Three NiCad and NiMH batteries can be used to produce 3V/3.3V supply voltage. This can be done by this linear regulator circuit. This circuit uses an ICL7611 micropower op amp and MAX872 voltage reference. This regulator can be used to replace the charge pump or a switching regulator. The dropout characteristics of this circuit depends on the characteristics of Q1. The Q1 must have a gate-threshold voltage below the lowest battery voltage when this circuit is used with low voltage like a three-cell battery. Here is the schematic diagram of the circuit:
This circuit requires input voltage from 3V to 15V. This circuit has two mode, high power mode and low power mode that can be selected by logic at the MODE SELECT input. With Vin 6.5V, the quiescent current is 70µA when operated in high power mode and decrease to 40µA when it is used in low power mode. This circuit has maximum load power of 5mA in low power mode and 1A in high power mode.