S-8540 SII [Seiko Instruments Inc], S-8540 Datasheet - Page 11

• Manufacturer Part Number: S-8540
• Description: STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
• Manufacturer: SII [Seiko Instruments Inc]

Rev.3.2
2.
STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
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.
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
shutdown pin.
A soft-start time (t
t
e.g. When R
Soft-start function
SS
_00
is calculated from the following formula:
t
SS
[ms]=R [k Ω ] × C [ μ F] × In (V
SS
Figure 9 Waveforms of output voltage and rush current at soft-start
Rush current
= 220 k Ω , C
SS
(0.5 A/div)
) is changed by R
(1 V/div)
V
OU T
3.0 V
1 A
0 A
0 V
S-8540A33FN (V
SS
= 0.047 μ F, V
SS
Seiko Instruments Inc.
, C
ON/OFF
IN
SS
= V
and the input voltage V
ON/OFF
ON / OFF
time (1 ms/div)
[V] / (V
= 0 → 5 V)
= 2.7 V , then t
ON/OFF
[V] − 1.8))
ON/OFF
SS
= 11.4 ms.
to R
SS
S-8540/8541 Series
SS
and C
.
SS
) connected to
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