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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 103
Maxim > Design Support > Technical Documents > Application Notes > Voltage References > APP 103
Keywords: P-FET, linear regulator, low dropout, p-channel, MOSFETs, amplifiers, comparator
APPLICATION NOTE 103
pFET Linear Regulator Has Low Dropout Voltage
Jul 09, 1998
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 R
DS(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 V
IN
to V
OUT
differentials
lower than 100mV.
This capability lets the Figure 2 circuit derive 5V ±10% from four battery cells, even when they've
discharged as 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.
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