R1212D Datasheet, PDF (11/41 Pages) RICOH electronics devices division – STEP-UP DC/DC CONTOLLER

R1212D
• Output Voltage Setting Method and Phase Compensation Making Method
• The feedback voltage is controlled into 1.0V. The output voltage can be set with divider resistors for voltage
setting, R1 and R2 as shown in typical application of the previous page. Refer to the next formula.
Output Voltage = VFB× (R1+R2)/R2
Output Voltage is adjustable with setting various resistor values combination.
R1+R2 should be equal or less than 500kΩ
As for the DC/DC converter, depending on the load current and external components such as L and C, phase
may loss around 180°. In such case, phase margin becomes less and may be unstable. To avoid this situation,
make the phase margin more. The pole is made with external components L and C.
Fpole∼1/{2×π× (L × C3) }
C4, C5, R3, and R4 shown in the diagram are for making phase compensation. The gain of the system can be
set with using these resistors and capacitors. Each value in the diagram is just an example.
R4 and C5 make zero (the backward phase).
Fzero∼1/(2×π×R4×C5)
Choose the R4 and C5 value so as to make the cutoff frequency of this zero point close to the cutoff frequency of
the pole by external components, L and C.
For example, supposed that L=10µH and COUT (C3) =10µF, the cutoff frequency of the pole is approximately
16kHz. Therefore make the cutoff frequency of the zero point close to 16kHz. Then R4=4.7kΩ and C5=1000pF
are appropriate values.
As for setting the gain, the ratio of the composite resistor (RT: RT=R1×R2/(R1+R2)) to R4 is the key.
If the R4 against the composite resistor, RT, is large, the gain becomes also large. If the gain is large, the
response characteristic is improved, however, too large gain makes the system be unstable.
If the spike noise of VOUT may be large, the spike noise may be picked into VFB pin, and the unstable operation
may result. In this case, a resistor R3, shown in typical application of the previous page. The recommended
resistance value of R3 is in the range from 1kΩ to 5kΩ. Then, noise level will be decreased.
Further, R1 and C4 makes another zero point (the backward phase).
Fzero∼1/(2×π×R1×C4)
Make the cutoff frequency of this zero point be lower than the cutoff frequency of the pole by external
components, or, L and C. Herein, R1=180kΩ and C4=1000pF are appropriate values.
• Select the Power MOSFET, the diode, capacitors and the inductor within ratings (Voltage, Current, Power) of
this IC. Choose the power MOSFET with low threshold voltage depending on the input voltage to be able to
turn on the FET completely. Choose the diode with low VF such as Shottky type with low reverse current IR,
and with fast switching speed. When an external transistor is switching, spike voltage may be generated
caused by an inductor, therefore recommended voltage tolerance of capacitor connected to VOUT is twice as
much as the setting voltage or more.
• The soft-start time and the maximum duty cycle setting method
The soft-start time and the maximum duty cycle can be set with R5, R6, and C7 values connected to the
VREFOUT pin and the DTC pin. (Refer to the timing chart: Soft-start operation.)
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