S-8540 Datasheet, PDF (11/36 Pages) Seiko Instruments Inc – STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.3.2_00
S-8540/8541 Series
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.
S-8540A33FN (VIN = VON / OFF = 0 → 5 V)
3.0 V
V OU T
(1 V/div)
0V
1A
Rush current
(0.5 A/div)
0A
time (1 ms/div)
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 (R_SS and C_SS) connected to
shutdown pin.
A soft-start time (t_SS) is changed by R_SS, C_SS and the input voltage V ON/OFF to R_SS.
t_SS is calculated from the following formula:
t_SS [ms]=R [kΩ] × C [μF] × In (V ON/OFF [V] / (V ON/OFF [V] − 1.8))
e.g. When R_SS = 220 kΩ, C_SS = 0.047 μF, V ON/OFF = 2.7 V , then t_SS = 11.4 ms.
Seiko Instruments Inc.
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