R1224N Datasheet, PDF (10/42 Pages) RICOH electronics devices division – PWM/VFM step-down DC/DC Converter

R1224N
OPERATION of step-down DC/DC converter and Output Current
The step-down DC/DC converter charges energy in the inductor when Lx transistor is ON, and discharges the
energy from the inductor when Lx transistor is OFF and controls with less energy loss, so that a lower output
voltage than the input voltage is obtained. The operation will be explained with reference to the following
diagrams:
<Basic Circuits>
<Current through L>
i1
IOUT
IL
ILmax
VIN
Lx Tr
L
VOUT
ILmin
topen
GND
SD
i2
CL
ton
toff
T=1/fosc
Step 1: Lx 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 Lx Tr.
Step 2: When Lx Tr. turns off, Schottky diode (SD) turns on in order that L maintains IL at ILmax, and current IL
(=i2) flows.
Step 3: IL decreases gradually and reaches ILmin. after a time period of topen, and SD 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 is 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.
Discontinuous Conduction Mode and Continuous Conduction Mode
The maximum value (ILmax) and the minimum value (ILmin) current which flow through the inductor is the
same as those when Lx Tr. is ON and when it is OFF.
The difference between ILmax and ILmin, which is represented by ∆I;
∆I=ILmax-ILmin=VOUT×topen/L=(VIN-VOUT)×ton/L ................................... Equation 1
wherein, T=1/fosc=ton+toff
duty (%)=ton/T×100=ton×fosc×100
topen ^<= toff
In Equation 1, VOUT×topen/L and (VIN-VOUT)×ton/L are respectively shown the change of the current at ON, and
the change of the current at OFF.
When the output current (IOUT) is relatively small, topen<toff as illustrated in the above diagram. In this case, the
energy is charged in the inductor during the time period of ton and is discharged in its entirely during the time
period of toff, therefore ILmin becomes to zero (ILmin=0). When Iout is gradually increased, eventually, topen
becomes to toff (topen=toff), and when IOUT is further increased, ILmin becomes larger than zero (ILmin>0). The
former mode is referred to as the discontinuous mode and the latter mode is referred to as continuous mode.
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