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LM338: power circuit from 12V (through LM338) or from 5V from USB connection

Rudy Moore
Prodigy 10 points
Part Number: LM338

I'd like to be able to power my circuit from either a 12V source or from a 5V USB connection. How would you recommend connecting to the 5VUSB supply?

Thanks!

  • 0
    TI__Genius 12511 points

    Hi Rudy,

    I'm a little confused by your question. If you would like to connect a 5V supply on the input, we recommend 1uF capacitor to help as a bypass cap.

    What exactly are you asking about for the device? What Iout/Vout are you looking for?

    Regards,

    John

  • 0
    Prodigy 10 points

    Sorry, I wasn’t clear. My Vin is 12V and Vout is 5V. I would like to be able to power the Vout side with a USB 5V.  I was thinking of just using a diode from USB 5V to my Vout bus. It’ll end up less than 5V but I wasn’t sure what the LM338 would do if back fed that way…

  • 0 in reply to Rudy Moore
    TI__Genius 12511 points

    Hi Rudy,

    So you are powering a 5V rail with a USB 5.0V supply, and you also want the LDO to supply the 5V rail if I'm interpreting you correctly. The LDO should be configured using the ADJ pin and the VOUT pin with a resistor divider.

    Here is the 5V constant regulator from the datasheet:

    If you are powering the 5V rail with an additional source, the 5V may have contention. In this case the LDO is not capable of sinking current. To protect the LDO from a reverse current condition, using a Schottky diode from out to in will protect the device in case the input falls and the device tries to sink current.

    Regards,

    John