S-8351A20MC-J2FT2X Datasheet, PDF (22/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER
S-8351/8352 Series
Rev.3.0_01
4. 2 Bipolar (NPN) Type
A circuit example using the CPH3210 (hFE = 200 to 560) from Sanyo Electric Co., Ltd. as a bipolar transistor
(NPN) is shown in Figure 24 to 26 in the “ Standard Circuits”. The hFE value and Rb value of the bipolar
transistor determine the driving capacity to increase the output current using a bipolar transistor. A peripheral
circuit example of the transistor is shown in Figure 20.
VOUT^*1
C_b
Pch
2200 pF
I_PK
EXT
R_b
1 kΩ
Nch
*1. VDD for D type.
Figure 20 External Transistor Peripheral Circuit
The recommended Rb value is around 1 kΩ. Actually, calculate the necessary base current (Ib) from the bipolar
transistor (hFE) using
I_b
=
I_PK
h_FE
,
and
select
the
smaller
Rb
value
than
R_b
=
V_OUT − 0.7
I_b
−
0.4
I_EXTH
*1_.
A small Rb value can increase the output current, but the efficiency decreases. Since a current may flow on the
pulse and the voltage may drop due to wiring resistance or other factors in the actual circuit, therefore the
optimum Rb value should be determined by experiment.
Connecting the speed-up capacitor (Cb) in parallel with the Rb resistance as shown in Figure 20, decreases
switching loss and improves the efficiency.
The Cb value is calculated according to
Cb
≤
1
2π • Rb • fosc • 0.7
.
Select a Cb value after performing sufficient evaluation since the optimum Cb value differs depending upon the
characteristics of the bipolar transistor.
*1. For D type,
R_b
=
V_DD − 0.7
I_b
−
0.4
I_EXTH
.
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