TPS54335A RT pin bypass capacitor

tateo yamashiro

Other Parts Discussed in Thread: TPS54335A

Hi,

My customer has question about TPS54335A.
Could you tell me about questoin?

[Question]
My customer considers to use RT pin bypass capacitor.
How much is the recommended value of RT pin bypass capacitor?
My customer wants to use 10pF to 33pF.
Is there any problem?

Best Regards
tateo

over 8 years ago

0 JohnTucker over 8 years ago

TI__Guru**** 191445 points

Why would you want to do that? I'm sure we do not have any test data to support capacitance on the RT pin. I doubt anyone has ever tried it.

0 tateo yamashiro over 8 years ago in reply to JohnTucker

Guru 10890 points

Thank you for your reply.

My customer set the switching frequency by RT resistor.
But when the device is enable, the switching frequency is higher than setting the frequency by RT resistor.
My customer try to set bypass capacitor.
And then the switching frequency became to appropriate value.

Best Regards
tateo

0 JohnTucker over 8 years ago in reply to tateo yamashiro

TI__Guru**** 191445 points

They may have a layout issue. I can inquire about bypass capacitance next week.

0 tateo yamashiro over 8 years ago in reply to JohnTucker

Guru 10890 points

Thank you for your help.
My customer mesured RT pin waveform. Waveform is below. Is these waveform normal?

- not setting RT pin bypass capacitor

- setting RT pin bypass capacitor 33pF

Best Regards

tateo

0 JohnTucker over 8 years ago in reply to tateo yamashiro

TI__Guru**** 191445 points

My suspicion is that they are picking up interference from the SW node (possibly too long a ground lead on the probe?). The RT pin output voltage is tightly regulated by an internal control loop. Adding external capacitance may cause it to become unstable. Since the RT pin was not intended to have capacitance added to it, this has not been characterized or evaluated. I cannot recommend it.

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TPS54335A RT pin bypass capacitor