S-8540 Datasheet, PDF (17/36 Pages) Seiko Instruments Inc – STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.3.2_00
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 h_FE value and the R_b value.
2SA1213-Y
VIN
Rb
Cb
EXT
V_IN
Figure 12
The R_b value is given by the following equation:
VIN − 0.7 0.4
Rb =
−
Ib
IEXTL
Calculate the necessary base current Ib using the h_FE value of the bipolar transistor from the relation,
I_b = I_PK/h_FE, and select a smaller value for R_b which is calculated from the above equation.
A small R_b 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 C_b is inserted in parallel with resistance R_b, as shown in Figure 12,
the switching loss will be reduced, leading to a higher efficiency.
by using the following equation :
Cb ≤
1
2π × Rb × f_OSC × 0.7
Select a C_b value after performing sufficient evaluation since the optimum C_b value differs depending
upon the characteristics of the bipolar transistor.
Seiko Instruments Inc.
17