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[General LDO] How can I connect USB power supply and a battery (LDO)?

Nuno Dias
Prodigy 190 points

I want to make a PCB board that can be powered via the USB connection or with an external power supply. The question is how I can make the power source is selection automatically?

I have not picked any voltage regulator but I intend to use a LDO (5V / 500mA)

Can i connect the USB-VCC directly to the LDO output when I don't have any type external supply connected to my board (Figure 1)? Or I need to use a jumper or a switch to disconnect the LDO output (Figure 2)?

Thanks

  • 0
    Prodigy 130 points

    I would advise to pick an LDO with reverse current protection since ordinary PMOS LDOs have an internal body diode which conducts whenever Vout > Vin.

    Best regards

    Pawel

  • 0 in reply to Pawel Holeksa
    Prodigy 190 points

    Thanks for your repply.

    I search some information about this theme and I found this:

    Reverse-Current Protection Feature: A typical LDO with a PMOS pass device has an intrinsic body diode between VIN and VOUT. When VIN is greater than VOUT, this diode is reverse-biased. If VOUT is greater than VIN, the intrinsic diode becomes forward-biased and conducts current from VOUT to VIN, potentially causing destructive power dissipation. Some LDOs have additional circuitry to protect against reverse current flow from VOUT to VIN. The reverse current protection circuitry detects when VOUT is greater than VIN and reverses the direction of the intrinsic diode connection, reverse-biasing the diode.

    I was thinking to use an LDO with an a small 3-pin encapsulation, such as SOT-233, but i think there aren't this kind of LOD with this protection.

  • 0 in reply to Nuno Dias
    Prodigy 190 points

    Hi guys,

    I can insert a diode between VOUT pin and VIN pin as a protection circuit. Therefore, if VOUT is higher than VIN, this  diode conducts in forward direction.

    Usually, the LDOs with Reverse Current Protection has a built-in diode to stop the reverse current from VOUT pin to VIN or GND pins when VOUT becomes higher than VIN.

  • 0 in reply to Nuno Dias
    TI__Guru 55940 points

    Hi Nuno,

    A reverse current protection diode can be added the way you show.  Please note that the trade off is a degraded PSRR.

    Very Respectfully,

    Ryan