lm5000sdx-6 National Semiconductor Corporation, lm5000sdx-6 Datasheet - Page 13

• Manufacturer Part Number: lm5000sdx-6
• Description: High Voltage Switch Mode Regulator
• Manufacturer: National Semiconductor Corporation

OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the max-
imum switch voltage minus a diode drop. In a flyback regula-
tor, the maximum output voltage is determined by the turns
ratio, N, and the duty cycle, D, by the equation:
The duty cycle of a flyback regulator is determined by the fol-
lowing equation:
Theoretically, the maximum output voltage can be as large as
desired—just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from in-
creasing to infinity. The physical limitations are capacitances
and inductances in the LM5000 switch, the output diode(s),
and the transformer—such as reverse recovery time of the
output diode (mentioned above).
INPUT LINE CONDITIONING
A small, low-pass RC filter should be used at the input pin of
the LM5000 if the input voltage has an unusually large amount
of transient noise. Additionally, the RC filter can reduce the
dissipation within the device when the input voltage is high.
V
OUT
N × V
IN
× D/(1 − D)
FIGURE 5. LM5000 Flyback Converter
13
Flyback Regulator Operation
The LM5000 is ideally suited for use in the flyback regulator
topology. The flyback regulator can produce a single output
voltage, or multiple output voltages.
The operation of a flyback regulator is as follows: 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
D5 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 1.259V reference. The
error amp output voltage is compared to a ramp voltage pro-
portional 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.
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20031972