R1212D

11

• 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

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×

(

)

3

C

L

×

}

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

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.

response characteristic is improved, however, too large gain makes the system be unstable.

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 1k

Ω 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

and with fast switching speed. When an external transistor is switching, spike voltage may be generated

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