TPS54821: Issue with the Switching node signal and to understand the root cause

Felix Arul Rajesh Francis

Part Number: TPS54821

https://e2e.ti.com:443/HI,We are using the TPS54821 device. Designed for 3.3V generation with 800KHz SW frequency. Suddenly we started to see a different behavior, when we probe the PH signal and Output signal after inductor, we are not seeing a constant voltage. Seeing a ramp up and down.We can see that from the attached waveform "3_3V_SWNode_Bad1". When we zoom the ramping area, we could see some switching happening, as shown in the waveform "3_3V_SWNode_Bad2".But on a good working board they are like the waveform "3_3V_SWNode_Good1".Kindly give some clarification behind the issue.Could be Inductor failure, MOSFET failure at the device, some auto frequency change (or) anything with Input voltage? We are seeing a constant 5V rails at the input.Regards,Felix.

over 6 years ago

0 Vincent Zhang(BSR) over 6 years ago

TI__Expert 4795 points

Hi,

So you have measured the good waveform, right? What's the different of the configuration between these two boards?

Could you also help measure the inductor current waveform? I think the inductor is small, maybe the current limit is triggered.

Regards,

Vincent

0 Felix Arul Rajesh Francis over 6 years ago in reply to Vincent Zhang(BSR)

Intellectual 470 points

Hi Vincent,

We could resolve the issue. Its not related either with Inductor become bad not the switcher MOSFET failure.
We could see an open at the feedback path (soldering issue). Once we touch up those points, the regulator started working fine and we could get proper voltage rail coming up.

Thanks for your input on this.

Regards,
Felix.

0 Vincent Zhang(BSR) over 6 years ago in reply to Felix Arul Rajesh Francis

TI__Expert 4795 points

Hi Felix,

Thanks for update. Good to see the issue fixed now.

BR, Vincent

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TPS54821: Issue with the Switching node signal and to understand the root cause