lm5000sdx-6ep National Semiconductor Corporation, lm5000sdx-6ep Datasheet - Page 14

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

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Operation
at the switch pin is caused by the output diode capacitance
and the transformer leakage inductance forming a resonant
circuit at the secondary(ies). The resonant circuit generates
the “ringing” voltage, which gets reflected back through the
transformer to the switch pin. There are two common meth-
ods to avoid this problem. One is to add an RC snubber
around the output rectifier(s). The values of the resistor and
the capacitor must be chosen so that the voltage at the
Switch pin does not drop below −0.4V. The resistor may
range in value between 10Ω and 1 kΩ, and the capacitor will
vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)
reduce the efficiency of the overall circuit.
The other method to reduce or eliminate the “ringing” is to
insert a Schottky diode clamp between the SW pin and the
PGND pin. The reverse voltage rating of the diode must be
greater than the switch off voltage.
OUTPUT VOLTAGE LIMITATIONS
The maximum output voltage of a boost regulator is the
maximum switch voltage minus a diode drop. In a flyback
regulator, 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
following 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
increasing to infinity. The physical limitations are capaci-
V
OUT
(Continued)
≈ N x V
IN
x D/(1 − D)
14
tances and inductances in the LM5000EP 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 LM5000EP 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.
Flyback Regulator Operation
The LM5000EP is ideally suited for use in the flyback regu-
lator 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 differ-
ence between the feedback voltage and a 1.259V 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.