TPS548B22: No smooth rising waveform during start-up

Stan Wu

Intellectual 580 points

Part Number: TPS548B22
Other Parts Discussed in Thread: TPS53318

Hello TI,

We use TPS548B22 converter for 12V to 0.8V solution. We found the start-up waveform is not smooth.

Seems the soft-start mechanism is not quite well because the first several duties are too wide during this period.

Is there any solution to avoid this? or it's not resolvable?

Regards.

Stan.

over 5 years ago

0 JohnTucker over 5 years ago

TI__Guru**** 191445 points

Stan,

Can you post your schematic?

0 Stan Wu over 5 years ago in reply to JohnTucker

Intellectual 580 points

Hi John,

Could you please give me your mail address? I want to send you the SCH directly since it's confidential based on our company compliance.

Regards.

Stan.

0 JohnTucker over 5 years ago in reply to Stan Wu

TI__Guru**** 191445 points

It is j(dash)tucker(at)ti(dot)com

0 Stan Wu over 5 years ago in reply to JohnTucker

Intellectual 580 points

Hi John,

Thank you. I've forwarded my SCH to you by mail. Sorry I have to correct my word of my last post.

It's minimum duty at the beginning of soft-start. The converter looks like operating at skip mode during rising period.

It changed to FCCM mode that we set after the soft-start completed.

Regards.

Stan.

0 JohnTucker over 5 years ago in reply to Stan Wu

TI__Guru**** 191445 points

Stan,

Yes it is a minimum on time and skip mode phenomenon. You could try operating in FCCM mode during start up. That said, I don't think your ripple is too severe. It looks worse than it is because of your low output voltage and sensitive amplitude scale of 200mV per division. You can also refer to this document. It is not directly targeting TPS548B22 but the information is general.

www.ti.com/.../slva866

0 Stan Wu over 5 years ago in reply to JohnTucker

Intellectual 580 points

HI John,

Thanks. Here is another question regarding the jitter performance at DCAP3 mode we set for TPS548B22.

The jitter (DCAP3) is worse than TPS53318 (DCAP). Is it inherent in DCAP3?

If it is, how do we judge such jitter is no problem? what's the criteria for PASS and FAIL?

Regards.

Stan.

0 JohnTucker over 5 years ago in reply to Stan Wu

TI__Guru**** 191445 points

Stan,

As you may know, there is no internal oscillator for DCAP, DCAP2, DCAP3, DCAP+, etc. For normal PWM style constant frequency converters, the internal oscillator keeps the frequency well controlled. You may see some jitter in the trailing edge. DCAP type devices are adaptive on time and hysteretic off time. You may see apparent jitter on both the rising and falling edge. The actual amount of jitter is mostly a function of ramp amplitude. DCAP3 has reduced ramp amplitude to improve Vout set point accuracy but increased jitter. There is no particular criteria for the amount of jitter as long as the circuit is stable.

0 Stan Wu over 4 years ago in reply to JohnTucker

Intellectual 580 points

Hi John,

Got it and thanks for you explanation, our customer also agrees with this view point.

Regards.

Stan.

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TPS548B22: No smooth rising waveform during start-up