AN3410

Circuit description

Doc ID 018890 Rev 1

7/28

the peak from this divider on C6. (Normally this voltage is used internally by the L6564 to

adjust its multiplier gain to accommodate a wide line voltage range.)

Obtaining the line current (the controlled variable)

The vast majority of input current flows through R22, the sense resistor for the PFC-flyback

converter. The average of the current in R22 and the average of the line voltage (minus the

LED voltage) will be used in the power calculation.

Calculating the average power

Scaling and addition of voltage and current is done by R17 and R14. The AC noise present

at their junction is removed by C12 - the DC voltage on C12 now represents the input power

as calculated by the linear approximation. This voltage will be regulated by the slow PFC

feedback loop.

Op amp U2 is wired as a non-inverting amplifier. The feedback loop requires only one

inversion, supplied by the op amp in U1.

U2 performs three different functions:

–

Derives a reference voltage from U1.

–

Provides gain for the relatively low voltage on C12.

–

Provides a point in the circuit to compensate for different LED voltages.

A DC reference voltage is derived from U1’s inverting input. This point will always be at 2.5 V

if the control loop is in steady-state, because there is no DC current path to any other

voltage source. The reference voltage is delivered to U2’s inverting input by divider R18-

R21. The voltage divider R18-R21 also sets the DC gain for U2.

If this circuit acted alone, the input power would be (approximately) regulated to a fixed

value, and the LED current would inversely track the LED voltage.

2.4

Setting the “DC” operating point

The control loop is to set the average current through R22 to deliver slightly more than the

desired LED current when both the line voltage and LED voltage are at design center.

Deviations of line and LED voltage from this point will then cause smaller deviations of LED

current.

approximation (of the constant-power curve) should give equal voltage from the average line

voltage and the average input current.

The value of R22 is determined from the usual calculations (see ST’s excellent application

note AN1059, reference 1).

The average input current (in R22) can now be calculated from the design center line

voltage, output power, and efficiency. At design center line voltage, LED current, and LED

voltage, the average voltage appearing across R17 due to current from R14 must match the

average voltage on R22.