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PASS TRANSISTOR OPTION FOR VOLTAGE ERROR AMPLIFIERS

IP.com Disclosure Number: IPCOM000005810D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2001-Nov-08
Document File: 2 page(s) / 97K

Publishing Venue

Motorola

Related People

Richard Meador: AUTHOR

Abstract

A voltage error amplifier is commonly used in a feedback circuit such as a 5 Volt regulator to compare a reference voltage with a sampled output voltage. The purpose of the error amplifier is to provide sufficient loop gain to minimize the error voltage (i.e., voltage difference between the reference voltage and the sampled voltage). The most common error amplifier consists of a differential amplifier with a large pass transistor in series with the output as shown in the figure below: Integrated Circuit Ref Voltage Figure A The Fig. A circuit combination is widely used in circuit design. For IC implementation, a problem exists in integrating the output stage of the error amplifier. The PNP pass transistor is required to operate over a wide output load varying from a new milliamps to over 1 amp. For most IC processes, a high performance on-chip PNP device is not available to the circuit designer. [An integrated PNP transistor is constructed in a later con- figuration which severely limits the DC current gain. To construct the PNP in avertical configuration would re- quire special processing which would be cost prohibitive.] Therefore, the circuit designer must use a vertically-oriented external PNP which is capable of operating at high currents with good current gain. [Note that using an on-chip NPN transistor for the output device would decrease the useful voltage range of the error amplifier.] The ext PNP transistor is shown in Fig. A.

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@ MOlOROLA

Technical Developments Volume 9 August 1989

PASS TRANSISTOR OPTION FOR VOLTAGE ERROR AMPLIFIERS

by Richard Meador

   A voltage error amplifier is commonly used in a feedback circuit such as a 5 Volt regulator to compare a reference voltage with a sampled output voltage. The purpose of the error amplifier is to provide sufficient loop gain to minimize the error voltage (i.e., voltage difference between the reference voltage and the sampled voltage). The most common error amplifier consists of a differential amplifier with a large pass transistor in series with the output as shown in the figure below:

--

Integrated Circuit

Ref Voltage

Figure A

   The Fig. A circuit combination is widely used in circuit design. For IC implementation, a problem exists in integrating the output stage of the error amplifier. The PNP pass transistor is required to operate over a wide output load varying from a new milliamps to over 1 amp. For most IC processes, a high performance on-chip PNP device is not available to the circuit designer. [An integrated PNP transistor is constructed in a later con- figuration which severely limits the DC current gain. To construct the PNP in avertical configuration would re- quire special processing which would be cost prohibitive.]

   Therefore, the circuit designer must use a vertically-oriented external PNP which is capable of operating at high currents with good current gain. [Note that using an on-chip NPN transistor for the output device would decrease the useful voltage range of the error amplifier.] The ext PNP transistor is shown in Fig. A.

   With the new BiCMOS IC technology, the opportunity to integrate the pass-transistor using an p-channel FET became a possibility. Circuit simulations showed that a large on-chip FET (15 X 20 mils) could source up to 50 mA of output current with a Vgs of less than 5 Volts. Operation at this current level is acceptable for many applications such as Portable Products but proves unacceptable for applications such as Mobile Products which typically require higher current sourcing. The ci...