TPS54331 slow start

kura

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Other Parts Discussed in Thread: TPS54331

Our customer consider to use TPS54331.

They would like to set SS time about 50ms.

We found following description in TPS54331 datasheet P11.

"The slow-start time should be set between 1 ms to 10 ms to ensure good startup behavior.

The value of the slow-start capacitor should not exceed 27 nF."

If they set SS time 50ms,does this make a bad effect on stability of this part in normal operation ?

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

SS time of 50 nsec requires 125 nF of capacitance at SS pin. The purpose of the limitation to 27 nF is that SS is discharged in the case of fault conditions such as over current. Why do you need such a long SS time? can you delay EN instead?

0 kura over 8 years ago in reply to JohnTucker

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TPS54331 circuit and start up waveform.pdfJohn-san

Thank you for your reply.

They can't control EN pin,because EN pin is opened in their circuit.

I attached their circuit and waveform.Please check it.

And they have one more restriction that is " Vin rising time take 30ms".

Circuit value can be changed. Please check attached and give us your advise.

0 JohnTucker over 8 years ago in reply to kura

TI__Guru**** 191445 points

Can you provide the time scale and V/div for the waveforms?

0 JohnTucker over 8 years ago in reply to JohnTucker

TI__Guru**** 191445 points

I probably know what is going on with this, but need the additional info. Can you provide the COMP pin voltage and switching node as well?

0 kura over 8 years ago in reply to JohnTucker

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John-san

Thank you for your reply.

About SW node waveform,do you need waveform at start up time or at steady state?

0 JohnTucker over 8 years ago in reply to kura

TI__Guru**** 191445 points

During the start up when the output voltage is non monotonic. It would help to se one long time scale and one zoomed in on the falling edge of one of the dips.

0 kura over 8 years ago in reply to JohnTucker

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Thank you for your support.
Customer modify VIN rising speed from slow to fast.They fixed prolem.

We have addtional question about TPS54331 gain/phase margin.
Please let us know what FCO value you use in Datasheet P18 EQ20.
www.ti.com/.../slvs839f.pdf
We can't get Gain –2.26 dB.

0 JohnTucker over 8 years ago in reply to kura

TI__Guru**** 191445 points

looks like 25 kHz was used. That whole procedure is rather convoluted. I would probably recommend you use Webench for calculation of the component values.

0 kura over 8 years ago in reply to JohnTucker

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John-san

Thank you for your reply.

I used folowing value.
pi:3.14
RSENSE:1 /12 ohm　（0.083）
FCO:25kHz
CO:94uF
Gain=-1.762111...

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TPS54331 slow start