SWITCHING BETWEEN 2 INPUT SOURCES AND LM2596

Perico Eldelospalotes

Other Parts Discussed in Thread: LM2596, TPS54540

Hi.

I want to select between two 5V power sources. One of the sources is 12V from a battery and a LM2596 regulator. The other source is direct from an USB port. I think a solution is the circuit I have attached. The circuit gives preference to the 12V+LM2596 option, so when both are present, the system is powered from this option. It seems to work ok, but I have a doubt about the reverse voltage on the LM2596 regulator. When only the USB 5V are present, the regulator is reverse biased, as 5V are present at its output and the input is grounded via the PNP base resistor. Is this a problem or the LM2596 will not be damaged?

The circuit schematic is attached as a Word file. Thank you in advance.

4150.Doc1.docx

over 10 years ago

0 Frank De Stasi over 10 years ago

TI__Guru 72145 points

I think that there may be a with a sneak path from the output of the regulator back to the input. This may tend to turn the PNP off. Also, you a correct about not having an output on the regulator with no input. Although some regulators may be OK, this device is very old and it is not known how it will react. You may place a diode in series with the regulator output and take the feedback from the cathode side. In any case you will need to test this very carefully.

0 Perico Eldelospalotes over 10 years ago in reply to Frank De Stasi

Intellectual 505 points

Hi.
Thank you very much for your help. Do you think it will work wih more modern regulators? What about with the TPS54540?

0 JohnTucker over 10 years ago in reply to Perico Eldelospalotes

TI__Guru**** 191445 points

Even with TPS54540, there will be a leakage path back to the input. I think you will need to actively control both 5V and 12V.

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SWITCHING BETWEEN 2 INPUT SOURCES AND LM2596