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LM2585 Datasheet(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2585
Description  SIMPLE SWITCHER 3A Flyback Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2585 Datasheet(HTML) 9 Page - National Semiconductor (TI)

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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 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 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-
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.
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.
As shown in
Figure 4, 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 5. Typical Performance Characteristics observed during the operation of this circuit are shown in Figure 6.
FIGURE 4. 12V Flyback Regulator Design Example
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
Horizontal: 2 µs/div
FIGURE 5. Switching Waveforms

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