R1224N Datasheet, PDF (9/42 Pages) RICOH electronics devices division – PWM/VFM step-down DC/DC Converter

R1224N
How to Adjust Output Voltage and about Phase Compensation
As for Adjustable Output type, feedback pin (VFB) voltage is controlled to maintain 1.0V.
Output Voltage, VOUT is as following equation:
VOUT: R2+R4=VFB: R2
VOUT=VFB×(R2+R4)/R2
Thus, with changing the value of R2 and R4, 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
C3.
Fpole ~ 1/2π L × C3
A zero (signal back to zero) is formed with R4 and C4.
≅Fzero~1/(2π×R4×C4)
For example, if L=27µH, C3=47µF, the cut off frequency of the pole is approximately 4.5kHz.
To make the cut off frequency of the pole as much as 4.5kHz, set R4=33kΩ and C4=1000pF.
If VOUT is set at 2.5V, R2=22kΩ is appropriate.
R3 prevents feedback of the noise to VFB pin, about 2.7kΩ is appropriate value.
L
C1
R1
5 VIN
PMOS
4
EXT
VFB 3
C4
R4
R3
1 _CE
C2
R1224N
GND
2
SD _R2
C3
LOAD
CE CONTROL
9