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P-channel MOSFETs (pFETs), though more expensive than pnp transistors, are free of the dissipation 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!
Maxim > App Notes > Amplifier and Comparator Circuits Voltage References

Keywords: P-FET, linear regulator, low dropout, p-channel, MOSFETs, amplifiers, comparator Jul 09, 1998

APPLICATION NOTE 103
pFET Linear Regulator Has Low Dropout Voltage


P-channel MOSFETs (pFETs), though more expensive than pnp transistors, are free of the dissipation 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%

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