AN4130 datasheet(25/41 Pages) STMICROELECTRONICS

25 / 41 page

AN4130

Design guidance

Doc ID 023315 Rev 1

25/41

4.9

COMP pin capacitor

Because the HVLED815PF is used only in current limit mode when driving LEDs, the usual

loop stability compensation network is not needed. Voltage limiting is used only when the

LED string is open (think of bench testing…) and the comp pin capacitor needs to be small

so that overvoltage response is quick. 1 nanofarad is a good value.

4.10

Current sense resistor

This resistor value is determined by the average LED current desired and turns ratio of the

transformer, according to a formula, as follows:

Equation 3

where n is the transformer turns ratio, VCLED the internal reference, and RSENSE the current

sensing resistor. Internally, VCLED is 0.2 V.

This is the ideal situation. In fact, because the transformer is not the ideal transformer,

imperfect coupling makes the actual turns ratio less than the designed value. Also the

voltage feed-forward compensation and demagnetizing time further reduce the actual LED

current from the calculated number. Typically, the coefficient K is around 0.85~0.9.

Therefore the formula to determine the resistor is:

Equation 4

For this demo board, with turns ratio of 2 and a 1.00

Ω, 1% current sensing resistor, we get

the LED current around 180 mA. For different designs, tweaks may be needed, but once the

final values are selected, repeatability from unit to unit is excellent.

4.11

AC injection divider (R3, R4)

In the US, line current total harmonic distortion must be kept below 20% of the 60 Hz

fundamental.

The input current is already distorted due to the use of a sinusoidal peak current envelope.

Input current harmonic distortion actually improves slightly when the line current goes to

zero for a short time around the voltage zero crossing. The distortion minimum only occurs

at one input voltage, increasing as the line voltage is moved away from that point. The

average of the injected AC waveform (not the RMS value) should be set approximately

equal to the DC level required to give the correct current to the LEDs. At nominal line the

average injected voltage is close to 0.95 V - the RMS voltage is 1.111 times the average, or

1.05 V, peak voltage is about 1.45 V. Again, tweaking may be necessary, but repeatability

between units is excellent once the values are selected.

AN4130 Datasheet(PDF) 25 Page - STMicroelectronics