TPS65131: negative output voltage abnormal

user021864

Intellectual 2145 points

Part Number: TPS65131

Hi,

I am using TPS65131 for the application to output +-7.5V.

There are some problem at negative rail during power save mode enable:

Then checking the waveform on SW:

I wanna know what may be the reason of this problem.

Output voltage become normal after disable the power save mode of negative rail.

Thanks for your help.

over 5 years ago

0 ChristianSchmoeller over 5 years ago

TI__Prodigy 565 points

Hello,
our experts for this device are out of office this week. We will respond to you next week. Thank you for your patience.

Best regards,
Christian

0 Brigitte over 5 years ago

TI__Guru 58225 points

Hello user021864,

This behavior could be caused by a layout issue. Please check if there is an area where noise can couple into FBN or CN. These 2 nodes need to be in a quiet zone on the layout.

Best regards,
Brigitte

0 user021864 over 5 years ago in reply to Brigitte

TI__Intellectual 2145 points

Hi Brigitte,

Thanks for your help to point out the part may need to improve.
May I ask that is there is a chance to improve the feedback noise without changing the layout by using other external component?

Thanks for your help.

0 Brigitte over 5 years ago in reply to user021864

TI__Guru 58225 points

Hello user021864,

In general this will be hard to impossible, but you could try to reduce the value of the resistor divider to the minimum (be careful not to draw more than 10uA out of VREF). In addition if you have a resistor in series to the capacitor that is placed from VNEG to VREF, you might be able to reduce the coupled noise by increasing this resistor, e.g. to 100k.
Nevertheless, please check on your layout what might cause the noise injection on FBN or CN.

Best regards,
Brigitte

0 user021864 over 5 years ago in reply to Brigitte

TI__Intellectual 2145 points

Hi Brigitte,

Thanks for your reply. After checking the difference between EVM and typical application, I found below difference:

Typical application of the datasheet:

schematic from the EVM:

May I know that

Why the TPS56131 datasheet reference schematic is different from TPS56131 EVM reference schematic?
The reason of series 51.5kohm resistor from VNEG to VREF is for reducing the coupled noise? Or for another reason?

Thanks for your help.

0 user021864 over 5 years ago in reply to user021864

TI__Intellectual 2145 points

One other things to mention:
The waveform have become normal after adding the 51.5K ohm resistor to my board.

0 Brigitte over 5 years ago in reply to user021864

TI__Guru 58225 points

Hello user021864,

This difference is to reduce the noise coupled into the FBN node. Thank you very much for the feedback that this solved your problem.

Best regards,
Brigitte

0 user021864 over 5 years ago in reply to Brigitte

TI__Intellectual 2145 points

Hi Brigitte,

May I ask that can the typical application in datasheet suggest to add this component to improve the noise immunity.
thank you.

0 Brigitte over 5 years ago in reply to user021864

TI__Guru 58225 points

Hello user021864,

Yes, with the next datasheet update we should include this.

Best regards,
Brigitte

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TPS65131: negative output voltage abnormal