S-8355 Seiko Instruments, S-8355 Datasheet - Page 30

• Manufacturer Part Number: S-8355
• Description: (S-8355 - S-8358) PWM/PFM SWITCHING CONTROL STEP-UP SWITCHING REGULATOR
• Manufacturer: Seiko Instruments

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STEP-UP, SUPER-SMALL PACKAGE, 600 kHz, PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8355/56/57/58 Series
3. Capacitor (C
4. External Transistor
A capacitor on the input side (C
current. Select a C
A capacitor on the output side (C
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 (R
range in this IC depends on the R
m
wiring, and the applications (output load). Therefore, fully evaluate the R
determine the best value.
Refer to the “ 3. Example of Ceramic Capacitor Application ” ( Figure 27 ) in the “
circuit example using a ceramic capacitor and the external resistance of the capacitor (R
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
Ω
Figure 18 is a circuit example using a MOS FET transistor (N-channel).
*1. For V
*2. With shutdown function.
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 (V
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 (V
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.
maximizes the characteristics. However, the best R
IN
DD
, C
/ V
L
)
OUT
IN
value according to the impedance of the power supply used.
separate type.
Figure 18 Circuit Example Using MOS FET (N-channel) Type
IN
) improves the efficiency by reducing the power impedance and stabilizing the input
L
ESR
) is used for smoothing the output voltage. For step-up types, the output voltage
. Although the inductance value (L value) is also a factor, an R
+
−
Seiko Instruments Inc.
VSS
ESR
EXT
ON
value may depend on the L value, the capacitance, the
/
VDD
OFF
VOUT
ESR
*1
*2
) for stable output voltage. The stable voltage
ESR
under the actual operating conditions to
+
−
V
OUT
OUT
), the MOS FET is driven more
ESR
Application Circuit ” for the
).
ESR
Rev.6.6
of 30 to 500
OUT
) and
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