TPS54325 Pre-Biased startup

T Kishi

Genius 4425 points

Other Parts Discussed in Thread: TPS54325

My customer has question at TPS54325_Pre-Biased operation.

Vin=5V

Vout=1.8V

Pre-Biased≒1V

At this time,SW stops at Startup.Please see the attached file.

This operation is starting specification at the time of Pre-Biased?

Regard.

T Kishi

TPS54325_PreBiased.pdf

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

It is hard to tell from that waveform. Is the slow start time completed already? Can you show both a zoomed out and zoomed in version? What is the load current?

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Hi John

Thank you for replay.

Load current =0mA.

The SW stop happen just before and just after the slow start is complete.

And,SW stop does not happen when the output is completely stable.

Attached the zoom version of waveform.

TPS54325_zoom.pdf

Regurds

0 JohnTucker over 9 years ago in reply to T Kishi

TI__Guru**** 191445 points

Actually I was looking for you to zoom in the other direction as well. I need to see that area of interest closer in. I may be able to answer anyway, but it would help.

Thanks.

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Hi
Let me confirm.
”I need to see that area of interest closer in”
⇒Measuring the waveform that looks the frequency of the ringing?

Regard
T Kishi

0 JohnTucker over 9 years ago in reply to T Kishi

TI__Guru**** 191445 points

Yes, zoom in on the area you have circled in the first waveform.

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Since were measured waveform, please check.

TPS54325_ringing.pdf

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Measured waveform of when there is no Pre-Biased.

Please see the attached file.

Is this will normal Operation?

TPS54325_no PreBiased.pdf

0 JohnTucker over 9 years ago in reply to T Kishi

TI__Guru**** 191445 points

It looks like the low side FET is turning off. I do not think I have seen this before and cannot think of a scenario which should cause it. Is this measured on a TI EVM?

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Yes. I measured at TI EVM.
Also, the customer is measured in prototype board.
Device is wrong?

0 JohnTucker over 9 years ago in reply to T Kishi

TI__Guru**** 191445 points

I will look into this further next week.

0 T Kishi over 9 years ago in reply to JohnTucker

Genius 4425 points

Hi john
What about status?
I want you to tell me one point.
we think the application is not the cause.What do you think?

Regard

T Kishi

0 JohnTucker over 9 years ago in reply to T Kishi

TI__Guru**** 191445 points

I could not duplicate this in the lab, but I have found the answer. It is normal operation. During the slow start time, there is a reverse current limit that is set close to zero. In your case, the reverse current limit is reached and the low side switches off. Briefly the high side body diode will conduct as shown by the higher than normal SW pulse. When the inductor current reaches zero, you will see the characteristic ring as shown until the high side switches back on.

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TPS54325 Pre-Biased startup