TPS54331: Output issue

Ning Li

Part Number: TPS54331

Dears,

Customer use the TPS54331,the input is 24VDC,output is 5VDC.

1.Now they find when the C180 is 8200pF and C178 is 68pF and C172 is 22pF,the output waveform shows ladder,pls find attachfie for waveform#1.

2.When the C180 is 104,C178 is 102 and C172 is 10pF,the ladder vanished,but have ripple,pls find attach for waveform#2.

BTW,the schematic#1 is to waveform#1,the schematic#2 is to waveform#2.

Pls kindly help to check the schematic have any issue or not?

Thanks

over 2 years ago

0 Miranda Gu over 2 years ago

TI__Genius 16632 points

Hi Ning,

Thanks for reaching to us.

I will do some tests in lab to verify your question. Will reply you by early next week.

0 Miranda Gu over 2 years ago in reply to Miranda Gu

TI__Genius 16632 points

Hello Ning,

Could you please capture SW and inductor current (IL) waveforms as well?

I could reproduce the Vout startup waveform using your first schematic Comp/L/Css values, as shown below.

TPS54331EVM-24Vin-5.33Vout-0A-power on:

About the voltage ladder you asked, where triggered device OC limit, and Vout boosted to a quite high value, no pulses for a while. With Vout gradually decreased, the control loop will sense the decreased voltage and re-start switching.

Zoom-in:

I found that your first BOM SW is unstable during CCM operation, as shown below. The loop design needs to be optimized.

24Vin-5.33Vout-3A:

Then I changed Ccomp (C178) from 68pF to 1nF, the SW became stable during CCM, and startup Vout waveform is smooth, I didn't see the ringing that you mentioned. Please check SW&IL waveforms in your side.

Change C178 to 1nF, 24Vin-5.33Vout-0A-power on

Change C178 to 1nF, 24Vin-5.33Vout-3A

0 Ning Li over 2 years ago in reply to Miranda Gu

Intellectual 750 points

Dear Miranda,

I feedback your suggestion to customer,they want to know that whether you have load or not,as when have no load,they find the output waveform is normal.

Thanks

0 Zixu Wang over 2 years ago in reply to Ning Li

TI__Expert 8100 points

Hi Ning,

Please read carefully for reply from Miranda. Loading condition is quite clear in the reply.

BRs

Zixu

0 Ning Li over 2 years ago in reply to Zixu Wang

Intellectual 750 points

Hi Zixu&Miranda,

Noted,thanks for your support,I will let customer provide the SW&IL waveform for #1 #2 Schematic,once I got it,will reply you..

Thanks

0 Ning Li over 2 years ago in reply to Miranda Gu

Intellectual 750 points

Dear Miranda,

I want to know that you change the C178 only?The C180&C172 is the same as schematic #1.

Thanks

0 Miranda Gu over 2 years ago in reply to Ning Li

TI__Genius 16632 points

Hi Ning,

Yes, for second test, only change C178. I presented all my test conditions in my first reply.

0 Ning Li over 2 years ago in reply to Miranda Gu

Intellectual 750 points

Hi Miranda,

When they do the test,the load is 450mA.

Below shows the waveform of schematic that no change.

Change C178 to 1nF waveform only,then found the output have ripple,so pls kindly help to suggest how to optimized?

Thanks

0 Miranda Gu over 2 years ago in reply to Ning Li

TI__Genius 16632 points

Hello Ning,

Did you check SW/IL waveforms during steadystate? You shouldn't only care about startup waveform, but also should care about steadystate waveforms. I saw obvious unstable SW waveform of your first design in steadystate.

Seems your loop design needs to be reconsidered. Could you completely change to webench recommended circuit?

24Vin-5Vout-Webench recommended circuit: