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R1224N272E Datasheet(PDF) 10 Page - RICOH electronics devices division |
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R1224N272E Datasheet(HTML) 10 Page - RICOH electronics devices division |
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10 / 42 page  R1224N 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> IOUT Lx Tr L SD VIN i1 VOUT CL i2 GND T =1/fosc ton toff topen ILmin ILmax IL 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. 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=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 are respectively shown the change of the current at ON, and the change of the current at OFF. When the output current (IOUT) 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. 10 |
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