Datasheet

DocID023432 Rev 4 15/40
LED2000 Application notes
40
The embedded compensation network is R
C
= 70 K, C
C
= 195 pF while C
P
and C
O
can be
considered as negligible. The error amplifier output resistance is 240 M
so the relevant
singularities are:
Equation 12
6.4 LED small signal model
Once the system reaches the working condition, the LEDs composing the row are biased
and their equivalent circuit can be considered as a resistor for frequencies << 1 MHz.
The LED manufacturer typically provides the equivalent dynamic resistance of the LED
biased at different DC currents. This parameter is required to study the behavior of the
system in the small signal analysis.
For instance, the equivalent dynamic resistance of the Luxeon III Star from Lumiled
measured with different biasing current level is reported below:
Equation 13
If the LED datasheet does not report the equivalent resistor value, it can be simply derived
as the tangent to the diode I-V characteristic in the present working point (see Figure 7).
f
Z
11 6 kHz= f
P LF
34 Hz=
r
LED
1,3 I
LED
350mA=
0,9 I
LED
700mA=


