I’ve been investigating ways to handle multiple supplies and consumers of +5V 500mA power in a single project, and it’s surprisingly complicated. My circuit may have up to three simultaneous external connections: a device needing +5V power at “A”, an external +5V supply at “B”, and something at “C” that might either supply +5V power or need +5V provided to it, depending on what mode the circuit is operating in. Any of these three could be present or absent, and the external +5V supplies could possibly be connected but turned off and not delivering power.
The simple solution would be to directly connect all of those +5V lines within my circuit. That would work, and maybe it’s what I’ll do, but it presents some risks of unintended current backflow into a supply. Imagine if B and C are both connected, with C configured as a supply. If one is turned off, the other would feed +5V into it. Depending on what that supply is and how it’s designed, feeding external +5V back into a device that’s turned off may cause weird behavior or damage. A similar problem could arise if a poorly-designed device on A actually powers the +5V line instead of drawing power from it, because it has batteries or an independent supply of its own.
I’m willing to handle the bidirectional nature of C with a physical switch. But I still need something that ensures current only flows in the intended direction in/out of A, B, and C, as shown in the diagram above. The ground connections aren’t shown, but they are all tied together. Current direction arrows mean diodes, right? I think diodes would theoretically work, but probably aren’t the best choice. A diode would cause a substantial voltage drop between the supply and whatever it was powering, and you need a diode that’s physically large in order to handle significant amounts of current.
A better solution might be a transistor. From studying similar circuits, a good choice might be a power MOSFET. These have a very low resistance when switched on, resulting in only a small voltage drop. A small-sized MOSFET could also handle all the 500mA current I need for this project. But I don’t have any experience working with single MOSFETs in this kind of application. I get a bit bogged down in discussions of N and P type, enhancement and depletion, high-side and low-side switching, and so forth. I’m not completely confident I’d know which specific MOSFET to select for my purpose, and how to connect it in the circuit, and what limitations or unexpected behaviors it might have.
An even better and simpler solution might be a power distribution switch IC, like the Microchip MIC2005A. This is basically a MOSFET, but one that’s designed for this specific purpose, with some extra features like soft start, current limiting, and thermal protection. And in the SOT-23 package I’d likely use, it’s not much more expensive than a generic MOSFET.
Power distribution ICs are designed to conditionally enable a power supply, and they have an /ENABLE pin to determine when to energize OUT from the supply at IN. But I would be using them always enabled, just to get the reverse current protection feature. Maybe that means this is the wrong tool for the job, and a different solution would be better?
A second question is whether a power distribution IC like the Microchip MIC2005A actually provides reverse current protection. It has terminals labeled IN and OUT, but it’s not clear if anything prevents current flowing from OUT to IN if OUT is energized by something else. I skimmed the datasheet, but couldn’t find any mention of this.
A final question is whether connecting the OUT terminals of several MIC2005A chips would cause problems. That’s what I’d need to do, if both B and C might function as +5V supplies. I can imagine this might cause a problem if one of the MIC2005A chips finds its OUT terminal at +5V even though its IN terminal is at 0. Or it might be a problem if both B and C were turned on at the same time, both delivering “5 volts”, but one was 4.9V and the other 5.1V. I didn’t see anything in the datasheet that addresses this either.
I’m trying very hard to keep the total cost of this solution low, like 50 cents or less. There are more complex power management ICs that might simplify things, but cost $2 or more apiece. Add the cost of assembly, and a retail markup sufficient to get acceptable margins, and the final sales price of the product would have to be increased by an unacceptable amount. I would probably choose to eliminate functionality instead, or just use the “connect all +5V supplies” approach, before I made a significant increase in product price just to support power management.
My diagram shows five current direction arrows, so potentially I would need five MOSFETs or MIC2005As. But I don’t actually need one on the line to the “circuit” cloud, because there’s no possibility of current flowing the wrong way. And the two arrows at C could probably be collapsed into a single arrow, if I used a DPDT switch that reversed the connections to the MOSFET or MIC2005A terminals. And I maybe don’t really need the arrow at A. If a device at A were to backflow +5V power, it would turn on the circuit, and possibly also supply power to C depending on the switch setting. That would be unexpected, but I don’t think it would harm anything. Applying all those simplifications, I might be able to get away with as few as two MOSFETs or MIC2005As, squeaking under my arbitrary $0.50 budget.