TPS54428: Maximum Soft Start Time

Allan Fan

Expert 6170 points

Part Number: TPS54428

Hi Expert,

Is there maximum soft start time spec limitation for TPS54428?

Thanks.

Allan

over 4 years ago

0 Jenson Lam over 4 years ago

TI__Prodigy 450 points

Hi Allen,

There is no maximum soft start specification.

Thanks,

jenson

0 Allan Fan over 4 years ago in reply to Jenson Lam

TI__Expert 6170 points

Hi Jenson,

Would you please review below circuit and clarify why customer only get 1.35Vout with Css=680pF but get normal 3.3Vout in 8200pF?

Thanks.

0 jamie kong over 4 years ago in reply to Jenson Lam

Prodigy 30 points

Hi Jenson:

Now i use SS cap 680pF , it can not out 3V3 just about 1V3,when i change the SS cap from 680pF to 8200pF

it can work 3V3 ,so the design have issue ,please help to check it!

0 Jenson Lam over 4 years ago in reply to jamie kong

TI__Prodigy 450 points

Hi,

I will help you checking it out tomorrow

regards,

jenson

0 Jenson Lam over 4 years ago in reply to jamie kong

TI__Prodigy 450 points

Hi Allen,

I don't see any reason why you do not get the correct Vout at 3.3V.

Besides changing CPC24 to 8200pF, can you also try to remove CPC25 then see if you can get the correct output?

On WEBENCH, there is suggestion for minimum soft time to be 1.15ms.

Regards,

jenson

54428_AllanFan.tsc

0 jamie kong over 4 years ago in reply to Jenson Lam

Prodigy 30 points

Hi Jenson:

Yes when i remove the CPC25 ,the output is OK.

So what the reason?

Jamie

0 jamie kong over 4 years ago in reply to jamie kong

Prodigy 30 points

Sorry ,it unstable ,it can not output

Please help to check on demon board

Jamie

0 Jenson Lam over 4 years ago in reply to jamie kong

TI__Prodigy 450 points

Hi Jamie,

Did you mean that removal of CPC25 does not fix the issue?

In this case, you need to increase the size of CPC24 because the soft time is probably too short

According to section 7.4.1, CPC24 > Tss(ms) x 6 / 0.765 / 1.1 = 7130pF

regards,

Jenson

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TPS54428: Maximum Soft Start Time