参数资料
型号: RP505K061B-TR
厂商: RICOH COMPANY LTD
元件分类: 稳压器
英文描述: SWITCHING CONTROLLER, 2500 kHz SWITCHING FREQ-MAX, PDSO8
封装: 2 X 2 MM, HALOGEN FREE AND ROHS COMPLIANT, DFN-8
文件页数: 2/32页
文件大小: 938K
代理商: RP505K061B-TR
RP505K
10
Operation of step-down DC/DC converter and Output Current
The 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>
Pch Tr
L
Nch Tr
VIN
i1
VOUT
CL
i2
GND
T=1/fosc
ton
toff
topen
ILmin
ILmax
IL
i1
i2
Step 1: Pch 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 Pch Tr.
Step 2: When Pch Tr. turns off, Synchronous rectifier Nch 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 Nch 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.
The maximum value (ILmax) and the minimum value (ILmin) of the current flowing through the inductor are the
same as those when Pch Tr. turns on and off.
The difference between ILmax and ILmin, which is represented by ΔI:
I = 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
In Equation 1, VOUT
× topen / L and (VIN VOUT) × ton / L respectively show the change of the current at "ON", and
the change of the current at "OFF".
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