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A Dual Mode Linear Voltage Regulator with Wide Load Operating Range

IP.com Disclosure Number: IPCOM000199814D
Publication Date: 2010-Sep-16
Document File: 4 page(s) / 484K

Publishing Venue

The IP.com Prior Art Database

Abstract

A voltage regulator capable of supplying both low and high current load to a System On Chip (SOC) is presented. It can operate standalone providing a current up to 60 mA, or in conjunction with an external ballast npn bipolar transistor providing up to 1 A. A novel compensation scheme allows the regulator to transition from a low load condition to a high load condition without becoming unstable.

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A Dual Mode Linear Voltage Regulator with Wide Load Operating Range

Abstract 

A voltage regulator capable of supplying both low and high current load to a System On Chip (SOC) is presented. It can operate standalone providing a current up to 60 mA, or in conjunction with an external ballast npn bipolar transistor providing up to 1 A. A novel compensation scheme allows the regulator to transition from a low load condition to a high load condition without becoming unstable.

Body 

General purpose Micro-Controller Units (MCU) can operate from either a 5V supply or a 3.3V supply, in most cases the system requires both. In order to reduce costs and simplify board design, it is desirable to have an integrated regulator capable of regulating a 3.3V supply from the available 5V rail. The regulated 3.3V supply can be used then to power internal peripherals that require some tens of mA, or even high speed I/O that require several hundreds of mA. Due to power budget limitations the internal regulator can only dissipate 100 mW, so for high current loads an external ballast device is required.

Designing a single amplifier regulator stable with or without an external ballast device is not viable, as it would require excessive power and area, or simply too expensive external components that would not justify the use of such a regulator.

However by splitting the regulator into two gain sections it is possible to design a system that is stable in the two different configurations and supports the full range load, with reasonable area and power.

Fig.1 Voltage regulator with internal driver, no external npn.

In the configuration with internal driver (no external npn) shown in Fig.1, the current load on the regulator ranges from 1 mA to about 60 mA. The open loop gain is practically constant and approximately equal to gm2/gds2 and higher than 40 dB. The two amplifiers use the same reference voltage, but gm2 has a lower feedback tap than gm1. This means that at low loads M2 will provide the initial current, e.g. up to 20 mA, while M0 and M1 will be off. The system has two poles in this configuration:

dominant pole ;

non dominant pole ,

Where gds(M2) is the output transconductance of M2, Cc is the compensation capacitance, go2 is the output transconductance of the amplifier gm2, Av2 is the gain between gate and drain of M2, or approximately gm(M2)/gds(M2).

Increasing the current load above 20 mA will cause the transistors M2 to enter triode region and the current to saturate, therefore the voltage on Vdd33 will lower  until M0 / M1 are turned on to provide the additional current. It is reasonable to assume that M0 and M1 are capable of providing equal or higher current than M2 (i.e. they are larger devices). As M2 enters triode, the poles P0 and P1 move to higher frequencies, and the relevant poles from a AC analysis point of view become

dominant pole ;

non dominant pole ,

Where gds(M0) – gds(M1) is the output transconductance of M0 and M1, Cc is the compensati...