R1224N Datasheet, PDF (11/42 Pages) RICOH electronics devices division – PWM/VFM step-down DC/DC Converter

R1224N
In the continuous mode, when Equation 1 is solved for ton and assumed that the solution is tonc,
tonc=T×VOUT/VIN..................................................................................... Equation 2
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
When Lx Tr. is ON:
(Wherein, Ripple Current P-P value is described as IRP, ON resistance of Lx Tr. is described as Rp the direct
current of the inductor is described as RL.)
VIN=VOUT+(Rp+RL)×IOUT+L×IRP/ton .................................................Equation 3
When Lx Tr. is OFF:
L×IRP/toff=VF+VOUT+RL×IOUT ............................................................ Equation 4
Put Equation 4 to Equation 3 and solve for ON duty, ton/(toff+ton)=DON,
DON=(VOUT+VF+RL×IOUT)/(VIN+VF−Rp×IOUT) ......................................Equation 5
Ripple Current is as follows;
IRP=(VIN−VOUT−Rp×IOUT−RL×IOUT)×DON/f/L........................................Equation 6
Wherein, peak current that flows through L, Lx Tr., and SD is as follows;
ILmax=IOUT+IRP/2............................................................................ Equation 7
Consider ILmax, condition of input and output and select external components.
Ì The above explanation is directed to the calculation in an ideal case in continuous mode.
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