Typical Applications (Continued)
DS008711-14
DS008711-13
=
=
Vin 5V
R4 0.15Ω
=
R5 270
9V ≤ Vin ≤ 15V
=
±
Vo 12V
C1 820 pF
=
=
C1 1820 pF
Vo 12V
=
=
±
Io
40 mA
C2 10 pF
=
C2 220 µF
Io = 100 mA
=
=
fosc 80 kHz
C3 220 µF
=
=
C3 20 pF
Vripple 50 mV
=
=
D1, D2, D3 1N5819
R1 10 kΩ
=
=
C4 0.0022 µF
fosc 50 kHz
=
=
T1 PE-64287
R2 240 kΩ
=
=
L1 220 µH
R1 110k
=
R3 240 kΩ
=
=
D1, D2 1N5819
R2 10k
FIGURE 23. RS-232 Line Driver Power Supply
=
=
Q1 D44
R3 0.15
=
R4 220k
Capacitor C1 sets the oscillator frequency and is selected
from Figure 1.
FIGURE 22. Buck-Boost Regulator
Capacitor C2 serves as a compensation capacitor for syn-
chronous operation and a value of 10 to 50 pF should be suf-
ficient for most applications.
A minimum value for an ideal output capacitor C3, could be
=
calculated as C Io x t/∆V where Io is the load current, t is
the transistor on time (typically 0.4/fosc), and ∆V is the
peak-to-peak output voltage ripple. A larger output capacitor
than this theoretical value should be used since electrolytics
have poor high frequency performance. Experience has
shown that a value from 5 to 10 times the calculated value
should be used.
For good efficiency, the diodes must have a low forward volt-
age drop and be fast switching. 1N5819 Schottky diodes
work well.
Transformer selection should be picked for an output transis-
tor “on” time of 0.4/fosc, and a primary inductance high
enough to prevent the output transistor switch from ramping
higher than the transistor’s rating of 750 mA. Pulse Engi-
neering (San Diego, Calif.) and Renco Electronics, Inc.
(Deer Park, N.Y.) can provide further assistance in selecting
the proper transformer for a specific application need. The
transformer used in Figure 23 was a Pulse Engineering
PE-64287.
15
www.national.com
NSC [ National Semiconductor ]
