S-8351A20MC-J2FT2X Datasheet, PDF (21/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

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).
V_OUT
+
−
EXT
+
ON / OFF ^{*1 VOUT}
−
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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