P-channel MOSFETs (pFETs), though more expensive than pnp transistors, are free of the dissi
pation loss associated with base drive in a pnp circuit. pFETs also have a lower saturation voltage at light loads (Figure 1). In fact, the low RDS(ON), logic-level, p-channel power MOSFETs currently available can regulate 5V from a 5.1V battery!
Figure 1. A p-channel MOSFET (Q1) allows this linear regulator to operate with VIN to VOUT differentials lower than 100mV.
This capability lets the Figure 2 circuit derive 5V ±10% from four battery cells, even when they've discharged a
s low as 4.6V. Low dropout voltage lets the circuit "ride down" on the declining battery voltage, finally falling out of regulation at approximately 4.6V. The low input-to-output differential at that time (0.1V) allows nearly 100% efficiency. Note that the output accuracy of IC1 (±0.6% over temperature) makes it suitable as a 2.5V system reference.
Figure 2. At low output current, these p-channel MOSFETs exhibit low source-to-drain voltage (i.e., dropout voltage in the Figure 1 circuit).
IC2's pin-programmable bias current makes possible a low-power mode in which the entire circuit draws less than 50µA. Five milliamps is available in this mode for circuitry such as backup RAM and a real-time clock. In high-power mode, the regulator can deliver 1A.
The 100µF output capacitor (C1) is chosen to accomodate the maximum load currents of 1A; for lighter loads you can scale the capacitor to a smaller value. Beware, though-the circuit's loop stability depends on lag compensation in which 1/2πRESRC1 > 14kHz, where RESR is C1's equ
ivalent series resistance. (Figure 1 recommends acceptable capacitor types for C1.)
Configured for a 5V output (R1 = 100kΩ), the circuit can deliver 500mA from five cells producing 7.5V, or 1A from four cells producing 6V. Configured for 3V (R1 = 20kΩ), it delivers 500mA from four cells producing 6V, or 1A from three cells producing 4.5V. The input voltage can range from 3V to 15V, subject to a limitation; with no heatsink on Q1, the MOSFET's package-dissipation rating limits the input voltage and output current as follows: IOUT × (VIN - VOUT) < 1.25W.