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参数资料
| 型号： | R1224N212L-TR |
| 英文描述： | Analog IC |
| 中文描述： | 模拟IC |
| 文件页数： | 8/36页 |
| 文件大小： | 503K |
| 代理商： | R1224N212L-TR |
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Rev. 1.12 - 8 -
I
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
Step 1: Lx Tr. 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 LX Tr.
Step 2: When Lx Tr. 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.
G
Discontinuous Conduction Mode and Continuous Conduction Mode
The maximum value (ILmax) and the minimum value (ILmin) current which flow through the inductor is the same
as those when Lx Tr. is ON and when it is 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
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 shown the change of the current at ON, and
the change of the current at OFF.
When the output current (I
OUT
) 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,
tonc=T
×
V
OUT
/V
IN
Equation 2
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
I
OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
When Lx Tr. is ON:
(Wherein, Ripple Current P-P value is described as I
RP
, ON resistance of LX Tr. is described as Rp the direct
current of the inductor is described as R
L
.)
SD
L
CL
V
OUT
I
OUT
V
IN
Lx Tr
i2
T=1/fosc
toff
ton
ILmax
ILmin
topen
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