TPS7A19: Vout become higher than set value

Hi Koyo,

That startup waveform is very odd, especially because output starts rising before Vin is fully raised. Vout also starts at >1V in the unstable Vout but <1V in the stable Vout. I'd like to know more.

Can you explain more on what you mean by VIN is unstable? Are the output capacitors on an upstream buck out of the stable range, etc?

What is happening on the EN pin? Is it shorted to VIN, constantly raised, etc?

Where are you measuring Vin and Vout?

What other conditions was the unstable Vout overserved under? How long since the output was disabled, etc?

Best,

Gregory Thompson

Hi Gregory

There is 127kohm top and 100kohm bot divider for EN from VIN to release UVLO at 4Vin.

I want to know possible root cause first. They just apply the Vin and the undesired 5V is generated. If you don't come up with possible root cause, try to apply similar Vin to your EVM.

Best regards
koyo

Hi Koyo,

Thank you for the clarification on the EN. It looks like VIN is triggering the EN and then falling down on the unstable waveform. I believe the weird output startup before the input fully rises on the unstable scope shot is related to the issue.

I do not have enough information to determine a root cause. I do not know what's different in setup between the two startups, I do not know enough about your upstream supply to determine what's driving the input up/down before it turns on.

Right now, the only thing I can do is take the EVM into the lab and provide an oscilloscope screenshot of startup working properly.

Best,

Gregory Thompson

Thanks. Please try to replicate the sympton. I think this should be best to find a root cause.

Hi Koyo,

Issue not replicated on EVM:

I need more information to perform further attempts:

• What other conditions was the unstable Vout overserved under? How long since the output was disabled, etc?
• What is the upstream supply?

Best,

Gregory Thompson

Hi Gregory

I know the sympton won't happen when you input clean Vin as you see my 2nd image in the initial post. Please input messy Vin like 1st image so that you may see the sympton. I don't know exact conditions how to replicate but I add how voltages are changed in case you don't understand the odd waveform.

Please apply higher than Ven and lower than Ven multiple times and then ramp up to 17V. You may see the sympton.

Best regards
koyo

Hi Koyo,

Thank you for labeling the image, it helps. 

• Can you provide a schematic?
• Is the issue replicable on their part or was it a single incident?
• Are you able to tell me more about the upstream supply that's driving this input voltage?

Best,

Gregory Thompson

Hi Gregory,

I replicated similar sympton on TPS7A19 EVM. Vout reaches to 5.5V though setting voltage is 3.3V.

The issue is found when Vin become 18V from 3.5V suddenly. It is stable Vout when initial Vin is higher than 4V or Vout is 0V. I assume this is regulation issue because 3.5Vin means Vout is dependent on Vin as typical dropout is 240mV.

Is this expected behavior? Please investigate actual root cause. How about other LDOs?

Are you trying to replicate the issue on your EVM? I need the reason why Vout can reach to 5V which is higher voltage than setting value.
I see the device doesn't work under 4Vin but Vout can be 3.3V when 3.5Vin. I think internal regulation doesn't work under 4Vin is the root cause.