參數(shù)資料
型號(hào)： R1232D301B
廠商： Ricoh Co., Ltd.
英文描述： PWM STEP-DOWN DC/DC CONVERTER WITH SYNCHRONOUS RECTIFIER
中文描述： PWM降壓型DC / DC變換器的同步整流
文件頁(yè)數(shù)： 10/24頁(yè)
文件大小： 819K
代理商： R1232D301B
R1232D
OPERATION of step-down DC/DC converter and Output Current
10
The step-down DC/DC converter charges energy in the inductor when L
X
transistor is ON, and discharges the
energy from the inductor when L
X
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>
Step 1: P-channel Tr. turns on and current IL (
=
i1) flows, and energy is charged into C
L
. At this moment,
IL increases from Ilmin (
=
0) to reach ILmax in proportion to the on-time period (ton) of P-channel Tr.
Step 2: When P-channel Tr. turns off, Synchronous rectifier N-channel Tr. turns on in order that L maintains IL at
ILmax, and current IL (
=
i2) flows.
Step 3: IL (
=
i2) decreases gradually and reaches IL
=
ILmin
=
0 after a time period of topen, and N-channel Tr.
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 increases 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.
Continuous Conduction Mode
I
OUT
Lx Tr
L
SD
V
IN
i1
V
OUT
CL
i2
The maximum value (ILmax) and the minimum value (ILmin) of the current flowing through the inductor are the
same as those when P-channel Tr. turns on and off.
The difference between ILmax and ILmin, which is represented by
I;
I
=
ILmax
ILmin
=
V
OUT
×
topen/L
=
(V
IN
V
OUT
)
×
ton/L ........................................................Equation 1
Where, 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 shown the change of the current at ON,
and the change of the current at OFF.
Even if the output current (I
OUT
) is, topen < toff as illustrated in the above diagram is not realized with this IC. At
least, topen is equal toff (topen
=
toff), and when I
OUT
is further increased, ILmin becomes larger than zero
(ILmin>0). The mode is referred to as the continuous mode.
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