Datasheet
DocID18279 Rev 5 15/37
ST1CC40 Application notes
37
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 current. This parameter is required to study the behavior of the
system in the small signal analysis.
For instance, the equivalent dynamic resistance of Luxeon III Star from Lumiled measured
with a different biasing current level is reported below:
In case the LED datasheet doesn’t 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).
Figure 7. Equivalent series resistor
f
Z
11 6 kHz= f
P LF
34 Hz=
r
LED
1,3 I
LED
350mA=
0,9 I
LED
700mA=
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