TPS56121 soft start function does not work

K. Sasaki14

Guru 10200 points

Other Parts Discussed in Thread: TPS56121

My customer observed that soft start function does not work.

They tested Vin on/off test.

EN/SS pin becomes open 20ms - 30ms after adding Vin.

Please refer to the attache file.

This phenomenon occurs about once every ten times.

Do you have any idea about this cause?

4743.TPS56121　Soft Start function does not work.xlsx

over 12 years ago

0 Matt Schurmann over 12 years ago

TI__Genius 11095 points

How is the enable pin configured? is it simply a capacitor to ground? Is there a discharge FET to pull EN Low?

It is not OK to drive the EN/SS pin with a voltage in this device due to the start-up sequence described on pg. 10 of the datasheet. The EN/SS pin must be floated in order to properly enable regulation.

0 K. Sasaki14 over 12 years ago in reply to Matt Schurmann

Guru 10200 points

EN/SS pin has a 22nF capacitor and FET to pull EN/SS Low.

But EN/SS pin just becomes open by FET turn off. EN/SS pin is not drived with voltage.

Is there possibility that device is broken?

0 Matt Schurmann over 12 years ago in reply to K. Sasaki14

TI__Genius 11095 points

Before we reach that conclusion...

Have you checked all of the solder connections? Is this happening on a single part/board or many?

If you swap the parts between boards, does the issue track the IC or the board?

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TPS56121 soft start function does not work