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TPS791: Fixed 1.8V LDO is not providing the correct output voltage 1.8V

Part Number: TPS791

Hi, 

I'm using Ultra-low-noise 100mA TPS79118 IC, but it is providing 1.2V on an empty PCB (without load), and 0.45V with 11*130uA unpowered ADCs as load. I connected all the pins as in the datasheet, with 0.1uF input capacitor, 1uF output capacitor and 0.1uF at the bypass node. 

There is no short between the output and ground, so I don't understand why the output voltage is so much lower than its nominal value. 

Sincerely, 

PX

  • Hi PX,

    In order for us to help debug, please provide a scopeshot with Vin, Ven, and Vout. We want to see how these rails interact; therefore, please make sure that you use a multichannel scope and set the timescale so that we can see as much detail on when you are attempting to startup the regulator.

    Often when a regulator does not start to the intended voltage it is caused by an unintentional leakage path. This could come from flux which is not properly cleaned from the PCB. As we debug, please do a visual inspection of the board for any unintentional leakage path particularly around OUT and BYPASS. Since Cbypass filters the internal reference for the regulator, an unintentional leakage path on BYPASS will directly alter the output voltage.

    Very Respectfully,
    Ryan
  • Hi Ryan,

    Thanks for the reply.

    It turned out that the output pin and bypass pin have been interchanged in the footprint I created. There is actually no problem with the LDO.

    Best,

    PX