S8550 datasheet(13/31 Pages) SII

13 / 31 page

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

Rev.5.0_01

S-8550 Series

2. Capacitors (CIN, COUT)

A ceramic capacitor can be used for the input (CIN) and output (COUT) sides. CIN lowers the power supply

impedance and averages the input current to improve efficiency. Select CIN according to the impedance of the

power supply to be used. The recommended capacitance is 4.7

μF for the S-8550 Series when a general

lithium ion rechargeable battery is used.

Select as COUT a capacitor with large capacitance and small ESR for smoothing the ripple voltage. The

optimum capacitor selection depends on the L value, capacitance value, wiring, and application (output load).

Select COUT after sufficient evaluation under actual use conditions.

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

With the S-8550 Series, VOUT can be set to any value by external divider resistors. Connect the divider

resistors across the VOUT and VSS pins. Because VFB

= 0.6 V typ., VOUT can be calculated by this equation.

VOUT =

(RFB1

+ RFB2)

RFB2

× 0.6

Connect divider resistors RFB1 and RFB2 as close to the IC to minimize effects from of noise. If noise does have

an effect, adjust the values of RFB1 and RFB2 so that RFB1

+ RFB2 < 100 kΩ.

CFB 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 feedback loop gains the phase margin. As a result, the stability can be obtained. In principle, to use

the portion how much the phase has feed back by the zero point effectively, define CFB referring to the following

equation.

CFB

≅

× π × RFB1 × 70 kHz

This equation is the reference.

The followings are explanation regarding the proper setting.

To use the portion how much the phase has feed back by the zero point effectively, set RFB1 and CFB so that the

zero point goes into the higher frequency than the pole frequency of L and COUT. The following equations are the

pole frequency of L and COUT and the zero point frequency by CFB and RFB1.

fpole

≅

× π × L × COUT

fzero

≅

× π × RFB1 × CFB

The transient response can be improved by setting the zero point frequency in the range of lower frequency.

However, since the gain becomes higher in the range of high frequency, the total phase of feedback loop delays

180

° or more by setting the zero point frequency in the significantly lower range. As a result, the gain cannot be

0 dB or lower in the frequency range thus the operation might be unstable. Determine the proper value after the

sufficient evaluation under the actual condition.

The typical constants by our evaluation are in Table 8.

Table 8 Constant for External Parts

VOUT(s) [V]

RFB1 [k

Ω]

RFB2 [k

Ω]

CFB [pF]

L [

μH]*1

COUT [

μF]*1

1.1

3.3

1.8

3.3

120

3.3

4.0

100

3.3

*1. The recommended parts in Table 5

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