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LP3875 problem

Viktor Takacs
Prodigy 20 points
Other Parts Discussed in Thread: LP3875

HI All,

I am trying to evaluate the LP3875 linear LDO, both 3.3V and 5.0V versions.

I have made a breakout PCB for them and tried both of them in the same breadboard. Using the typical application circuit from datasheet.

The 3.3V version works perfectly, as expected, but the 5.0V version causes a major headache for me: at the Vout pin I measure the same voltage as at Vin. I have already replaced the chip for another one just in case, but the same result. The input voltage is about 6.3V.

Any idea what the problem might be?

Thanks for any pointers in advance!

v

  • 0
    TI__Mastermind 22610 points

    There is no internal electrical reason why one would work fine (LP3875-3.3), and the other version would have Vout=Vin (LP3875-5.0). That leaves the external connections as the most likely problem area.

    1) Vin pin and Vout pin are shorted together. (You don't mention which package you are using so it is difficult to assess the probability, but this is the most common reason for Vout=Vin)

    2) SENSE pin is not connected to Vout pin.

    3) GND pin is not connected to ground.

    4) Device is internally damaged. (Not a very likely possibility since you replaced the device and problem remained)

     

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Hi Donald,

    Thanks for the quick answer. I also thought of the same possibilities, checked almost all of them but found nothing.

    However, after a day of headaches, thanks to your reply, I found the problem: the SENSE pin was not connected. ONce I connected it it was working as expected.

    I also found out the reason for the 3.3 version working: When ordering the samples, by mistake, I ordered an LP3875-5 and an LP3872-3.3 and that works happily without a SENSE pin. That's why one worked and one didn't.

    Thanks for your help!

     

    v