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Part Number: TPS54331
My customer has a TPS54331 design, it is used to get an output of 3.9V from the input 15V.
But there is a backup power from the battery, which is also 3.9V, directly connected together to the output of TPS54331.
In this case, if there's no 15V input to the TPS54331, The battery will provide 3.9V to the loading, as well as the output PIN of the TPS54331.
Then in this test, they found that the input PIN of the TPS54331 is ~3.5V.
So here comes the problem:
1. Risk assessment: is it all right to directly give the TPS54331 output PIN with 3.9V while no input voltage?
2. why there would be 3.5V in the VIN of the TPS54331 when output has 3.9V. The voltage of the input PIN seems coming from the chip internal sourcing from the output 3.9V.
The input capacitor is charged by the battery via inductor and body diode of high-side MOSFET. You need to ensure there is no issue for your system if the input voltage is 3.5V. For TPS54331, you can set the UVLO threshold voltage higher than 3.9V so that the device is disabled.
In addition, you can add a diode to prevent the input capacitor from being charged.
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In reply to Shawn Nie:
Thanks Shawn for the response!
As this is mature design, there's no diode can be added in the power rail, that's why they'd like to learn the risk of this design.
So based on your comment, it is all right to connect 3.9V to the TPS54331 when there's no voltage input. The TPS54331 won't be damaged by this 3.9V output power rail. The only thing is that we need to take care of the 3.5V of the input Pin, sourcing from the 3.9V output. Ensure the circuit in front of the TPS54331 won't be damaged by the 3.5V is all right.
Can you confirm? Thank you!
In reply to Steven Liu1:
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