R1221N15AA RICOH [RICOH electronics devices division], R1221N15AA Datasheet - Page 8

• Manufacturer Part Number: R1221N15AA
• Description: Step-down DC/DC Converter with Voltage Detector
• Manufacturer: RICOH [RICOH electronics devices division]

l Use capacitors with a capacity of 22mF or more for V
l Choose an inductor that has sufficiently small D.C. resistance and large allowable current and is hard to reach
l Use a diode of a Schottky type with high switching speed, and also pay attention to its current capacity.
l
P The performance of power source circuits using these ICs extremely depends upon the peripheral circuits.
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 :
Step 1 : LxTr turns on and current IL(=i1) flows, and energy is charged into CL. At this moment, IL increases from
Step 2 : When LxTr turns off, Schottky diode(SD) turns on in order that L maintains IL at ILmax, and current IL(=i2)
Step 3 : IL decreases gradually and reaches ILmin after a time period of topen, and SD turns off, provided that
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
same as those when LxTr is ON and when it is OFF.
Rev. 1.11
as tantalum capacitors. We recommend you to use capacitors with an allowable voltage which is at least twice as
Use an inductor with appropriate inductance.
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.
magnetic saturation. And if the value of inductance of an inductor is extremely small, the I
absolute maximum rating at the maximum loading.
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.
The maximum value(ILmax) and the minimum value(ILmin) of the current which flows through the inductor are the
The difference between ILmax and ILmin, which is represented by DI ;
DI =ILmax –ILmin =V
wherein T=1/fosc=ton+toff
Do not use this IC under the condition at V
V
12345
IN
ILmin(=0) to reach ILmax in proportion to the on-time period(ton) of LXTr.
flows.
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).
duty(%)=ton/T´100=ton´fosc´100
topen£toff
<Basic Circuit>
Lx Tr
SD
OUT
´topen/L=(V
i2
L
IN
-V
OUT
IN
i1
CL
voltage less than minimum operating voltage.
)´ton/L×××Equation 1
V
I
OUT
OUT
- 8 -
OUT
Pin, and with good high frequency characteristics such
<Current through L>
ILmin
ton
T=1/fosc
ILmax
toff
LX
topen
may exceed the