UC29432: REF unstable

Daisuke Higa

Part Number: UC29432

Hi support team.

My customer has a problem with poor REF output regulation.

They have experimentally confirmed that connecting a capacitor to the REF output with respect to GND results in very poor regulation from a certain capacitance.

Is it not recommended to connect a capacitor to GND at the REF output terminal?

Best regards

Higa

over 5 years ago

0 Ben C over 5 years ago

TI__Guru 56345 points

Hi Higa-san,

What type of capacitor is the customer using? Do you have a schematic that I can view? The poor regulation could be coming from the capacitor being very leaky. The datasheet says the REF pin can source or sink 10uA.

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Hi Ben-san

Thank you for your reply.

I don't have schematic however I will try to get it from my customer.

I have additional questions.

Is there a possibility that the REF output will oscillate when a ceramic capacitor is connected?

Best regards,

Higa

0 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

It is possible. How much capacitance is on the REF pin?

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Hi Ben-san

The capacitance is 0.1uF.

regards,

Higa

0 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

Is the REF output stable with no capacitor?

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Ben-san

Yes.

Regards,

Higa

0 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

Has the customer tried a resistor that is placed in series with the capacitor?

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Hi Ben-san

Thank you for your proposal.

However, it is inevitable that the customer will confirm how to design the resistance value.

Could you give me some advice on determining the resistance value that way?

Best regards,

Higa

0 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

I would suggest a few ohms and see if the stability goes away. If not, have them increase the resistance to until stability is achieved. Since this is a very old part, there is very little data for me to work on.

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Hi Ben-san

Thank you for your kind support.

Is it correct if it is stable without a capacitor connected to the REF pin? (Can you judge if you get the schematic?)

Or is it better to insert a series resistor and connect a capacitor?

My customer says that inserting a series resistor requires changing the board, which is unacceptable.

The method of not connecting the capacitor is acceptable because it only removes it.

Best regards,

Higa

+1 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

Based on what has been provided, the REF pin seems to be stable without an output capacitor. I would suggest that the customer performs the necessary tests to ensure that the part is stable throughout the conditions that the application is being applied.

Ben

0 Daisuke Higa over 5 years ago in reply to Ben C

Genius 4935 points

Hi Ben-san

Thank you for your advise.

I understand.

I would like to negotiate with the customer so that it can be resolved.

Thank you for your support.

Regards,

Higa

0 Ben C over 5 years ago in reply to Daisuke Higa

TI__Guru 56345 points

Hi Higa-san,

Good luck to you and I am glad to be of help!

Ben

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UC29432: REF unstable