S-8520 Seiko Instruments, S-8520 Datasheet - Page 13

• Manufacturer Part Number: S-8520
• Description: (S-8520 / S-8521) PWM Control and PFM Control Step DOwn Switching Regulator Controllers
• Manufacturer: Seiko Instruments

Rev.7.4
5.2 Bipolar PNP type
your application, choose a MOS FET with as low an on-resistance as possible.
As for the current rating, select a MOS FET whose maximum continuous drain current rating is higher
than the peak current I
For reference purpose, some efficiency data has been included in this document. For applications with
an input voltage range of 10 V or less, data was obtained by using TM6201 of Toyoda Automatic Loom
Works, Ltd. IRF7606, a standard of International Rectifier, was used for applications with an input
voltage range over 10 V. Refer to "Reference Data."
Figure 7 shows a sample circuit diagram using Toshiba 2SA1213-Y for the bipolar transistor (PNP). The
driving capacity for increasing the output current by means of a bipolar transistor is determined by the
h
The R
Find the necessary base current Ib using the h
and select a smaller R
A small R
the efficiency. Moreover, in practice, a current may flow as the pulses or a voltage drop may take place
due to the wiring resistance or some other reason.
experimentation.
In addition, if speed-up capacitor C
switching loss will be reduced, leading to a higher efficiency.
Select a C
transistor. Optimize the C
However, the practically-reasonable C
FE
_10
PWM Control & PWM/PFM Control Step-Down Switching Regulator-Controllers
-value and the R
b
-value is given by the following equation:
R
b
=
b
b
C
-value will certainly contribute to increasing the output current, but it will also adversely affect
-value by using the following equation as a guide:
b
≤
V
IN
2 π xR
I
−0.7
b
b
-value of that bipolar transistor.
b
b
pk
-value.
x f
−
.
1
OSC
b
|I
value based on the experiment result.
EXTL
0.4
Seiko Instruments Inc.
x 0.7
|
b
is inserted in parallel with resistance R
V
2SA1213-Y
b
R
IN
b
value differs depending upon the characteristics of the bipolar
FE
- value of bipolar transistor by the equation, I
C
Figure 7
EXT
VIN
b
Determine an optimum value through
b
, as shown in Figure 7, the
S-8520/8521 Series
b
= I
pk
/h
FE
,
13