Browse Prior Art Database

LOW DROP OUT MOSFET VOLTAGE REGULATOR

IP.com Disclosure Number: IPCOM000006204D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2001-Dec-13
Document File: 3 page(s) / 118K

Publishing Venue

Motorola

Related People

Christopher Brice: AUTHOR

Abstract

This form of regulator is novel in its use of a MOSFET transistor as its high current control ele- ment. The design of this type of regulator has only recently been possible due to the emergence of low channel resistance MOSFETs. This form of regulator is not directly equivalent to the standard BJT series regulator design, as the BIT regulator is current con- trolled, whereas the MOSFET regulator is voltage controlled.

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@ MOTOROLA INC.

Technical Developments Volume 13 July 1991

LOW DROP OUT MOSFET VOLTAGE REGULATOR

by Christopher Brice

  This form of regulator is novel in its use of a MOSFET transistor as its high current control ele- ment. The design of this type of regulator has only recently been possible due to the emergence of low channel resistance MOSFETs. This form of regulator is not directly equivalent to the standard BJT series regulator design, as the BIT regulator is current con- trolled, whereas the MOSFET regulator is voltage controlled.

  The major advantage of this type of regulator is that it can offer series voltage regulation with a very low dropout voltage. The usual voltage headroom required for a standard BIT regulator (i.e. LM317T) being 2.2volts and that for a high current low dropout regulator (i.e. LM2941) being 1 volt a further 0.7volts has to be added if series pass transistors are used. Whereas the dropout voltage for the MOSFET regula- tor with similar design characteristics is 0.15Volts. Under battery powered condition in the base station, this would allow the MOSFET regulator a few more hours of valid operation over its BIT equivalent.

  The only possible equivalents that can offer a sim- ilarly low dropout voltage are switching regulators and shunt regulators. I have ruled these two other forms of regulators out for the following reasons. Switching regulators will produce harmonics which would cause beat frequencies, they also physically require a large volume. A shunt regulator although it could meet the electrical requirements at worse case would be dissi- pating 50 watts compared to the 6.4 watts of the MOS- FET.

  Also as the MOSFET requires a very low control current, its control elements are much less sensitive to self heating. The voltage controlled characteristics of MOSFET regulator can theoretically allow the circuit to regulate even below its dropout voltage. A Figure 2 has been included so the output characteristics of the two types of regulators can be compared. My con- cluding note would be that this design allows the regu- lator to operate beyond the range of previous designs without sacrificing operational efficiency.

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OPERATION OF TiIE BASIC FORM OF MOSFET REGULATOR

The notes on the operation of this design refer to diagram figure 1.

  The maximum power dissipation and dropout voltage are governed by the choice of MOSFET. The maximum operating frequency is mainly influenced by the choice of operational amplifier (though it is influenced also by the MDSFET). The choice of oper- ational amplifie...