S8540 datasheet(17/36 Pages) SII

17 / 36 page

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

Rev.4.0_01

S-8540/8541 Series

5. 2 Bipolar (PNP) transistor

Figure 12

shows 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 determined

by the hFE value and the Rb value.

VIN

EXT

2SA1213-Y

VIN

Figure 12

The Rb value is given by the following equation:

EXTL

0.4

0.7

−

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 the

efficiency. Determine the optimum value through experiment since the base current flows as pulses

and 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 :

0.7

OSC

≤

Select a Cb value after performing sufficient evaluation since the optimum Cb value differs depending

upon the characteristics of the bipolar transistor.

S8540 Datasheet(PDF) 17 Page - Seiko Instruments Inc