AN3410

Circuit description

Doc ID 018890 Rev 1

11/28

To find the value of R21, we first arbitrarily select 25.2 k

Ω for R18. R21’s value is then

Equation 8

Use 1.10 k

Ω.

Now we examine the operating points when the LEDs are at nominal voltage. The peak

voltage at U1's Vff terminal will now be

Equation 9

So at nominal input power (20 W) the average voltage applied to C12 and U2's non-inverting

input is the voltage from R22 plus the divided voltage from R14 and R17:

Equation 10

R12 must be selected to deliver this voltage at U2's inverting input.

The equivalent voltage and resistance presented by U2's 2.50 V output and the R18-R21

divider is:

Equation 11

R12 must bring the voltage on U2's inverting input up to 0.175 V from 1.06 V when the

reflected LED voltage is at its nominal value.

Neglecting diode and R2 drops, the voltage at the top of R12 is the LED voltage times the

turns ratio of L1's auxiliary winding:

Equation 12

R12 must provide enough current to bring the voltage up to 0.175V. The added voltage due

to R12 must be 0.175 V - 0.104 V = 0.071 V.

R12's value is then

Equation 13

We will use 240 k

Ω.

Some trimming may be required to hit the 350 mA * 1.012 target to center the operating

point in the error band.

=1.115 k

Ω