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LP2989 Output is not 3.3

Stephen Doll
Prodigy 40 points
Other Parts Discussed in Thread: LP2989

I'm a little stumped here.  I purchased several LP2989IMM-3.3 regulators.  I'm using the 10uF Input and 4.7uFOutput ceramic capacitance values and the actual circuit is right out of the data sheet. 

I get a very stable 5.01 volts out of the part on the proto-boards.  It is supposed to be 3.3 volts.

I thought I had received the wrong part, but the top marking says LA2B, which seems to be correct for a 3.3 volt part.

It is the only regulator on the board, and should actually be tasked to provide 140mA at 3.3V from a 7.4V battery supply.

The only difference I can see between my circuit and the typical application circuit in the spec sheet is that I have a protection diode between input and output lines.

What am I missing?

  • 0
    TI__Mastermind 22610 points

    I'm stumped as well. LP2989 is pretty straight forward. Stated top marking is correct for 3.3V version.

    Try going to just basic connections ... remove the protection diode, remove the pull-up resistor from the /ERROR pin, and remove the filter capacitor from the BYPASS pin, make sure the SENSE pin is connected directly to the OUTPUT pin, leave the Cin and Cout capacitors in place, and see what the difference might be.

    One thought ... It's possible to force Vout to a higher voltage by adding resistance between SENSE pin and OUTPUT pin, instead of direct a connection. I don't have the list needed values to get the 3.3V to 5.0V on hand at the moment, but he odds of getting so close to 5.00V at random would be very slim.

  • 0 in reply to Donald Jones
    Prodigy 40 points

    Thanks Donald,

    I did all of that and the voltage output did not change, but it pointed me in the right direction.  I wired up a LP2989 just by itself with no pcb and the output was a stable 3.3.  At that point I knew it was something on the board.  I lifted the output and sense pin and the node on the pcb remained at 5V.  Aha moment, the problem is not the 2989 at all. 

    Started to get the feeling that this was one of those awkward inept moments I sometimes have........Yep, it was......I eventually found a solder splash at the other end of the board tying the battery supply voltage through another component to my 3.3V bus.  So, if anyone out there runs into this, make sure you do not have another voltage impinged on your output line.

  • 0 in reply to Stephen Doll
    TI__Mastermind 22610 points

    Good to hear that you got it resolved. Thanks for the feedback.