LM1577, LM2577
SNOS658D –JUNE 1999–REVISED APRIL 2013
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The primary of a flyback transformer draws discontinuous pulses of current from the input supply. As a result, a
flyback regulator generates more noise at the input supply than a step-up regulator, and this requires a larger
bypass capacitor to decouple the LM1577/LM2577 VIN pin from this noise. For most applications, a low ESR, 1.0
μF cap will be sufficient, if it is connected very close to the VIN and Ground pins.
Transformer
Type
Input
Dual
Output
Voltage
±10V
±12V
±15V
±10V
±12V
±15V
±10V
±12V
±15V
±10V
±12V
±15V
Maximum
Output
Current
325 mA
275 mA
225 mA
700 mA
575 mA
500 mA
800 mA
700 mA
575 mA
900 mA
825 mA
700 mA
Voltage
LP = 100 μH
5V
1
2
3
N = 1
5V
5V
10V
10V
10V
12V
12V
12V
15V
15V
15V
LP = 200 μH
N = 0.5
LP = 250 μH
N = 0.5
Table 6. Flyback Transformer Selection Guide
Transformer
Manufacturers' Part Numbers
Type
AIE
Pulse
Renco
RL-2580
RL-2581
RL-2582
1
2
3
326-0637
330-0202
330-0203
PE-65300
PE-65301
PE-65302
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 interfere 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 LP ≥ 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 43, the snubber consists of a fast recovery diode, and a parallel RC. The RC values are selected for
switch clamp voltage (VCLAMP) that is 5V to 10V greater than VSW(OFF). Use the following equations to calculate R
and C;
(29)
Power dissipation (and power rating) of the resistor is;
(30)
The fast recovery diode must have a reverse voltage rating greater than VCLAMP
.
26
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