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Selecting the right linear regulator

Posted: 16 Oct 2006 ?? ?Print Version ?Bookmark and Share

Keywords:power management? linear regulator? npn regulator? pnp regulator? FET regulator?

Each linear regulator type has advantages and disadvantages. Ultimately, it is the designer who determines whether a certain type is appropriate for the application based on its dropout voltage, ground current and stability-compensation method requirements.

A linear regulator's pass element establishes its dropout voltage and ground current, which determine the types of applications for which the regulator is suitable. Each of the five main linear regulators used today has a different pass element and unique characteristics that make it suitable for a given application.

The standard npn regulator has the advantages of steady ground current roughly equal to the base current of the pnp transistor, along with unconditional stability without an output capacitor. It is useful in applications where the I/O voltage differential is high. However, its high dropout voltage makes it undesirable for embedded applications.

For embedded applications, the npn pass transistor regulator is a good choice because of its lower dropout voltage and ease-of-use. However, it does not have a low enough dropout voltage for battery-powered applications with lower voltage-differential budgets. Its high-gain npn pass transistor results in a steady ground current of typically several milliamps. Moreover, its common collector configuration has low output impedance.

The pnp pass transistor is a low-dropout regulator with a pnp transistor as its pass element. Its dropout voltage is usually between 0.3V and 0.7V. Because of its low dropout voltage, the pnp pass transistor regulator is useful for battery-powered embedded applications. Its high ground current, however, shortens battery life. Additionally, the pnp transistor has low gain, resulting in unsteady ground current of typically several milliamps, and it has a high output impedance because of its common emitter configuration. This means that an output capacitor within a certain range of capacitance and equivalent-series resistance (ESR) is required for stability.

FET option
Because of its low dropout voltage and ground current, the p-channel FET regulator is popular for many of today's battery-powered devices. This type of regulator uses a p-channel FET as its pass element. The regulator's dropout voltage is low because the drain-source resistance is easily adjusted to a low value by sizing the FET. It has low ground current, due to the low "gate current" of the p-channel FET. However, because of the p-channel FET's relatively high gate capacitance, it requires an output capacitor with a certain range of capacitance and ESR for stability.

The n-channel FET regulator is suitable for applications requiring low dropout voltage, low ground current and high load current. An n-channel FET is used as a pass element, and the regulator's dropout voltage and ground current are low. It requires an output capacitor for stability, but the capacitance can be low, and the ESR is not critical. N-channel FET regulators require a charger pump to establish the gate bias voltage, resulting in a more complex circuitry. Luckily, an n-channel FET is up to 50 percent smaller than a p-channel FET with the same load current.

- Qi Deng
Sr. Applications Engineer, Microchip Technology Inc.

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