R1224N122G RICOH [RICOH electronics devices division], R1224N122G Datasheet - Page 10

• Manufacturer Part Number: R1224N122G
• Description: PWM/VFM step-down DC/DC Converter
• Manufacturer: RICOH [RICOH electronics devices division]

10
OPERATION of step-down DC/DC converter and Output Current
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: Lx Tr. turns on and current IL (=i1) flows, and energy is charged into CL. At this moment, IL increases
Step 2: When Lx Tr. turns off, Schottky diode (SD) turns on in order that L maintains IL at ILmax, and current IL
Step 3: IL decreases gradually and reaches ILmin. after a time period of topen, and SD turns off, provided that in
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
same as those when Lx Tr. is ON and when it is OFF.
wherein, T=1/fosc=ton+toff
In Equation 1, V
the change of the current at OFF.
When the output current (I
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 I
former mode is referred to as the discontinuous mode and the latter mode is referred to as continuous mode.
R1224N
The maximum value (ILmax) and the minimum value (ILmin) current which flow through the inductor is the
The difference between ILmax and ILmin, which is represented by ∆I;
∆I=ILmax-ILmin=V
The step-down DC/DC converter charges energy in the inductor when Lx transistor is ON, and discharges the
GND
V
from ILmin. (=0) to reach ILmax. in proportion to the on-time period (ton) of Lx Tr.
(=i2) flows.
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
IN
< =
OUT
Lx Tr
toff
<Basic Circuits>
×topen/L and (V
SD
OUT
×topen/L=(V
OUT
) is relatively small, topen<toff as illustrated in the above diagram. In this case, the
i2
L
IN
IN
-V
-V
CL
i1
OUT
OUT
OUT
)×ton/L are respectively shown the change of the current at ON, and
)×ton/L ................................... Equation 1
is further increased, ILmin becomes larger than zero (ILmin>0). The
I
V
OUT
OUT
IL
ILmin
<Current through L>
ton
T=1/fosc
ILmax
toff
topen