R1234D Datasheet, PDF (14/26 Pages) RICOH electronics devices division – PWM/VFM Step-Down DC/DC CONVERTER with Synchronous Rectifier

R1234D
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/(2 π ×R1×Cb)
First, choose the appropriate value of R1, R2 and Cb.
Set R1+R2 value 100kΩ or less.
For example, if L=10µH, COUT=10µF, the cut off frequency of the pole is approximately 16kHz.
To make the cut off frequency of the zero higher than 16kHz, set R1=42kΩ and Cb=100pF.
If VOUT is set at 1.5V, R2=48kΩ is appropriate.
If a ceramic capacitor is desirable as COUT in your application, nonetheless of the usage of both the fixed
output voltage type and adjustable output type, add 0.2Ω or more resistance to compensate the ESR. Further, if
a ceramic capacitor is desirable to use as COUT without adding another resister to compensate the ESR, phase
should be back drastically. To make it, R2 value should be smaller compared to R1. As a result, the set output
voltage may be large. For example, to make VOUT=1.5V, constants are R1=42kΩ, R2=48kΩ, and Cb=100pF. If the
ceramic capacitor is used, under the heavy load condition, oscillation may be result. On the other hand, if R2 =
12kΩ and VOUT=3.6V, phase back becomes also large, and even if the device is used with a heavy load, the
operation will be stable.
Rb is effective for reducing the noise on VFB. However, it is not always necessary. If it is necessary, use a
resistance as much as 30kΩ as Rb.
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