R1282D002A Datasheet, PDF (17/18 Pages) RICOH electronics devices division – 2CH PWM DC/DC CONTROLLER

R1282D002A
EXTERNAL COMPONENTS
1. How to set the output voltages
As for step-up side, feedback (VFB1) pin voltage is controlled to maintain 1V, therefore,
VOUT1: R1+R2=VFB1: R2
Thus, VOUT1=VFB1×(R1+R2)/R2
Output Voltage is adjustable with R1 and R2.
As for Step-down side, Feedback (VFB2) pin voltage follows the next formula,
VOUT2: R3+R4=VFB2 : R4
Thus, VOUT2=VFB2×(R3+R4)/R4
Output Voltage is adjustable with R3 and R4.
2. How to set Soft-Start Time and Maximum Duty Cycle
Soft-start time is adjustable with connecting resistors and a capacitor to DTC pin.
Soft starting time, TSS1 and TSS2 are adjustable. Soft-start time can be set with the time constant of RC.
Soft-start time can be described as in next formula.
TSS1≅RO1×C4
If R10=0Ω, then,
TSS2≅R9×C5×ln((Vrefout−VDTC2)/Vrefout)
Maximum Duty Cycle is set with the voltage to DTC1 and DTC2.
Maximum duty cycle is described as follows;
CH1 (Step-up side)
Maxduty1≅ (R8/(R7+R8) ×Vrefout−0.2)/(1.2−0.2) ×100 (%)
Step-up side maximum duty cycle should be set equal or less than 90%. If the maximum duty cycle is set at
high percentage, operation will be unstable.
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