I am attempting to build a simple, mini USB UPS for my Rasberry Pi 3 to survive sudden, short-term voltage drops. I have built the working circuit below on a breadboard.
When the 5V supply is removed and the voltage level (from supercaps) drops to 4.3V (between Multi+ and Multi-), the circuit does not recover when the 5V supply returns.
At 4.4V and above, the circuit recovers and the supercaps recharge back to ~5.35V. If the circuit is stuck at 4.3V or below, and I remove the USB power to the RPi, the circuit recovers and the s-caps recharge to ~5.35V.
What am I missing or doing wrong, and/or how can I fix? Why wont the system bounce back to ~5V after the voltage level drops to 4.3V or below? Thank you in advance!
Update: schematic updated based on answer 13 JUN 17