Datasheet
Table Of Contents
- Figure 1. Typical application circuit
- 1 Pin settings
- 2 Maximum ratings
- 3 Electrical characteristics
- 4 Functional description
- 5 Application notes - buck conversion
- 5.1 Closing the loop
- 5.2 GCO(s) control to output transfer function
- 5.3 Error amplifier compensation network
- 5.4 LED small signal model
- 5.5 Total loop gain
- 5.6 Compensation network design
- 5.7 Example of system design
- 5.8 Dimming operation
- 5.9 Component selection
- 5.10 Layout considerations
- 5.11 Thermal considerations
- 5.12 Short-circuit protection
- 5.13 Application circuit
- 6 Application notes - alternative topologies
- 7 Package mechanical data
- 8 Ordering information
- 9 Revision history
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
r
LED
1.3Ω I
LED
350mA=
0.9Ω I
LED
700mA=
⎩
⎨
⎧
AM13492v1
1
0.1
1
2
3
[V]
4
[A]
working point
R
LOAD
n
LED
r
LED
R
SENSE
+⋅=