LM1577 datasheet(21/27 Pages) NSC

21 / 27 page

Application Hints (Continued)

Transformer

Input

Dual

Maxi-

mum

Type

Voltage

Output

Voltage

Current

P = 100 µH

±10V

325 mA

1N = 15V

±12V

275 mA

±15V

225 mA

10V

±10V

700 mA

10V

±12V

575 mA

P = 200 µH

10V

±15V

500 mA

N = 0.5

12V

±10V

800 mA

12V

±12V

700 mA

12V

±15V

575 mA

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

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

326-0637

PE-65300

RL-2580

330-0202

PE-65301

RL-2581

330-0203

PE-65302

RL-2582

FIGURE 20. Flyback Transformer Selection Guide

DS011468-19

FIGURE 21. Snubber Circuit

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