Use an inductor with appropriate inductance.
l
Use a diode of a Schottky type with high switching speed, and also pay attention to its current capacity.
l
Do not use this IC under the condition at V
IN
voltage less than minimum operating voltage.
P
The performance of power source circuits using these ICs extremely depends upon the peripheral circuits.
Pay attention in the selection of the peripheral circuits. In particular, design the peripheral circuits in a way that the
values such as voltage, current, and power of each component, PCB patterns and the IC do not exceed their
respected rated values.
n
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>
i1
I
OUT
L
V
IN
Lx Tr
SD
i2
CL
ton
T=1/fosc
Step 1 : LxTr 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 LXTr.
Step 2 : When LxTr 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.
l
Discontinuous Conduction Mode and Continuous Conduction Mode
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 when it is OFF.
The difference between ILmax and ILmin, which is represented by
DI
;
DI
= ILmax – ILmin = V
OUT
´
topen / L = (V
IN
-V
OUT
)´ton/L×××Equation 1
wherein T=1/fosc=ton+toff
duty(%)=ton/T´100=ton´fosc´100
topen
£
toff
In Equation 1, V
OUT
´topen/L
and (V
IN
-V
OUT
)´ton/L are respectively show 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.
In the continuous mode, when Equation 1 is solved for ton and assumed that the solution is tonc,
toff
V
OUT
ILmin
topen
<Current through L>
ILmax
12345
Rev. 1.11
-8-
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
