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## S8540 Datasheet(PDF) 17 Page - Seiko Instruments Inc

 Part # S8540 Description STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER Download 36 Pages Scroll/Zoom 100% Manufacturer SII [Seiko Instruments Inc] Direct Link http://www.sii.co.jp Logo

## S8540 Datasheet(HTML) 17 Page - Seiko Instruments Inc

 17 / 36 pageSTEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLERRev.4.0_01S-8540/8541 Series175. 2 Bipolar (PNP) transistorFigure 12shows a circuit diagram using Toshiba Corporation 2SA1213-Y for the bipolar transistor(PNP). Using a bipolar transistor, the driving capacity for increasing the output current is determinedby the hFE value and the Rb value.VINEXT2SA1213-YVINRbCbFigure 12The Rb value is given by the following equation:EXTLINI0.4I0.7VRbb−−=Calculate the necessary base current Ib using the hFE value of the bipolar transistor from the relation,Ib = IPK/hFE, and select a smaller value for Rb which is calculated from the above equation.A small Rb value will certainly contribute to increase the output current, but it will also decrease theefficiency. Determine the optimum value through experiment since the base current flows as pulsesand voltage drop may takes place due to the wiring resistance and so on.In addition, if speed-up capacitor Cb is inserted in parallel with resistance Rb, as shown in Figure 12,the switching loss will be reduced, leading to a higher efficiency.by using the following equation :0.7fR21COSCbb×××≤πSelect a Cb value after performing sufficient evaluation since the optimum Cb value differs dependingupon the characteristics of the bipolar transistor.

## Similar Part No. - S8540

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## Similar Description - S8540

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