to lower FET being on for too long (current flowing backwards), and the voltage does not reach intended level and sometimes the converter is overheating and burning up. What I would like to get an answer on is the lower FET staying on is by design and I am not doing anything wrong on my side.

Thanks,

Yong

over 9 years ago

0 Peter Meaney over 9 years ago

TI__Mastermind 31580 points

Hi Yong,

This post belongs in the non-isolated forum. I will move it and one of my colleagues will pick it up there.

Regards

Peter

0 Yong Chen72 over 9 years ago in reply to Peter Meaney

Prodigy 80 points

Bump. Any body can help? Thanks.

0 Vijay86929 over 9 years ago in reply to Yong Chen72

TI__Genius 11490 points

Can you share the schematic and test conditions for the failure?

0 Yong Chen72 over 9 years ago in reply to Vijay86929

Prodigy 80 points

Hi Vijay,

All I need to know is the state of the upper and lower FETs when the internal over current protection is triggered. From what I saw, it appears upper FET is OFF and lower FET is ON, and I would like to know if that's the intended behavior by design. I couldn't find any documentation on this.

Thanks,
Yong

0 Vijay86929 over 9 years ago in reply to Yong Chen72

TI__Genius 11490 points

That is correct. This is the intended behavior by design.

0 Yong Chen72 over 9 years ago in reply to Vijay86929

Prodigy 80 points

Hi Vijay,

Thank you for your clarification. I just would like to comment that this is indeed a bad design because it causes a lot of circulating current (proportional to the off time) on the IC, inductors and caps. To avoid that circulating current, I had to bypass the synchronous feature by adding a series diode to the output. It would make much more sense to have both switches remain OFF when an over current event occurs.

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LM5017 over current behavior