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S-8337ACDB-P8T1G Datasheet(PDF) 18 Page - Seiko Instruments Inc |
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S-8337ACDB-P8T1G Datasheet(HTML) 18 Page - Seiko Instruments Inc |
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18 / 51 page  STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER S-8337/8338 Series Rev.4.0_01 18 4. External transistor A bipolar (NPN) or enhancement (N-channel) MOS FET transistor can be used as the external capacitor. 4. 1 Bipolar (NPN) type The driving capability when the output current is increased by using a bipolar transistor is determined by hFE and Rb of the bipolar transistor. Figure 11 shows a peripheral circuit. Nch Pch Rb VIN IPK EXT Cb 2200 pF 1 k Ω Figure 11 External Transistor Periphery 1 k Ω is recommended for Rb. Actually, calculate the necessary base current (Ib) from hFE of the bipolar transistor as follows and select an Rb value lower than this. Ib = hFE IPK Rb = Ib VIN – 0.7 – IEXTH 0.4 A small Rb increases the output current, but the efficiency decreases. Actually, a pulsating current flows and a voltage drop occurs due to the wiring capacitance. Determine the optimum value by experiment. A speed-up capacitor (Cb) connected in parallel with Rb resistance as shown in Figure 11 decreases the switching loss and improves the efficiency. Select Cb by observing the following equation. Cb ≤ 2 π • Rb • fOSC • 0.7 1 However, in practice, the optimum Cb value also varies depending on the characteristics of the bipolar transistor employed. Therefore, determine the optimum value of Cb by experiment. |
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