AN3410

Circuit description

Doc ID 018890 Rev 1

9/28

Figure 7.

Error for 3 levels of load power

Note that the curves coincide at the nominal line voltage, where LED voltage compensation

is perfect. Also, for nominal LED voltage, line voltage compensation is at its best, +/-1.5%.

Over the entire range of line voltage (120 V +10%,-20%) and LED (+/-10%) voltage, the

variation of LED current is less than 4.5% (+1.5%, -3%).

2.5

Designing the “DC” control loop

Finding values for the resistors in this loop is surprisingly easy.

(For this procedure, diode drops will be ignored for simplicity.)

Let us assume that the average LED current is to be 350 mA, and a string of 18 LEDs drops

about 54.6 V. Output power is 18 watts. Input power will be assumed to be 20 W (90%

efficiency, actually conservative).

Assume the input current and voltage waveforms are sine waves (power factor is high, so

this is a good assumption).

The converter output current waveform is a triangle wave with an (assumed) rectified

sinusoidal upper envelope, and a lower envelope at zero. The short-term average of the

triangle wave is half its height. The rectified sinusoid has an average current of 63% of its

peak current.

At the peak of the input voltage, the peak current delivered to the LEDs is therefore.

Equation 1

We will allow some margin for the relatively slow fall time of the drain voltage due to the zero

current detection, so the peak current will be slightly higher. We will design the inductor for

1.4 A peak current in the main winding. The L6564 control chip has an upper voltage limit on

its current sense input of 1.08 V for linear operation. So the maximum value of R22 is

Equation 2