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TPS77633

Brian Cavanagh97
Prodigy 190 points
Other Parts Discussed in Thread: TPS776

Hi, I'm using the TPS77633D (SOIC8) LDO, 5Vin and 3V3 output.

Unfortunately, I'm getting 4.3VDC out instead of 3V3.

The attached schematic shows Pins 7 and 8, (Pin 7 FB/NC) and (Pin 8 RESET'/PG) not connected.

After further reviewing the datasheet I placed a 250k resistor from Pin 8 to Pin 6.  Still the voltage remains at 4.3VDC out the output.

Can someone help me determine what I'm missing here?

Thanks,

Brian

7181.TPS77633D.pdf

  • 0
    TI__Guru 55940 points
    Hi Brian,

    Often when the output of an LDO is above the intended set value there is an unintentional leakage path or an external bias on the output. Due to topology, TPS776 does not sink current and will not pull down the over voltage. Are you able to isolate the output from downstream devices? If so, is the output still at 4.3V?

    If you do not use PG, then you do not need to place a pullup resistor. Your original design should work as the use of the PG functionality does not impact the output voltage.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 190 points

    Hi Ryan, you were correct.  I cut the traces, see attached, and only connect to the 10uF capacitor, now my voltage is 3V3.  Can I place a schottky diode on the output of the 3V3 to prevent the biasing you mentioned?  I attached my circuit the 3V3 is feeding.Thermo Section.pdf

  • 0 in reply to Brian Cavanagh97
    TI__Guru 55940 points
    Hi Brian,

    A Schottky can be used to block the biased voltage; however, for the best effect you will need to isolate where exactly the bias is coming from. This voltage bias will not actually cause any issues with the LDO as Vin is above the biased voltage (no reverse current). Therefore, you may first want to determine if this biased voltage is causing any issues with anything else on the rail. These (if anything) are what will need the diode protection.

    Very Respectfully,
    Ryan