R1234D Datasheet, PDF (13/26 Pages) RICOH electronics devices division – PWM/VFM Step-Down DC/DC CONVERTER with Synchronous Rectifier

R1234D
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 P-channel Tr. of LX is ON:
(Wherein, Ripple Current P-P value is described as IRP, ON resistance of P-channel Tr. and N-channel Tr. of LX
are respectively described as RONP and RONN, and the DC resistor of the inductor is described as RL.)
VIN=VOUT+(RONP+RL)×IOUT+L×IRP/ton .............................................................................. Equation 3
When P-channel Tr. of LX is "OFF"(N-channel Tr. is "ON"):
L×IRP/toff=VF+VOUT+RONN×IOUT ....................................................................................... Equation 4
Put Equation 4 to Equation 3 and solve for ON duty of P-channel transistor, Don=ton/(toff+ton),
DON=(VOUT−RONN×IOUT+RL×IOUT)/(VIN+RONN×IOUT−RONP×IOUT) ........................................... Equation 5
Ripple Current is as follows;
IRP=(VIN−VOUT−RONP×IOUT−RL×IOUT)×DON/fosc/L .............................................................. Equation 6
wherein, peak current that flows through L, and LX Tr. is as follows;
ILmax=IOUT+IRP/2 .......................................................................................................... Equation 7
Consider ILmax, condition of input and output and select external components.
ÌThe above explanation is directed to the calculation in an ideal case in continuous mode.
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