5V power sharing

[Richard Lewis]

I've run into issue where I've had 2 (or more) competing 5V supply sources on a single card (USB, FTDI, RC2014 bus, etc) and was worried about blowing something up. Obviously solution is to use a Schottky diode, crowbars and/or jumpers to prevent this from happening. 

One solution is to let the 5V supplies play together with a diode OR circuit. Problem is that: 1. there is a voltage drop across the diodes (of course there are Schottky's with very small forward voltage drops), 2. Diode OR only works if both power supplies are at almost exactly the same voltage and that the diodes are well matched. Otherwise 1 supply will end up supplying all of the current. 

I've been playing around with MOSFET ideal diodes and ran across this chip: LTC4370CMS which does exactly what I need. Takes 2 supplies and balances the load between them. So I can plug in a USB, JTAG or FTDI cable while the card is plugged into the bus and instead of blue smoke  (a bad thing) I get extra current  (a good thing) and I don't get 1 supply feeding back into the other. I've taken one of the application schematics from the data sheet and turned it into a PCB that is available on OSH Park: 5V Load balance. The board is around 9cm^2

KiCad source is here: 5V load balancer

[Richard Lewis]

Got the board back yesterday from JLPCB, assembled  the 250mw (SO-232 MOSFETS) design and tested it at 5V with a 50ohm load. The range resistor is setup to trigger at +/- >6% of the supply voltage difference. The current load is equally balanced between the 2 supplies until the trigger point is crossed . 

In this case I increase 1 side to 5.3V which is at the trigger point and so side1 is now suppling 70% of the current. 5V on output is being maintained. 

In the last example 1 supply drops out completely. The other supply provides all the load and the "sharing off" LED is illuminated:

 

Will use this in my FPGA design that has both a USB JTAG and FTDI interface as well as the 5V signal coming for the RC2014 bus. Only downside (and a big one) is that this chip brand new (i.e. no Chinese clones yet) so it is $11 each :-(

-Richard