Browse Prior Art Database

LOW DROPOUT REGULATOR

IP.com Disclosure Number: IPCOM000008133D
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-21
Document File: 2 page(s) / 86K

Publishing Venue

Motorola

Related People

John Pigott: AUTHOR

Abstract

A common requirement in systems is to generate a regulated supply with a small minimum differential voltage between the input and regulated voltages. It is also a requirement for the regulator to regulate even with a very high input voltage. Thus a high- voltage transistor must be used in the input stage.

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MOTOROLA Technical Developments

LOW DROPOUT REGULATOR

by John Pigott

  A common requirement in systems is to generate a regulated supply with a small minimum differential voltage between the input and regulated voltages. It is also a requirement for the regulator to regulate even with a very high input voltage. Thus a high- voltage transistor must be used in the input stage.

  Two approaches have been used previously. One uses a high-voltage n-channel MOSFRT as the regulating element (Figure 1). the other uses a high- voltage p-channel MOSFET.

Fig. 1 N-Channel Regulator

  The n-channel regulator requires a charge pump which can supply the DC bias current for the gate drive load. In addition, this constant DC load reduces the maximum voltage that can be supplied to the gate of the driver device, raising the on-resis- tance of this device.

  The p-channel regulator does not require a charge pump, but does require a high voltage p-channel pass device. This device is not available in all technologies, and when it is available, its a real efficiency (R,,,,*Area product) is not very good.

Fig. 2 P-Channel Regulator

In addition, its use requires an opamp output

that can be driven as high as the input voltage. This requires a wide bandwidth output stage to maintain regulation during fast input voltage changes.

  Because both these regulators require that the opamp drive a large high-voltage transistor, it is dif- ficult to keep the internal loop bandwidth high in these circuits. This means that for stability, the dominant loop pole must be at a relatively low frequency. This r...