LM2585

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SNVS120F – APRIL 2000 – REVISED APRIL 2013

FLYBACK REGULATOR OPERATION

The LM2585 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 21, or multiple output voltages. In Figure 21, 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 operation of a flyback regulator is as follows (refer to Figure 21): 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 primary. When the switch turns off, the magnetic field collapses, reversing the voltage

polarity of the primary and secondary 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.

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 difference 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 during 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.

Figure 21. 12V Flyback Regulator Design Example

As shown in Figure 21, the LM2585 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 22. Typical Performance

Characteristics

observed

during

the

operation

of

this

circuit

are

shown

in

Figure

23.

A: Switch Voltage, 20 V/div

B: Switch Current, 2 A/div

C: Output Rectifier Current, 2 A/div

D: Output Ripple Voltage, 50 mV/div

AC-Coupled

Horizontal: 2

μs/div

Figure 22. Switching Waveforms

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