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R1232D191B Datasheet(PDF) 10 Page - RICOH electronics devices division |
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R1232D191B Datasheet(HTML) 10 Page - RICOH electronics devices division |
10 / 24 page R1232D 10 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 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. • Continuous Conduction Mode 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 Where, 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. Even if the output current (IOUT) 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 IOUT is further increased, ILmin becomes larger than zero (ILmin>0). The mode is referred to as the continuous mode. |
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