TPS54327 SS behavior

Wu JW

Other Parts Discussed in Thread: TPS54227, TPS54527, TPS54427, TPS54327

My customer have two questions for the SS behavior in our EVM. As in following pic, Why the Vout not start up during the T1? Why the SS ramp up so quickly during the T2?

over 14 years ago

0 JohnTucker over 14 years ago

TI__Guru**** 191445 points

I verified that this is the correct behavior. I do not know the mechanism for T1 but I can see if I can find out. T2 begins after 1.7 x ss time. After that time, the SS pin is pulled up to the internal VREG5 voltage.

0 JohnTucker over 14 years ago in reply to JohnTucker

TI__Guru**** 191445 points

From the IC designer:

"I add the one Vbe voltage shift function of the SS pin for the latest CS version.

Because high output current caused rapid SS voltage up due to the parasitic inductance.

So I added the one bipolar transistor in order to protect the noise.

As a result, SS start up is somewhat delayed.

As you may know, T1 time of TPS54227, TPS54527 and TPS54427 is shorter

than TPS54327 case. You can watch the these SS pin waveform.

I think minimum on time at your T1 is caused by the internal ground bounce."

0 Paul Yeh over 10 years ago in reply to JohnTucker

TI__Genius 16125 points

Hi John,
My customer also met the same issues like this, can you give me suggestion to response customer.
Why there is T1 period symptom?
Thanks

0 JohnTucker over 10 years ago in reply to Paul Yeh

TI__Guru**** 191445 points

See the response from the IC designer above.

0 Paul Yeh over 10 years ago in reply to JohnTucker

TI__Genius 16125 points

Hi John,
Yes, I already see this comments but I still have one question.
Is this T1 symptom caused by IC internal and can't avoid in system design?

Abd I also didn't know what's meaning of below statement, can you help to explain more detail as well.
"I add the one Vbe voltage shift function of the SS pin for the latest CS version.

Thank you and regards

0 JohnTucker over 10 years ago in reply to Paul Yeh

TI__Guru**** 191445 points

Yes the design is internal to the IC.

Vbe is base to emitter junction, about 0.7 V.

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TPS54327 SS behavior