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Injecting voltage into the SW pin of LM22670?

Norman Ballard
Prodigy 20 points
Other Parts Discussed in Thread: LM22670

Hi

I use an LM22670-5.0 to generate 5V from 12V.  I also have a USB input that can supply 5V.  I want to connect the USB supply to the 22670 output through a diode with cathode facing the 22670.  The idea is that the user can power the gadget from 12V or from USB and either way I see approx 5V nicely filtered by the caps on the output of the 22670.  If the switcher fails, the USB is protected by the diode.

When the 22670 is not powered via 12V, the USB supply will provide approx 5V to the SW pin on the 22670.  Will the 22670 survive this?

Thanks, Norman

  • 0
    TI__Guru 60755 points

    There is a diode betweenn the switch pin and the input pin of the LM22670. (like most regulators)

    This diode would be forward biased when the input supply is lost and the 5V from the USB is applied.

    Another diode to block this path should solve the problem.

    Frank D