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S-8333CBHA-T8T1y Datasheet(PDF) 15 Page - Seiko Instruments Inc |
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S-8333CBHA-T8T1y Datasheet(HTML) 15 Page - Seiko Instruments Inc |
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15 / 44 page  STEP-UP, FOR LCD BIAS SUPPLY, 1-CHANNEL, PWM CONTROL SWITCHING REGULATOR CONTROLLER Rev.4.2_01 S-8333 Series 15 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. L f ) V V (V I 2 I osc IN D OUT OUT PK × − + = ) mode ous discontinu ( ................................................. (17) L f ) V (V 2 V ) V V (V I V V V I osc D OUT IN IN D OUT OUT IN D OUT PK × × + × × − + + × + = mode) s (continuou ....................................................... (18) fOSC = Oscillation frequency, VD ≅ 0.4 V. 2. Diode 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 m Ω 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. |
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