LED5000 datasheet(16/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

16/51

Doc ID 023951 Rev 1

5.4

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 different biasing current levels is reported below:

In case the LED datasheet does not provide the equivalent resistor value, it can be easily

derived as the tangent to the diode I-V characteristic in the present working point (see

Figure 8).

Figure 8.

Equivalent series resistor

Figure 9 shows the equivalent circuit of the LED constant current generator.

The equivalent loading resistor in the LEDs working point is:

Equation 12

LED

1.3

LED

350mA

0.9

LED

700mA

⎩

⎨

⎧

AM13492v1

0.1

[V]

[A]

working point

LOAD

LED

SENSE

⋅

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