UCC28950 / abnormal pulse skipping

S.Satoshi

Other Parts Discussed in Thread: UCC28950

Hi,

Our customer are evaluating UCC28950 on their preproduction board and then they found abnormal operation as attached waveform. Following is its explanation.

- One side operate with max duty and the other side with 0% duty as attached waveform (Look like pulse skipping)

- This condition happens at heavy load

- The frequency of this operation about one time per second but suddenly and irregularly (Depend on load)

- It was not confirmed at all samples but confirmed about 30pcs per 100pcs.

- This abnormal condition is fixed by changing RTMIN register from 47kohm to 12kohm (Schematic stated after changing)

I would like to have your advice for;

- Estimated root cause for this abnormal operation

- The reason why changing RTMIN register fix this abnormal operation

Best Regards,

Sonoki / Japan Disty

UCC28950 abnormal pulse skipping.pdf

over 12 years ago

0 S.Satoshi over 12 years ago

Guru 21685 points

Hi,

Let me add information for CS pin voltage at abnormal operation.

Please let me know if you need more information.

Best Regards,

Sonoki / Japan Disty

0 John Bottrill over 12 years ago in reply to S.Satoshi

TI__Mastermind 25130 points

Sonoki,

I will see if I can find out.

John

0 S.Satoshi over 12 years ago in reply to John Bottrill

Guru 21685 points

Hi John-san,

Thank you for your response.

I appreciate if I can have your further comment.

Best Regards,

Sonoki

0 John Bottrill over 12 years ago in reply to S.Satoshi

TI__Mastermind 25130 points

Hi Sonoki-san,

Since the failure is intermittent in nature I would suggest that you look at each of the pins looking for noise issues. Does the voltage on any of the pins go below ground by more than 0.3 volts. If so that can be turning on internal parasitic devices.

Can you capture the waveforms COMP and CS at the time of the failures?

Regards,

John

0 S.Satoshi over 12 years ago in reply to John Bottrill

Guru 21685 points

Hi John-san,

Sorry for delayed response.

I'm requesting customer to check the voltage on COMP pin but they can not have a time to check it so far. So can you assume why this pulse skipping issue is happened and it is improved by changed the RTMIN resistor from 47kohm to 13kohm?

Best Regards,

Sonoki

0 John Bottrill over 12 years ago in reply to S.Satoshi

TI__Mastermind 25130 points

Sonoki-san

I have requested that the design engineer see if he can answer this.

0 John Bottrill over 12 years ago in reply to S.Satoshi

TI__Mastermind 25130 points

Sonoki-san,

The engineeering group has requested teh following information to enable them to see what is going on:

I would like to see screenshots of:

OUTA, OUTB, OUTC, OUTD

OUTA, OUTB, OUTC COMP

OUTA, OUTB, TMIN, COMP

OUTA, OUTB, SYNC, COMP

This will show operation more on the controller inputs/outputs. SYNC will likely give an idea of the internal oscillator operation

John

0 S.Satoshi over 12 years ago in reply to John Bottrill

Guru 21685 points

Hi John-san,

Sorry for my delayed response.

I'm requesting customer the information you requested right now.

Please wait for my update.

Best Regards,

Sonoki

0 S.Satoshi over 12 years ago in reply to S.Satoshi

Guru 21685 points

Hi John-san,

I'm requesting customer to check the waveform you requested, but the engineer I'm communicating make a business trip by end of Jan. so I will get it next month. I will update when I will be able to get it.

Best Regards,

Sonoki

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UCC28950 / abnormal pulse skipping