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TPS717: TPS71733 Fixed output = 3.92V not 3.3V

Adam Mitchell
Prodigy 75 points
Part Number: TPS717

Hello, 

I am using the TPS71733 LDO fixed at 3.3V but I am actually getting 3.92V. 

I have a 0.1 and 1.0uF input ceramic caps. 1.0uF output ceramic cap and a 0.1uF noise reduction capacitor. 

Input voltage is 4.975V from a TPS7B6950 supplied rail. 

The diode footprint is included in the event reverse current protection was needed. In or out of circuit this does not change the output of the TPS71733.

The trace that goes to the left of the output goes to a programming header for programmer supplied voltage. 

  • 0
    TI__Guru 55940 points
    Hi Adam,

    Many times when an LDO output is higher than the set voltage it is the result of an unintentional bias on the output of the LDO. Many times in devices that require multiple voltage rails, there is an unintentional path between the power rails. Often this path is through protection diodes. An additional common unintentional bias is flux that has not been removed after soldering. Check for flux around the LDO and surrounding capacitors.

    Traditional LDOs do not sink current; therefore, when the output is biased high, it is up to the load itself to pull the output down to the set voltage.

    In order to debug, are you able to isolate the LDO from the load?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 75 points
    Hi Ryan,

    I have cut the two traces either side of the output capacitor and I now see 3.31V at the output.

    Each power rail has two ESD Diodes to ground, one at each end of an FFC cable. Are you saying this is the source of the voltage discrepancy?

    Thank you for your time,

    Adam
  • 0 in reply to Adam Mitchell
    Prodigy 75 points
    Hi again,

    I have narrowed it down to an IO pin on the load MCU getting a 5volt input signal. So the path is through the MCU, I thank you for the explanation and the hints that put me on the right track.

    Thank you,

    Adam