I require a voltage source which holds a large capacitor at a stable voltage. The following secenarios must be supported:
- Something is feeding current into the capacitor. The voltage source needs to sink current to keep the voltage constant
- Something is draining current from the capacitor. The voltage source needs to provide current to keep the voltage constant
I only need a moderate amount of current (a supported range of +10mA to -10mA should suffice). The voltage is very low, in the range of 1.5V.
Solutions which I've considered:
LDO
simulate this circuit – Schematic created using CircuitLab
Upsides:
- Very low quiescent current
- Stable voltage
Downsides:
- LDO outputs can't sink current, therefore this doesn't work
LDO with constant load
Upsides:
- Very stable voltage
- Works
Downsides:
- 10mA of quiescent current. This is prohibitive since the circuit is battery-operated
- Unpleasant surprises when the load current goes even slightly below -10mA
Zener diode voltage divider
Upsides:
- Low complexity, cheap
- Works
Downsides:
- Very unstable (+/- 100mV over the current range with the shown 700 Ohm resistor)
- High quiescent current (10mA with the shown 700 Ohm resistor)
This solution is inferior to the "LDO with constant load" variant: You can adjust the resistor, but then you either get even more quiescent current, or you get even more instability.
OpAmp in voltage follower configuration
Upsides:
- Easily adjustable (could be controlled by uC etc.)
- Low quiescent current
- Very stable voltage
Downsides:
- OpAmp outputs are not stable when connected to capacitive loads; therefore this doesn't work.
Any alternate ideas?