lapses, reversing the voltage polarity of the primary and sec-
ondary windings. Now rectifier D1 is forward biased and
current flows through it, releasing the energy stored in the
transformer. This produces voltage at the output.
Flyback Regulator Operation
The LM2587 is ideally suited for use in the flyback regulator
topology. The flyback regulator can produce a single output
voltage, such as the one shown in Figure 4, or multiple out-
put voltages. In Figure 4, the flyback regulator generates an
output voltage that is inside the range of the input voltage.
This feature is unique to flyback regulators and cannot be
duplicated with buck or boost regulators.
The output voltage is controlled by modulating the peak
switch current. This is done by feeding back a portion of the
output voltage to the error amp, which amplifies the differ-
ence between the feedback voltage and a 1.230V reference.
The error amp output voltage is compared to a ramp voltage
proportional to the switch current (i.e., inductor current dur-
ing the switch on time). The comparator terminates the
switch on time when the two voltages are equal, thereby
controlling the peak switch current to maintain a constant
output voltage.
The operation of a flyback regulator is as follows (refer to
Figure 4): when the switch is on, current flows through the
primary winding of the transformer, T1, storing energy in the
magnetic field of the transformer. Note that the primary and
secondary windings are out of phase, so no current flows
through the secondary when current flows through the pri-
mary. When the switch turns off, the magnetic field col-
DS012316-10
As shown in Figure 4, the LM2587 can be used as a flyback regulator by using a minimum number of external components. The switching waveforms of this
regulator are shown in Figure 5. Typical Performance Characteristics observed during the operation of this circuit are shown in Figure 6.
FIGURE 4. 12V Flyback Regulator Design Example
Typical Performance Characteristics
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A: Switch Voltage, 10 V/div
B: Switch Current, 5 A/div
C: Output Rectifier Current, 5 A/div
D: Output Ripple Voltage, 100 mV/div
AC-Coupled
Horizontal: 2 µs/div
FIGURE 5. Switching Waveforms
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