LM3578A MWC National Semiconductor, LM3578A MWC Datasheet - Page 15

• Manufacturer Part Number: LM3578A MWC
• Description: Switching Regulator
• Manufacturer: National Semiconductor

Typical Applications
9V
V
I
V
f
R1 = 110k
R2 = 10k
R3 = 0.15
R4 = 220k
o
osc
o
ripple
= 100 mA
= 12V
= 50 kHz
FIGURE 22. Buck-Boost Regulator
V
= 50 mV
in
15V
R5 = 270
C1 = 1820 pF
C2 = 220 µF
C3 = 20 pF
C4 = 0.0022 µF
L1 = 220 µH
D1, D2 = 1N5819
Q1 = D44
(Continued)
DS008711-13
15
Capacitor C1 sets the oscillator frequency and is selected
from Figure 1 .
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 = I
the transistor on time (typically 0.4/f
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/f
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.
FIGURE 23. RS-232 Line Driver Power Supply
V
V
I
f
R1 = 10 k
R2 = 240 k
R3 = 240 k
o
osc
in
o
=
±
= 5V
= 80 kHz
±
12V
40 mA
o
x t/ V where I
osc
, and a primary inductance high
R4 = 0.15
C1 = 820 pF
C2 = 10 pF
C3 = 220 µF
D1, D2, D3 = 1N5819
T1 = PE-64287
o
is the load current, t is
osc
), and
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V is the
DS008711-14