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LM1577 数据表(PDF) 21 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部件名 LM1577
功能描述  SIMPLE SWITCHER Step-Up Voltage Regulator
PDF  27 Pages
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制造商  NSC [National Semiconductor (TI)]
网页  http://www.national.com
标志 NSC - National Semiconductor (TI)

LM1577 数据表(HTML) 21 Page - National Semiconductor (TI)

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Application Hints (Continued)
Transformer
Input
Dual
Maxi-
mum
Type
Voltage
Output
Output
Voltage
Current
L
P = 100 µH
5V
±10V
325 mA
1N = 15V
±12V
275 mA
5V
±15V
225 mA
10V
±10V
700 mA
10V
±12V
575 mA
2L
P = 200 µH
10V
±15V
500 mA
N = 0.5
12V
±10V
800 mA
12V
±12V
700 mA
12V
±15V
575 mA
3L
P = 250 µH
15V
±10V
900 mA
N = 0.5
15V
±12V
825 mA
15V
±15V
700 mA
In addition to this bypass cap, a larger capacitor (
≥ 47 µF)
should be used where the flyback transformer connects to
the input supply. This will attenuate noise which may inter-
fere with other circuits connected to the same input supply
voltage.
6. Snubber Circuit
A “snubber” circuit is required when operating from input
voltages greater than 10V, or when using a transformer with
L
P ≥ 200 µH. This circuit clamps a voltage spike from the
transformer primary that occurs immediately after the output
switch turns off. Without it, the switch voltage may exceed
the 65V maximum rating. As shown in
Figure 21, the snub-
ber consists of a fast recovery diode, and a parallel RC. The
RC values are selected for switch clamp voltage (V
CLAMP)
that is 5V to 10V greater than V
SW(OFF). Use the following
equations to calculate R and C;
Power dissipation (and power rating) of the resistor is;
The fast recovery diode must have a reverse voltage rating
greater than V
CLAMP.
FLYBACK REGULATOR CIRCUIT EXAMPLE
The circuit of
Figure 22 produces ±15V (at 225 mA each)
from a single 5V input. The output regulation of this circuit is
shown in
Figure 23 and Figure 25, while the load transient
response is shown in
Figure 24 and Figure 26. Switching
waveforms seen in this circuit are shown in
Figure 27.
Transformer
Manufacturers’ Part Numbers
Type
AIE
Pulse
Renco
1
326-0637
PE-65300
RL-2580
2
330-0202
PE-65301
RL-2581
3
330-0203
PE-65302
RL-2582
FIGURE 20. Flyback Transformer Selection Guide
DS011468-19
FIGURE 21. Snubber Circuit
www.national.com
21



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