LM1577 datasheet(21/31 Pages) NSC

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Application Hints (Continued)

FLYBACK REGULATOR

A Flyback regulator can produce single or multiple output

voltages that are lower or greater than the input supply

voltage. Figure 18 shows the LM1577/LM2577 used as a

flyback regulator with positive and negative regulated out-

puts. Its operation is similar to a step-up regulator, except the

output switch contols the primary current of a flyback trans-

former. Note that the primary and secondary windings are

out of phase, so no current flows through secondary when

current flows through the primary. This allows the primary to

charge up the transformer core when the switch is on. When

the switch turns off, the core discharges by sending current

through the secondary, and this produces voltage at the

outputs. The output voltages are controlled by adjusting the

peak primary current, as described in the step-up regulator

section.

Voltage and current waveforms for this circuit are shown in

Figure 17, and formulas for calculating them are given in

Figure 19.

FLYBACK REGULATOR DESIGN PROCEDURE

1. Transformer Selection

A family of standardized flyback transformers is available for

creating flyback regulators that produce dual output volt-

ages, from ±10V to ±15V, as shown in Figure 18. Figure

20lists these transformers with the input voltage, output

voltages and maximum load current they are designed for.

2. Compensation Network (C

C,RC) and

Output Capacitor (C

OUT) Selection

As explained in the Step-Up Regulator Design Procedure,

C,RC and COUT must be selected as a group. The following

procedure is for a dual output flyback regulator with equal

turns ratios for each secondary (i.e., both output voltages

have the same magnitude). The equations can be used for a

single output regulator by changing

∑I

LOAD(max) to ILOAD(max)

in the following equations.

A. First, calculate the maximum value for R

Where

∑I

LOAD(max) is the sum of the load current (magni-

tude) required from both outputs. Select a resistor less than

or equal to this value, and no greater than 3 k

Ω.

B. Calculate the minimum value for

∑C

OUT (sum of COUT

at both outputs) using the following two equations.

The larger of these two values must be used to ensure

regulator stability.

01146817

FIGURE 17. Flyback Regulator Waveforms

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