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RB070M-30TR

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM / PFM SWITCHABLE SWITCHING REGULATOR

Seiko Instruments Inc 

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM/PFM SWITCHABLE SWITCHING REGULATOR

S-8363 Series

Rev.2.0_01

4. Output voltage setting resistors (RFB1, RFB2), capacitor for phase compensation (CFB)

For the S-8363 Series, VOUT can be set to any value by using external divider resistors. Connect the divider

resistors between the VOUT and VSS pins.

Because VFB = 0.6 V typ., VOUT can be calculated by using the following equation :

VOUT =

RFB1 + RFB2

RFB2

× 0.6

Connect divider resistors RFB1 and RFB2 as close to the IC as possible to minimize the effects of noise. If noise has

an effect, adjust the values of RFB1 and RFB2 so that RFB1 + RFB2 < 100 kΩ.

CFB, which is connected in parallel with RFB1, is a capacitor for phase compensation.

By setting the zero point (the phase feedback) by adding capacitor CFB to output voltage setting resistor RFB1 in

parallel, the phase margin increases, improving the stability of the feedback loop. To effectively use the feedback

portion of the phase based on the zero point, define CFB by using the following equation :

CFB ≅

L × COUT

3 × RFB1

×

VOUT

VDD

This equation is only a guide.

The following explains the optimum setting.

To efficiently use the feedback portion of the phase based on the zero point, specify settings so that the phase

feeds back at the zero point frequency (fzero) of RFB1 and CFB according to the phase delay at the pole frequency

(fpole) of L and COUT. The zero point frequency is generally set slightly higher than the pole frequency.

The following equations are used to determine the pole frequency of L and COUT and the zero point frequency set

using RFB1 and CFB.

fpole ≅

2×π×

1

L × COUT

×

fzero ≅

1

2 × π × RFB1 × CFB

VDD

VOUT

The transient response can be improved by setting the zero point frequency in a lower frequency range. If, however,

the zero point frequency is set in a significantly lower range, the gain increases in the range of high frequency and

the phase margin decreases. This might result in unstable operation. Determine the proper value after sufficient

evaluation with actual application.

The typical constants based on our evaluation are shown in Table 11.

Table 11 Example of Constant for External Parts

VOUT(S) [V]

VIN [V]

RFB1 [kΩ]

RFB2 [kΩ]

CFB [pF]

1.8

1.2

30

15

82

2.48

1.2

47

15

68

3.32

1.8

68

15

47

4.2

1.8

90

15

39

5.0

1.8

110

15

39

20

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