Hi,
I'm working on a little project where I need to support powering my circuit from three different voltage sources:
- an internal 12V-Li-Ion battery (always there)
- the charger for the internal battery (can be disconnected)
- an externally connected 12V battery (can also be disconnected)
To do this, I'm using an LTC 4417 power path controller, which allows me to specify the priority of the different voltage sources. To power the device on and off, I'm using the shutdown line of the LTC 4417. So far, this works very well.
But in addition to this switchable power, I also need a little bit of stand-by power (at most 100mA if all the LEDs are on), to power a battery gauging circuit and some logic things that should always work. To this end I used a simple diode-OR circuit using three LSM115J schottky-diodes connected directly to the three inputs. Now there seems to be a problem, because within a few seconds, the input capacitors on the inputs where nothing is connected get charged by (i assume) the current leaking in reverse through the diodes. Then, the potential on these inputs gets high enough that the LTC4417 (and my logic stuff) thinks, that there is some voltage source connected, even tough it's not.
Now, I noticed, that the max. reverse leakage current on the LSM115J is quite high (10mA). But even if i choose a different diode, this will only delay the build-up of the potential on non-connected inputs, but eventually they will still be recognized as present..
Are there better solutions than the OR circuit, that don't require a much lager footprint on the pcb than the 3 diodes in a DO-214BA package?
For now, I removed the diodes for all inputs except the internal battery, so the stand-by power is always provided by the internal battery. But i fear, that if the battery is completely empty, the logic stuff won't work any more, and then the charger-detection logic would not connect the charger thourgh to the battery...