I am trying to design a power supply for portable device and stumbled upon a problem that is probably trivial, but it's a tough one for me (having no formal electrical education).
The device is powered from battery, but if I plug USB connector, the P channel MOSFET in Q1 (MF5853CS) closes and disconnects battery and the device is powered through Schottky diode in Q1. I pretty much copied the design in Microchip's AN1149 (Designing A Li-Ion Battery Charger and Load Sharing System With Microchip’s Stand-Alone Li-Ion Battery Charge Management Controller).
The design is working fine when battery is fully charged, but if it's not charged (3.8 V and lower) and I unplug the USB connector, there is a short drop in voltage to around 3.04 V which is too low for U2 (power supply supervisor CAT809, RESET is low if voltage lower than 3.08 V) and it in turn turns off my buck converter for around 140 ms and that causes other problems further down in my design. My load is approx. 100 mA
In my quest to solve this, I placed Schottky diode D1 and lowered value of resistor R18, but that introduced another problem and that is upon connecting USB power, Schottky diode in U8 gets shorted out and now conducts in both directions (diode testing with my meter show 0.19 V in one and 0.75 V in opposite direction. Why that happens is beyond me.
I could probably get around it if I connect the U2 supervisor directly to battery, but it doesn't seem right. Or maybe if I lower the inrush current for capacitor C7 with a diode and resistor would that help mitigate the drop of voltage?
And now after I wasted number of U8 components (MOSFET + Schottky) I'm more than ever curious why that happened and how to prevent that.