Datasheet

LM1577, LM2577
SNOS658D –JUNE 1999–REVISED APRIL 2013 www.ti.com
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 V
IN
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 V
IN
and Ground pins.
Transformer Input Dual Maximum
Type Voltage Output Output
Voltage Current
L
P
= 100 μH 5V ±10V 325 mA
1 N = 1 5V ±12V 275 mA
5V ±15V 225 mA
10V ±10V 700 mA
10V ±12V 575 mA
2 L
P
= 200 μH 10V ±15V 500 mA
N = 0.5 12V ±10V 800 mA
12V ±12V 700 mA
12V ±15V 575 mA
3 L
P
= 250 μH 15V ±10V 900 mA
N = 0.5 15V ±12V 825 mA
15V ±15V 700 mA
Table 6. Flyback Transformer Selection Guide
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
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 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 43, the snubber 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;
(29)
Power dissipation (and power rating) of the resistor is;
(30)
The fast recovery diode must have a reverse voltage rating greater than V
CLAMP
.
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