AN2129 APPLICATION NOTE

6/9

Both equations (Equation 1 and Equation 2) must be valid together, i.e. two equations for three

before and than the other resistors calculated from the equations mentioned. This process

should be iterative (calculated for different chosen resistors) to get resistor values as close to

the industrial standard values as possible. The Table 2 can help for work simplification, be-

cause it contains resistor values for the most common super high brightness LEDs.

Table 2. Pre-calculated standard values for feedback loop

*

ILED is a nominal LED current obtained with minimum dimming voltage (Vdimm=0V)

**

VdimMAX means dimming voltage for maximum dimming i.e. zero output current (ILED=0A)

5 Measurement

A couple of measurements have been performed on the board; the results are on the graphs

below. One up to six LEDs in serial string have been used as load (Golden Dragon LW W5SG

from OSRAM)

In Figure 9 there is a LED current waveform during dimming with PWM signal at 100Hz fre-

quency. It could be noticed a waveform rounding during turning-on and off, which is caused by

charging the output capacitor C2. If the sharper on and off edges are needed a smaller capac-

itor should be used (e.g. 1

µF), but on the other side it must be taken in account that it will rise

the current ripple.

R4 [

Ω]R5 [Ω]R6 [Ω]

350

3.3

330

1000

8200

27000

700

3.3

200

910

2400

20000

1000

3.3

120

910

5600

24000

Figure 9. PWM dimming (50%)

Figure 10. Current ripple

(1 LED, 15V input, 0% dimming)