S8540 datasheet(11/36 Pages) SII

Part #	S8540

Description	STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

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Manufacturer	SII [Seiko Instruments Inc]

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STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

Rev.4.0_01

S-8540/8541 Series

11

2.

Soft-start function

The S-8540/8541 series has a built-in soft-start circuit. This circuit enables the output voltage to rise

gradually over the specified soft-start time to suppress the overshooting of the output voltage and the

rush current from the power source when the power is switched on or the power-off pin is set to "H"

The soft-start function of this IC, however, can not suppress rush current to the load completely (Refer

to Figure 9). The rush current is affected by the input voltage and the load. Please evaluate the rush

current under the actual test condition.

time (1 ms/div)

VOUT

(1 V/div)

Rush current

(0.5 A/div)

0 V

3.0 V

0 A

1 A

S-8540A33FN (VIN

= VON /OFF = 0 → 5 V)

Figure 9 Waveforms of output voltage and rush current at soft-start

The soft-start function of the IC is achieved by raising internal reference voltage gradually, which is

caused by the raising of shutdown pin voltage through RC components (RSS and CSS) connected to

shutdown pin.

A soft-start time (tSS) is changed by RSS, CSS and the input voltage V ON/OFF to RSS.

tSS is calculated from the following formula:

tSS [ms]=R [k

Ω] × C [μF] × In (V ON/OFF [V] / (V ON/OFF [V] − 1.8))

e.g. When RSS = 220 k

Ω, CSS = 0.047 μF, V ON/OFF = 2.7 V , then tSS = 11.4 ms.

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