R5220x

7

OPERATION of step-down DC/DC converter and Output Current

voltage than the input voltage is obtained. The operation will be explained with reference to the following

diagrams:

<Basic Circuits>

<Current through L>

Pch Tr

L

Nch Tr

i1

CL

i2

GND

T=1/fosc

ton

toff

topen

ILmin

ILmax

IL

Step 1: P-channel Tr. turns on and current IL (

=i1) flows, and energy is charged into CL. At this moment, IL

increases from ILmin (

=0) to reach ILmax in proportion to the on-time period (ton) of P-channel Tr.

Step 2: When P-channel Tr. turns off, Synchronous rectifier N-channel Tr. turns on in order that L maintains IL

at ILmax, and current IL (

=i2) flows.

Step 3: IL (

=i2) decreases gradually and reaches IL=ILmin=0 after a time period of topen, and N-channel Tr.

Turns off. Provided that in the continuous mode, next cycle starts before IL becomes to 0 because toff

time is not enough. In this case, IL value increases from this ILmin (>0).

In the case of PWM control system, the output voltage is maintained by controlling the on-time period (ton), with

the oscillator frequency (fosc) being maintained constant.

The maximum value (ILmax) and the minimum value (ILmin) of the current flowing through the inductor are the

same as those when P-channel Tr. turns on and off.

wherein, T

=1/fosc=ton+toff

duty (%)

=ton/T×100=ton×fosc×100

change of the current at "OFF".