S-8520A20MC-AVFT2X Datasheet, PDF (20/44 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8520/8521 Series
Rev.9.0_01
5. 2 Bipolar PNP type
Figure 11 shows the sample of circuit diagram using Toshiba Corporation 2SA1213 as the bipolar transistor
(PNP). The hFE value and the Rb value of that bipolar transistor determine the driving capacity, which is used for
increasing the output current.
Toshiba Corporation
2SA1213
VIN
R_b
C_b
EXT
V_IN
Figure 11
The Rb value is given by the following equation:
Rb =
VIN − 0.7
Ib
−
0.4
IEXTL
Calculate the necessary base current (Ib) using the (hFE) value of bipolar transistor by the equation, Ib =
IPK
hFE
,
and select a smaller Rb value.
A small Rb value increases the output current, but it also decreases efficiency. Determine the optimum value
through experiment, since the base current may flow on the pulse, or voltage may drop due to wiring resistance.
In addition, if speed-up capacitor Cb is connected in parallel with resistor Rb, as shown in Figure 11, the loss in
switching will be reduced, leading to higher efficiency.
Determine the Cb value using the following equation:
Cb ≤
1
2 × π × Rb × fosc × 0.7
Select the Cb value after performing sufficient evaluation since the optimum Cb value differs depending upon the
characteristics of the bipolar transistor.
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