I wish to allow powering my circuit using at least one of 3 connectable periphals. For example, my application could have 3 usb ports that each connect to different usb hosts. Any combination with at least one connected periphal should be able to power the circuit, but obviously no current is allowed to flow back into any of the 3 supplies. The circuit should be able to draw 200mA at 5v.
Unfortunately, the voltage drop even from schottky diodes is unacceptable, so OR-ing diodes is not an option.
I've come across a lot of power mux IC's, such as the the LTC4412[1] and TPS22933a[2]. The LTC4412 still requires a diode at one of the power sources, and is only a 2-input solution. Still quite far from ideal. The TPS22933a seems like a better component for me due to the 3 inputs, but the maximum output current is rated at only 50mA (doesn't that make this thing utterly useless?).
Many of these power muxes offer niceties that I do not need:
- no need for any kind of load balancing between the power sources
- no need for a wide range of input voltages. All of them will be 5v.
- no need to source from the highest available voltage source. Bias is fine.
- not looking at a 3-input solution per se, expandability to n-inputs is desirable.
Given this, what kind of solution should I be looking at? Is there an IC that nicely accomodates my (seemingly simple) needs?
Best Answer
I believe you can use the LTC4412 for your purpose, specifically, three of them, one for each power input. Please look at figure 5 in the datasheet. This shows two of them doing load sharing for two inputs with a diode connecting a third input. Clearly, we can remove the diode and third input (equivalent to the "wall adapter" not being connected). This leaves two 4412s and MOSFETs acting as ideal diodes for two inputs. Isn't it clear that you can simply add a third 4412 and MOSFET for a third ideal diode and input?