S8355 datasheet(29/72 Pages) SII

29 / 72 page

STEP-UP, SUPER-SMALL PACKAGE, 600 kHz, PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

Rev.8.1_01

S-8355/56/57/58 Series

3. Capacitor (CIN, CL)

A capacitor on the input side (CIN) improves the efficiency by reducing the power impedance and stabilizing the input

current. Select a CIN value according to the impedance of the power supply used.

A capacitor on the output side (CL) is used for smoothing the output voltage. For step-up types, the output voltage

flows intermittently to the load current, so step-up types need a larger capacitance than step-down types. Therefore,

select an appropriate capacitor in accordance with the ripple voltage, which increases in case of a higher output

voltage or a higher load current. The capacitor value should be 10

μF or more.

Select an appropriate capacitor the equivalent series resistance (RESR) for stable output voltage. The stable voltage

range in this IC depends on the RESR. Although the inductance value (L value) is also a factor, an RESR of 30 to 500

Ω maximizes the characteristics. However, the best RESR value may depend on the L value, the capacitance, the

wiring, and the applications (output load). Therefore, fully evaluate the RESR under the actual operating conditions to

determine the best value.

Refer to the “3. Example of Ceramic Capacitor Application” (Figure 26) in the “ Application Circuit” for the

circuit example using a ceramic capacitor and the external resistance of the capacitor (RESR).

4. External Transistor

An enhancement (N-channel) MOS FET type or A bipolar (NPN) type can be used as the external transistor.

4.1 Enhancement (N-channel) MOS FET Type

Figure 17

is a circuit example using a MOS FET transistor (N-channel).

VOUT

EXT

VOUT

−

OFF

VSS

VDD

−

*1.

For VDD / VOUT separate type.

*2.

With shutdown function.

Figure 17 Circuit Example Using MOS FET (N-channel) Type

An N-channel power MOS FET should be used for the MOS FET. Because the gate voltage and current of the

external power MOS FET are supplied from the stepped-up output voltage (VOUT), the MOS FET is driven more

effectively.

A large current may flow during startup, depending on the MOS FET selection. So perform sufficient evaluation

using the actual devices. Also recommend to use a MOS FET with an input capacitance of 700 pF or less.

Since the ON-resistance of the MOS FET might depend on the difference between the output voltage (VOUT) and

the threshold voltage of the MOS FET, and affect the output current as well as the efficiency, the threshold

voltage should be low. When the output voltage is low, the circuit operates only when the MOS FET has a

threshold voltage lower than the output voltage.

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