Fuentes Lineales Ns

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National Semiconductor

Chester Simpson Member of Technical Staff Power Management Applications

Linear and Switching Voltage Regulator Fundamentals
Abstract
This paper will enable the user to understand the operation of switching and linear voltage regulators. The most commonly used regulating modes will be covered. For linear regulators, the Standard, Low-Dropout, and QuasiLow-Dropout regulators will be covered (along with circuit examples). In the switching regulator section, the Buck, Buck-boost, Boost, and Flyback topologies will be detailed. Some examples will be given of products available for the design and implementation of switching converters.

LINEAR VOLTAGE REGULATORS
Introduction
The linear regulator is the basic building block of nearly every power supplyused in electronics. The IC linear regulator is so easy to use that it is virtually foolproof, and so inexpensive that it is usually one of the cheapest components in an electronic assembly. This paper will present information that gives the user greater understanding of how a linear regulator works, and will help to de-mystify regulator specifications and applications. Some typical circuits will bepresented to highlight the commercial regulators that are currently available. The primary focus of the new product examples is in the area of Low-dropout regulators, which offer great advantages over standard regulators in many applications. 1

Linear Voltage Regulator Operation
Introduction
Every electronic circuit is designed to operate off of some supply voltage, which is usually assumedto be constant. A voltage regulator provides this constant DC output voltage and contains circuitry that continuously holds the output voltage at the design value regardless of changes in load current or input voltage (this assumes that the load current and input voltage are within the specified operating range for the part).

The Basic Linear Regulator
A linear regulator operates by using avoltage-controlled current source to force a fixed voltage to appear at the regulator output terminal (see Figure 1).

Voltage-Controlled Current Source V IN

I(v)

V OUT VSense Sense/Control Circuitry I LOAD R LOAD

FIGURE 1. LINEAR REGULATOR FUNCTIONAL DIAGRAM

The control circuitry must monitor (sense) the output voltage, and adjust the current source (as required by the load) to holdthe output voltage at the desired value. The design limit of the current source defines the maximum load current the regulator can source and still maintain regulation. The output voltage is controlled using a feedback loop, which requires some type of compensation to assure loop stability. Most linear regulators have built-in compensation, and are completely stable without external components.Some regulators (like Low-Dropout types), do require some external capacitance connected from the output lead to ground to assure regulator stability. Another characteristic of any linear regulator is that it requires a finite amount of time to "correct" the output voltage after a change in load current demand. This "time lag" defines the characteristic called transient response, which is a measureof how fast the regulator returns to steady-state conditions after a load change. 2

Control Loop Operation
The operation of the control loop in a typical linear regulator will be detailed using the simplified schematic diagram in Figure 2 (the function of the control loop is similar in all of the linear regulator types).

V IN

PASS DEVICE Q1 ERROR AMP R1

V OUT IL

RL V REF R2FIGURE 2. DIAGRAM OF A TYPICAL LINEAR REGULATOR

The pass device (Q1) in this regulator is made up of an NPN Darlington driven by a PNP transistor (this topology is a Standard regulator, as detailed in the following section). The current flowing out the emitter of the pass transistor (which is also the load current IL) is controlled by Q2 and the voltage error amplifier. The current through the...