LM5000

SNVS176D – MAY 2004 – REVISED MARCH 2007

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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.

Figure 7. LM5000 Flyback Converter

ITEM

PART NUMBER

DESCRIPTION

VALUE

C

1

C4532X7R2A105MT

Capacitor, CER, TDK

1µ, 100V

C

2

C4532X7R2A105MT

Capacitor, CER, TDK

1µ, 100V

C

3

C1206C224K5RAC

Capacitor, CER, KEMET

0.22µ, 50V

C

4

C1206C104K5RAC

Capacitor, CER, KEMET

0.1µ, 50V

C

5

C1206C104K5RAC

Capacitor, CER, KEMET

0.1µ, 50V

14

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