TPS54821: Vout drop issue

Jeff YANG

Mastermind 7845 points

Part Number: TPS54821

Hi,

Mu customer have TPS54821 vout drop issue.

Please see below waveform.

CH1: Iout

CH2: Vout

CH3: SW

But when Iout increase, Vout drop to 1.324V. And trig PWRGD low threshold.

we also check inductor should be meet spec.

Below is our schematic setting. We have follow wenbench simulation result modify the comp and input/output capacitor design.

over 3 years ago

0 Jeff YANG over 3 years ago

Mastermind 7845 points

Hi,

Below is Input capacitor and layout for you reference.

TOP

Bottom

In2 layer

0 Miranda Gu over 3 years ago in reply to Jeff YANG

TI__Genius 16632 points

Hi Jeff,

Thanks for reaching to us.

Could you check the output cap location and values? I didn't find it in the schematic. And C2085 is the feed-forward cap, not output cap, shouldn't use such large capacitance (68uF*2), please change to -pF level.

Increasing output capacitance and decreasing inductance are the common ways to improve transient performance (make Vout stable). You may try that.

0 Jeff YANG over 3 years ago in reply to Miranda Gu

Mastermind 7845 points

Hi Miranda,

Sorry, I have modify wrong local for output capacitor.

We remove the feed-forward cap, and put output cap about ~68uF *2.

And we found it have fail rate about 5/70, some MB found this issue and follow the TPS54821,

We may apply FA for this case. Or you have any idea for this case? thanks!

Jeff

0 Miranda Gu over 3 years ago in reply to Jeff YANG

TI__Genius 16632 points

Hi Jeff,

Have you tried to decrease L or increase Cout?

Currently I don't think it's a failure, changing external components may solve this problem.

0 Jeff YANG over 3 years ago in reply to Miranda Gu

Mastermind 7845 points

Hi Miranda,

Let us check if decrease L or increase Cout can improve or not.

So the compensation value are okay?

Is it may caused abnormal voltage drop?

Thanks!

Jeff

0 Miranda Gu over 3 years ago in reply to Jeff YANG

TI__Genius 16632 points

Hi Jeff,

The compensation circuit affects loop response, related to transient performance.

But I suggest to keep webench recommended values, change L/C values first to see whether the drop reduced, then tune the compensation values if needed.

Current 4.7uF is kind of large, please try to use 2.2uH.

0 Jeff YANG over 3 years ago in reply to Miranda Gu

Mastermind 7845 points

Hi Miranda,

We have tried to change output cap from 560uF to 820uF can improve this issue.

But why need lots of output cap for this application?

I used webench simulation set to Vin 4.5~5.5V Vout 1.538V, Iout 4A, switching freq 300khz, it shows only need 100uF *2.

Or there have better way to reduce the output capacitor? Increase switching frequency?

Thanks!

0 Miranda Gu over 3 years ago in reply to Jeff YANG

TI__Genius 16632 points

Hi Jeff,

Larger Cout could storage more charges to handle loading changes, considering dV=I*dt/C. The trade-off here is better transient VS larger BOM cost.

Have you tried to decrease L? It should be also workable.

+1 Miranda Gu over 3 years ago in reply to Miranda Gu

TI__Genius 16632 points

Hi Jeff,

Any updates? Let me know if you need more support. Or I will close this thread.

0 Jeff YANG over 3 years ago in reply to Miranda Gu

Mastermind 7845 points

Hi Miranda,

They have increase output cap to can solve this issue, I will close this case. thank you!

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TPS54821: Vout drop issue