BASIC SWITCHING REGULATOR THEORY

AND APPLICATIONS

The basic circuit of a step-down switching regulator circuit is

shown in Figure 12, along with a practical circuit design

using the LM3524D in Figure 15.

The circuit works as follows: Q1 is used as a switch, which

has ON and OFF times controlled by the pulse width modu-

lator. When Q1 is ON, power is drawn from V

to the load through L1; V

reverse biased, and C

inductor L1 will force V

in it, D1 will start conducting and the load current will flow

through D1 and L1. The voltage at V

C

o

filter giving a clean DC output. The current flowing

through L1 is equal to the nominal DC load current plus

some

∆I

good rule of thumb is to set

∆I

Neglecting V

∆I

L

∆I

L

whereT=Total Period

The above shows the relation between V

cycle.

as Q1 only conducts during t

The efficiency,

η, of the circuit is:

ηMAX will be further decreased due to switching losses in

Q1. For this reason Q1 should be selected to have the

maximum possible f

times.

CALCULATING INDUCTOR L1

Since

∆I

∆I

L

o

Solving the above for L1

00865016

FIGURE 12. Basic Step-Down Switching Regulator

00865017

FIGURE 13. Relation of Switch Timing to Inductor Current in Step-Down Regulator

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