Circuit description

AN3410

6/28

Doc ID 018890 Rev 1

2.3

Controlling the LED average current

The control circuit works by controlling average input power. For this explanation, it is

assumed that the power converter efficiency is constant over the range of line voltage and

LED voltage. Therefore, average output power is also controlled.

Linear approximation of input power

Over a narrow range of line voltage, the sum of scaled average line voltage and scaled

average line current closely approximates a constant power curve. If the sum is held

constant, the input power can be held approximately constant by a feedback circuit.

Figure 4.

Constant-power V-I curve and linear approximation

Figure 5 below shows the approximation’s error over a typical line voltage range.

Figure 5.

Linear approximation error

This will be explained in more detail later.

Obtaining the line voltage reference

In previous work it was found that the reference waveform for the inverted step-down

switching regulator should be taken from the negative output terminal for best power factor.

This point gives a line current waveform that goes to zero around the zero crossings and

rises (and falls) more rapidly than the line voltage sine wave. (The converter input current

goes to zero when line voltage falls below the DC output voltage.) The resulting line current

can be seen in the scope photos. It’s ugly, but the power factor is excellent and THD is

acceptable.

Average line voltage (minus the LED voltage) is derived from the peak voltage at the bottom

of the LED string by R6, R15, and R20. U1 contains a precision peak detector, which places