R1232D Datasheet, PDF (12/24 Pages) RICOH electronics devices division – PWM STEP-DOWN DC/DC CONVERTER WITH SYNCHRONOUS RECTIFIER

R1232D
How to Adjust Output Voltage and about Phase Compensation
As for Adjustable Output type, feedback pin (VFB) voltage is controlled to maintain 0.8V.
Output Voltage, VOUT is as following equation;
VOUT R1+R2=VFB:R2
VOUT=VFB×(R1+R2)/R2
Thus, with changing the value of R1 and R2, output voltage can be set in the specified range.
In the DC/DC converter, with the load current and external components such as L and C, phase might be
behind 180 degree. In this case, the phase margin of the system will be less and stability will be worse. To
prevent this, phase margin should be secured with proceeding the phase. A pole is formed with external
components L and COUT.
fpole ~ 1/2π LCOUT
A zero (signal back to zero) is formed with R1 and Cb.
≅fzero ~ 1/(2p×R1×Cb)
First, choose the appropriate value of R1, R2 and Cb.
Set R1+R2 value 100kΩ or less.
For example, if L=4.7µH, COUT =10µF, the cut off frequency of the pole is approximately 23kHz.
To make the cut off frequency of the zero by R1, R2, and Cb be higher than 23kHz,
set R1=33kΩ and Cb=100pF.If VOUT is set at 2.0V, R2=22kΩ is appropriate.
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