Circuit description

AN3410

8/28

Doc ID 018890 Rev 1

Stirring in the LED voltage

Compensation for LED voltage changes follows two paths in this design:

–

by direct subtraction of the LED voltage from the line voltage (path 1)

–

from the transformed LED voltage on C9 through R12 (path 2).

Path 1

An increase of LED voltage will reduce the voltage at U1 pin 5. This reduces current through

R14. The feedback loop will then call for more line current to compensate.

Path 2

The voltage compensation obtained from the 18-LED load is not quite sufficient to flatten the

LED current over the expected range of LED voltage, so the second path through R12 is

also implemented. The LED voltage (multiplied by L1’s turns ratio) is available on D4’s

cathode, filtered by C9. Current proportional to this voltage is delivered to U2’s inverting

input by R12. The reference for the operating point is thus compensated by the LED voltage.

The figures below show the results of compensating the input current setting for load

voltage.

Figure 6.

Input current compensated for 3 levels of output power

In Figure 6 the reference has been compensated by adding a current proportional to the

LED voltage. Three levels of input power are shown, corresponding to LED voltages of 10%

below nominal, nominal, and 10% above nominal. Figure 7 below shows the error between

the approximations and the ideal values.