LM5000

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SNVS176D – MAY 2004 – REVISED MARCH 2007

this voltage spike is caused by the transformer leakage inductance and/or the output rectifier recovery time. To

“clamp” the voltage at the switch from exceeding its maximum value, a transient suppressor in series with a

diode is inserted across the transformer primary.

If poor circuit layout techniques are used, negative voltage transients may appear on the Switch pin. Applying a

negative voltage (with respect to the IC's ground) to any monolithic IC pin causes erratic and unpredictable

operation of that IC. This holds true for the LM5000 IC as well. When used in a flyback regulator, the voltage at

the Switch pin can go negative when the switch turns on. The “ringing” voltage 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 methods 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:

(21)

The duty cycle of a flyback regulator is determined by the following equation:

(22)

Theoretically, the maximum output voltage can be as large as desired—just keep increasing the turns ratio of the

transformer. However, there exists some physical limitations that prevent the turns ratio, and thus the output

voltage, from increasing 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.

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 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.

Copyright © 2004–2007, Texas Instruments Incorporated

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