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 10

Ω and1kΩ, 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:

V

OUT

≈ NxV

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-

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.

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