S8351 datasheet(21/51 Pages) SII

21 / 51 page

STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER

Rev.3.0_01

S-8351/8352 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.

A capacitor at the output side (CL) is used for smoothing the ripple voltage. Select an appropriate capacitor with a

small equivalent series resistance (RESR) and a large capacitance. The capacitor value should be 10

μF or mpre. A

tantalum electrolytic capacitor and an organic semiconductor capacitor are especially recommended because of their

superior low-temperature and leakage current characteristics.

4. External Transistor (S-8352 Series)

For the S-8352 Series, connecting an external transistor increases the output current. 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 19

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

VOUT

EXT

VOUT

−

OFF

VSS

−

*1.

For A type.

Figure 19 Circuit Example Using MOS FET (N-channel) Type

An N-channel power MOS FET should be used for the MOS FET. In particular, the EXT pin can drive a MOS

FET with a gate capacitance of around 1000 pF. 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. The S-8352 Series does not

feature overcurrent protection for the external MOS FET, 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 as low as 2.0 V, like in the S-8352A20, the circuit operates only

when the MOS FET has a threshold voltage lower than the output voltage.

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