Datasheet

LM2588
SNVS117D –APRIL 1998–REVISED APRIL 2013 www.ti.com
The operation of a flyback regulator is as follows (refer to Figure 20): 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.
As shown in Figure 20, the LM2588 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 21. Typical Performance Characteristics
observed during the operation of this circuit are shown in Figure 22.
Figure 20. 12V Flyback Regulator Design Example
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