TPS53355: VFB is 0.63 rather than 0.60

amin cheraghi

Part Number: TPS53355

Hi Guys, I have an issue with the TPS53355 regulator and I was wondering if anyone has a suggestion for me.

I used the WEBENCH to design a 12V to 3.3V DC-DC converter. But now my output voltage is 3.5V. The reason is that the VFB is 0.634V instead of 0.6V. Any idea?

I have used this part before and never had this issue.

Here is the schematics:

Thanks,

Amin

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

Amin,

TPS53355 is uses DCAP control. The 0.6 V is the valley voltage at teh VFB pin. Spo the DC value may be quite a bit higher. Can you post the waveform?

0 amin cheraghi over 7 years ago in reply to JohnTucker

Prodigy 200 points

Hi John,

Thanks for your reply. Here is the waveform:

So, are saying that it is normal? 0.63 is out of the range according to the datasheet.

Do you suggest me just to fix the output by just adjusting the voltage divider resistors?

Thanks,

Amin

0 JohnTucker over 7 years ago in reply to amin cheraghi

TI__Guru**** 191445 points

Amin,

That looks like the SW node. If you look at VFB, you should see a triangular waveform. The valley point should be at 0.6 V.

0 amin cheraghi over 7 years ago in reply to JohnTucker

Prodigy 200 points

Yes, that was the output node. here is the VFB:

0 amin cheraghi over 7 years ago in reply to amin cheraghi

Prodigy 200 points

The swing looks very big. Am I right?

0 JohnTucker over 7 years ago in reply to amin cheraghi

TI__Guru**** 191445 points

Amin,

Yes, way too much ripple on VFB. 30 mV should be more than enough. Are all your output capacitors ceramic types? If so, please check you calculations for the ripple injection components.

0 amin cheraghi over 7 years ago in reply to JohnTucker

Prodigy 200 points

John,

You are right. The injection ripple was high (But why the simulation tool suggest these values?). I changed the R810 (R7 on the datasheet) to 19k and it lowers the VFB DC level to 0.609. Now my output is correct. Here is the VFB waveform. Does it look good? or still high swing?

Thanks,

Amin

0 JohnTucker over 7 years ago in reply to amin cheraghi

TI__Guru**** 191445 points

Amin,

Looks much better. I think you are good to go.

0 amin cheraghi over 7 years ago in reply to JohnTucker

Prodigy 200 points

Thanks John for your help.

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TPS53355: VFB is 0.63 rather than 0.60