7

RS5RJ

OPERATION OF STEP-UP DC/DC CONVERTER

Step-up DC/DC Converter charges energy in the inductor when Lx Transistor (LxTr) is ON, and discharges the

energy with the addition of the energy from Input Power Source thereto when LxTr is off, so that a higher output

voltage than the input voltage is obtained.

The operation will be explained with reference to the following diagrams:

< Current through L >

< Basic Circuits >

i2

SD

i1

L

CL

IL

IL min

IL max

topen

ton

toff

t

T=1/ fosc

Step.1 : LxTr is turned on and current IL (= i1 ) flows, so that energy is charged in L. At this moment, IL(=i1 ) is

increased from ILmin (= 0) to reach ILmax in proportion to the on-time period (ton) of LxTr.

Step.2 : When LxTr is turned off, Schottky diode (SD) is turned on in order that L maintains IL at ILmax, so that

current IL (= i2) is released.

Step.3 : IL (=i2) is gradually decreased, and in the case of discontinuous mode, IL reaches ILmin (=0) after a time

period of topen, so that SD is turned off.

In the case of the VFM control system, with the on-time period (ton) maintained constant, the output voltage

is maintained constant by controlling the oscillator frequency (fosc).