TPS568215: SS Capacitor Causing Peaking and Output error?

Louie

Expert 3180 points

Part Number: TPS568215
Other Parts Discussed in Thread: TPS62088, TPS63036

Hello,

My customer has a system where:

12V -> efuse -> TPS568215 -> 5V.

The device is in FCCM, 400kHz Mode with a 0.047uF capacitor on the SS pin.

They noticed that when this capacitor is on there, there is overshoot when the device is starting up:

But when that SS capacitor is removed, this is what the output looks like:

Is there any correlation with regards to the SS capacitor and the peaking that occurred? The loads are TPS63036 and TPS62088, which are power sequenced after the TPS568215 is powered up correctly.

Thank you,

Louie

over 7 years ago

0 Gavin Wang (Zejing) over 7 years ago

TI__Expert 3480 points

1. What is the ss cap part number?
2. Can you help test the waveforms of SW pin and Vout for 0.047uF case?
3. How about disconnect the load TPS63036 and TPS62088 ?

0 Louie over 7 years ago in reply to Gavin Wang (Zejing)

TI__Expert 3180 points

Hello,

1. We have a couple of options on this cap. I need to contact CM to know which exact cap they used on the board.

Here is one of the options,

KEMET, P/N: C0603C473K5RAC

2. Waveform attached.

3. No, two downstream regulators attach with it.

Thank you,

Louie

0 Gavin Wang (Zejing) over 7 years ago in reply to Louie

TI__Expert 3480 points

1. Can you help test the waveforms putting SW pin and Vout together with 1ms/div range for 0.047uF case?
2. What is your load condition?

0 Louie over 7 years ago in reply to Gavin Wang (Zejing)

TI__Expert 3180 points

Since the issue was resolved by removing the 0.047uF capacitor, the engineer does not want to revisit this.

However, could you provide us with some theories of why have a longer delay would cause this kind of peaking?

Thank you,

Louie

0 Gavin Wang (Zejing) over 7 years ago in reply to Louie

TI__Expert 3480 points

There is no issue for our EVM using 0.047uF. I must check your waveforms and see what is the root cause.

0 Daniel Li14 over 6 years ago in reply to Louie

TI__Mastermind 27470 points

Hi Louie:
one suspect from my side is there is some load transient at your 5V load or other place make Io or Vo change in this time period . but when you Vo build up quick enough, the condition will change so that we don't see any Vo transient when you remove the SS capacitor.
what's the result if you change to 0.068uF, will it happen in the same time after Vo start build up?
if you can provide whole waveform including SW, inductor current and Vo with 1ms/div, we maybe can help you identify the problem.

Thanks

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TPS568215: SS Capacitor Causing Peaking and Output error?