Diodes AL5811 - Manual (Page 7)

AL5811
Document number: DS35255 Rev. 1 - 2
7 of 13
www.diodes.com
January 2013
© Diodes Incorporated
NEW PROD
UC
T
AL5811
Application Information
(cont.)
PWM Dimming
LED current dimming can be achieved by driving the R
SET
pin via the current setting resistor (R
SET
) and series MOSFET switch to ground
(Figure 5). The R
SET
pin current is then effectively switched on and off causing the LED current to turn on and off.
Figure 5. PWM Dimming
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
LED Current vs. Duty Cycle
Frequency = 100Hz; Duty Cycle = 0.1% to 100%;
R = 10K; 2LEDs in Series
SET
LE
D
CURRENT
(
m
A
)
Figure 6. PWM Dimming Linearity
Thermal Considerations
When designing linear LED drivers careful consideration must be given to:
1.
the power dissipation within the LED driver
and
2. PCB layout/heat sinking.
A Linear LED driver has to be able to handle the large potential input voltage variations due to the supply voltage tolerance and also the variation in
LED forward voltage due to binning and temperature.
This can result in a large potential difference across the LED driver resulting in a larger than anticipated power dissipation.
For example in n 12V powered system with a 5% output voltage tolerance; the input voltage could typically vary from 12.6V down to 11.4V driving
3 LEDs with a voltage varying from 3V to 3.5V at 75mA. This means that the LED driver has to cope with a voltage drop across varying from
approximately 3.6V to 0.9V. This means that the power dissipation of the AL5811 could be as much as 270mW.
Figure 7 below shows how the AL5811’s power dissipation capability varies with package; these values will vary with PCB size and area of metal
associated with the ground plane used for heat sinking. By increasing the area on the top layer the thermal impedance of both packages could be
improved.