Ricoh RH5RH13B - Manual (Page 14)

10
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, 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 >
RH5RH
i2
L SD I OUT
V OUT
CL Lx Tr
i1 V IN
IL
ILmin
ILmax
topen
t
ton toff
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. However, in the case of a continuous mode which will be mentioned
later,the time period (toff) runs out before IL reaches ILmin (=0), so that LxTr is turned ON in the next
cycle, and SD is turned OFF. In this case, ILmin does not reach zero, and IL (=i1) increases from ILmin (> 0).
In the case of PWM control system, the output voltage is maintained constant by controlling the on-time peri-
od (ton), with the oscillator frequency (fosc) being maintained constant.
• Discontinuous Conduction Mode and Continuous Conduction Mode
In the above two diagrams, the maximum value (ILmax) and the minimum value (ILmin) of the current which
flows through the inductor are the same as those when LxTr is ON and also when LxTr is OFF.
The difference between ILmax and ILmin, which is represented by
∆
I, is :
∆
I=ILmax–ILmin=V IN · ton/L=(V OUT –V IN ) · topen/L .........................................Equation 1
wherein T=1/fosc=ton+toff
duty (%)=ton/T · 100=ton · fosc · 100
topen
≤
toff
In Equation 1, V IN · ton/L and (V OUT –V IN ) · topen/L are respectively show the change in the current at ON, and the
change in the current at OFF.