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R5220K181A Datasheet(PDF) 7 Page - RICOH electronics devices division |
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R5220K181A Datasheet(HTML) 7 Page - RICOH electronics devices division |
7 / 25 page R5220x 7 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> <Current through L> Pch Tr L Nch Tr VIN i1 VOUT CL i2 GND T=1/fosc ton toff topen ILmin ILmax IL Step 1: P-channel 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 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. 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=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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