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STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8460
Rev.1.2_00
12
Seiko Instruments Inc.
Continuous mode at step-up operation
I
VV
V
I
(V
V
V ) V
2(V
V ) fosc L
PK
OUT
F
IN
OUT
OUTFIN
IN
OUT
F
=
+
×+
+
×
×+
×
Continuous mode at step-down operation
II
V(V
V
)
2fosc L V
PK
OUT
IN
OUT
IN
=+
×
×× ×
Where fOSC (
= 300 kHz) is the oscillation frequency, L is the inductance value of the coil, and VF is the diode
forward voltage. VF should be approximately 0.4 V.
2. Diode
Use an external diode that meets the following requirements :
Low forward voltage (Schottky barrier diode is recommended.)
High switching speed (50 ns)
The current rating is larger than IPK.
The reverse-direction withstand voltage is higher than VIN or VOUT for SD1.
The reverse-direction withstand voltage is higher than VIN for SD2.
3. Capacitors
3. 1 Input and Output Capacitors (CIN, COUT)
A capacitor inserted 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. Select a
capacitor that has low ESR (Equivalent Series Resistance) and large capacitance. Approximately 47 to 100
F is
recommended for a capacitance depending on the impedance of the power source and load current value.
For the output side capacitor (COUT), select a large capacitance with low ESR for smoothing the ripple voltage.
When the input voltage is extremely high or the load current is extremely large, the output voltage may become
unstable. In this case the unstable area will become narrow by selecting a large capacitance for an output
capacitor. A tantalum electrolyte capacitor is recommended since the unstable area widens when a capacitor with
a large ESR, such as an aluminum electrolyte capacitor, or a capacitor with a small ESR, such as a ceramic
capacitor, is chosen.
Fully evaluate input and output capacitors under the actual operating conditions to determine the best value.
3. 2 Internal Power Source Stabilization Capacitor (CVL)
The main circuits of the IC work on an internal power source connected to the VL pin. CVL is a bypass capacitor
for stabilizing the internal power source. CVL is a 4.7
F ceramic capacitor and it should be wired nearby and at a
low impedance.
4. External Transistors
Enhancement N-channel MOS FETs should be used for the external transistors.
SW1 is driven by the bootstrapped voltage. If a bipolar transistor is used for SW1, SW1 is not turned on since the
charge in the capacitor (CBST) for bootstrap is discharged.