R1221N Datasheet, PDF (8/21 Pages) RICOH electronics devices division – Step-down DC/DC Converter with Voltage Detector

l Use capacitors with a capacity of 22mF or more for VOUT Pin, and with good high frequency characteristics such
as tantalum capacitors. We recommend you to use capacitors with an allowable voltage which is at least twice as
much as setting output voltage. This is because there may be a case where a spike-shaped high voltage is
generated by an inductor when an external transistor is on and off.
l Choose an inductor that has sufficiently small D.C. resistance and large allowable current and is hard to reach
magnetic saturation. And if the value of inductance of an inductor is extremely small, the ILX may exceed the
absolute maximum rating at the maximum loading.
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 VIN 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 Circuit>
<Current through L>
i1
ILmax
L
VIN
Lx Tr
IOUT
VOUT
ILmin
topen
i2
SD
CL
ton
toff
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), sith 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 =VOUT´topen/L=(VIN-VOUT)´ton/L×××Equation 1
wherein T=1/fosc=ton+toff
duty(%)=ton/T´100=ton´fosc´100
topen£toff
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Rev. 1.11
-8-