Hi, I'm working on a circuit which can either be powered via a Micro-USB, or a pre-regulated 5V input. If both are present I want the 5V to come from the regulated source.

I need some help finding a simple circuit which can help me accomplish this.

I've seen one variant which just uses a diode in series with each input, but that will incur a small voltage drop which I would like to avoid.

I've also seen a couple of variants which uses opamps and p-channel mosfets, but those seem to require that the input voltage is higher than 5V.

How can I build a circuit that takes in 5V from one or two sources, and outputs 5V?

 

You can use two Schottky diodes such as 1N5817 for reduced voltage drop.

 

So, you mean like this then?

 

Correct.

 

Hi, I'm working on a circuit which can either be powered via a Micro-USB, or a pre-regulated 5V input. If both are present I want the 5V to come from the regulated source.

Here's an ideal diode IC that selects a priority input.

 

That last one was a little expensive.

What would be the simplest circuit based around a FET that could be used? I see the ones Arduino boards use normally also include a op-amp. Can this be done without the opamp?

 

LTC4412 is simpler and less expensive.

 

You might want to try this with a little diode-resistor logic:
1.5A Current Limit High Side Switch
The cost abut 24¢ U.S. per channel in single unit quantities.

If you provide more detail of your requirements it might help. For example, how much current?

 

How much current does your load draw..

 

The circuit is primarily an ESP32, two PWM controlled servos, a few leds, a MEMS accelerometer, and some IR sensors. Not sure about what the total current consumption will be, but I assume I'm under the 500mA that USB can supply.

Maybe a silly question, but why does current consumption matter?

 

I found some LTC4411 lying around here, I assume it is close enough to LTC4412 to do the same job?

 

LTC4411 is even simpler since it contains the high-side p-channel MOSFET
You don't need the 470k pullup on STAT if you are not using the STAT output.

 

Cant a relay simply be used for this circuit..
Just that I havent seen a 5vdc powered relay

 

T

The circuit is primarily an ESP32, two PWM controlled servos, a few leds, a MEMS accelerometer, and some IR sensors. Not sure about what the total current consumption will be, but I assume I'm under the 500mA that USB can supply.

Maybe a silly question, but why does current consumption matter?

To avoid any contradiction between the two voltage sources supplying more current (that is if the circuit has no current limiter) when both sources are available

 

To avoid any contradiction between the two voltage sources supplying more current (that is if the circuit has no current limiter) when both sources are available

There is no contradiction or contention. The higher of the two supply voltages will take over the load. You don't get double the current.

I still think that the dual Schottky 1N5817 diodes is the simplest solution.
At 500mA the voltage drop will be about 0.3V.

 

Current consumption is need to select the appropriate FEts/diodes. There is a difference in components needed for a 10mA switch and a 10A one. A relay could be used to switch the supplies but there would be a short time on no connect during switching that would have to be taken into consideration. Also the driving current of the relay coil. A FET switch is better.
And to answer falade47, yes, there are plenty of 5V relays around. Most relays models are available in a range of coil voltages.
For example...
http://www.mouser.com/ds/2/316/ds_61B05_en_js-469954.pdf

 

Some text removed for clarity.

Maybe a silly question, but why does current consumption matter?

Most solutions can handle a limited amount of current and we did not know how much you need.

 

If both are present I want the 5V to come from the regulated source.

To meet that requirement you can use two LTC4411's.
LTspice simulation shown below:
Note that when both voltages are high, only the Reg voltage is delivering current, even though V(Reg) is less than V(Usb).

 

So, you mean like this then?

That seems to be the idea, but you incur a small forward volt drop. Shottky Barrier is the lowest Vf I can think of (usually less than 200mV). To a small extent; an excessively large diode will drop slightly less voltage under load.

USB regulation should be pretty good - unless the current is so high you need another supply, the other supply probably isn't any better.

 

Thanks, I'll try the two-schottky-diode setup first, with a couple of STPS2L30A or SSA33L's I have lying around.