Power Supply, two voltages

[brue]

Hi there,

I have to design a device which can be powered from two different voltages, 5V and 3.3V on different input pins and common ground pin.
The user can apply either the 5V on the 5V input or the 3.3V on the 3.3V input pin. If both voltages are present (Voltage on 5V Pin is > as 3,3V Pin), the device should take the 5V to power the device. We talk about <5 watts. Finally I need 3.3V for the controller, so the voltage drop of a simple diode in the 3,3V supply would be to high. And I need to have a regulator for the 5V. Any reverse current is not allowed.

I'm looking for a nice way to design that, any ideas?
Thanks!

[sdg]

I believe Linear makes some controllers that do precisely what you're looking for.

[jpb]

I'm not sure if I understand your requirements correctly and I don't know if you need this all to be small and well integrated, but it sounds like you just need some sort of switch operated by the 5V supply to turn off the 3.3V supply if it is there (a reed relay to avoid any diode drops in the 3.3V supply?). So my rather basic circuit would be 5.5V in to 3.3V regulator with a connection to the reed relay on the 5V side of the regulator and the 3.3V in would just be passed via the reed relay.

Perhaps reed relays are too noisy or power hungry - but it is just a thought.

Even simpler would be a mechanical switch attached to the 5V input socket (in the same way head phone jack sockets have a switch to disconnect the speakers).

[Kalvin]

LTC4415 might be something you want to take a look at. There are also other "ideal diode" products available, which may be suitable for your application.

"LTC4415 - Dual 4A Ideal Diodes with Adjustable Current Limit"
http://www.linear.com/product/LTC4415

[brue]

Hi,

Thank you for your suggestions.

Something like the LTC4415 or 4413 would fit pretty good I think, it's probably just a bit expensive solution for mass production.
The circuit has to be relatively small, I have enough space for some ICs or some transistor/FET circuits but a relay would not fit, apart from that it is not reliable enough.

Probably someone implemented something discretely?

[ajb]

There are options from other manufacturers as well.  Check out the TI TPS211x series, for instance, which is a tiny little 8TSSOP part.