LM1577, LM2577
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SNOS658D –JUNE 1999–REVISED APRIL 2013
A: Switch pin voltage, 10 V/div
B: Switch pin current, 2 A/div
C: Inductor current, 2 A/div
D: Output ripple voltage, 100 mV/div (AC-coupled)
Horizontal: 5 μs/div
Figure 40. Switching Waveforms of Step-Up
Regulator of Figure 37
FLYBACK REGULATOR
A Flyback regulator can produce single or multiple output voltages that are lower or greater than the input supply
voltage. Figure 42 shows the LM1577/LM2577 used as a flyback regulator with positive and negative regulated
outputs. Its operation is similar to a step-up regulator, except the output switch contols the primary current of a
flyback transformer. 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 (BOOST) REGULATOR section.
Voltage and current waveforms for this circuit are shown in Figure 41, and formulas for calculating them are
given in Table 5.
FLYBACK REGULATOR DESIGN PROCEDURE
1. Transformer Selection
A family of standardized flyback transformers is available for creating flyback regulators that produce dual output
voltages, from ±10V to ±15V, as shown in Figure 42. Table 6 lists these transformers with the input voltage,
output voltages and maximum load current they are designed for.
2. Compensation Network (CC, RC) and
Output Capacitor (COUT) Selection
As explained in the Step-Up Regulator Design Procedure, CC, 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
∑ILOAD(max) to ILOAD(max) in the following equations.
A. First, calculate the maximum value for RC.
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