S-8333CAAA-I8T1U Datasheet, PDF (15/44 Pages) Seiko Instruments Inc – PWM CONTROL SWITCHING REGULATOR CONTROLLER

STEP-UP, FOR LCD BIAS SUPPLY, 1-CHANNEL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.4.2_01
S-8333 Series
 External Parts Selection
1. Inductor
The inductance has a strong influence on the maximum output current (IOUT) and efficiency (η).
The peak current (IPK) increases by decreasing L and the stability of the circuit improves and IOUT increases. If L
is decreased further, the efficiency falls, and IOUT decreases if the current drive capability of the external
transistor is insufficient.
The loss of IPK by the switching transistor decreases by increasing L and the efficiency becomes maximum at a
certain L value. Further increasing L decrease the efficiency due to the loss of the DC resistance of the inductor.
IOUT also decreases.
If the oscillation frequency is higher, a smaller L value can be chosen, making the inductor smaller. In the S-8333
Series, the oscillation frequency can be varied within the range of 280 kHz to 1.08 MHz by the external resistor,
so select an L value best suited to the frequency. The recommended value is between 2.2 μH and 22 μH.
When selecting an inductor, note the allowable current of the inductor. If a current exceeding this allowable
current flows through the inductor, magnetic saturation occurs, substantially lowering the efficiency and
increasing the current, which results in damage to the IC.
Therefore, select an inductor so that IPK does not exceed the allowable current. IPK is expressed by the following
equations in the discontinuous mode and continuous mode.
IPK =
2 IOUT(VOUT + VD − VIN)
fosc × L
( discontinuous mode ) ................................................. (17)
IPK
=
VOUT +
VIN
VD
× IOUT +
(VOUT + VD − VIN) × VIN
2 × (VOUT + VD) × fosc × L
fOSC = Oscillation frequency, VD ≅ 0.4 V.
2. Diode
(continuous mode) ....................................................... (18)
Use an external diode that meets the following requirements.
• Low forward voltage
• High switching speed
• Reverse breakdown voltage: VOUT + [Spike voltage] or more
• Rated current: IPK or more
3. Capacitors (CIN, CL)
The capacitor on the input side (CIN) can lower the supply impedance and level the input current for better
efficiency. Select CIN according to the impedance of the power supply to be used.
The capacitor on the output side (CL) is used to smooth the output voltage. Select an appropriate capacitance
value based on the I/O conditions and load conditions. A capacitance of 10 μF or more is recommended.
By adjusting the phase compensation of the feedback loop using the external resistor (RZ) and capacitor (CZ), a
ceramic capacitor can be used as the capacitor on the output side. If a capacitor whose equivalent series
resistance is between 30 mΩ and 500 mΩ is used as the output capacitor, the adjustable range of the phase
compensation is wider; however, note that other characteristics may be affected by ripple voltage or other
conditions at this time. The optimal capacitor differs depending on the L value, capacitance value, wiring, and
application (output load), so select the capacitor after performing sufficient evaluation under the actual usage
conditions.
15