S-8521 datasheet(14/44 Pages) SII

14 / 44 page

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

S-8520/8521 Series

Rev.9.0_01

2. Soft start function

The S-8520/8521 Series has a built-in soft start-circuit. This circuit enables the output voltage (VOUT) to rise

gradually over the specified soft start time (t), when power-on or when the ON/OFF

______

pin is at the “H” level. This

prevents the output voltage from overshooting.

However, the soft-start function of this IC is not able to perfectly prevent a rush current from flowing to the load

(Refer to Figure 8). Since this rush current depends on the input voltage and load conditions, Perform sufficient

evaluation with actual device for design.

S-8520A33MC (VIN: 0 V

→4.0 V)

t [1 ms/div]

VOUT

[1 V/div]

Rush current

[0.5 A/div]

0 A

1.5 A

3 V

0 V

Figure 8 Waveforms of output voltage and rush current at soft start

3. ON/OFF

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pin (Shutdown pin)

The ON/OFF

______

pin stops or starts step-up operation.

Setting the ON/OFF

______

pin to the “L” level stops operation of all the internal circuits and reduces current consumption

significantly. The EXT pin’s voltage is set to the VIN voltage level so that the switching transistor is turned off.

Do not use the ON/OFF

______

pin in a floating state because it has the structure shown in Figure 9 and is not pulled up

or pulled down internally. Do not apply a voltage of between 0.3 V and 1.8 V to the ON/OFF

______

pin because applying

such a voltage increases the current consumption. When not using the ON/OFF

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pin, connect it to the VIN pin.

Table 8

ON/OFF

______

CR Oscillation Circuit

Output voltage

“H”

Operation

Set value

“L”

Stop

VSS

VIN

VSS

ON/OFF

______

Figure 9

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