Diodes AL5812 - Manual (Page 7)

AL5812
Document number: DS35616 Rev. 2 - 2
7 of 11
www.diodes.com
November 2013
© Diodes Incorporated
NEW PROD
UC
T
AL5812
Application Information
(cont.)
PWM Dimming
The AL5812 can be used to provide LED current dimming driving the R
SET
pin via the current setting resistor (R
SET
) and a series MOSFET
switch to ground (Figure 17). The R
SET
pin current is then effectively switched on and off causing the LED current to turn on and off. The linearity
is shown in Figure 18.
Figure 17 PWM Dimming
T = +25°C
2 LEDs
R
= 7.5k
f = 500Hz
A
SET
PWM
Ω
10 20 0 90 100 80 40 50 30 70 60
DUTY CYCLE (%)
Figure 18 PWM Dimming Linearity
110
100
90
80
70
60
50
40
30
20
10
0
LE
D
C
U
R
R
E
N
T
(
m
A
)
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 a 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 100mA. 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 AL5812 could be as much as 366mW.
Figure 19 below shows how the AL5812’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.
-40 -25 -10 5 20 35 50 65 80 95 110 125
AMBIENT TEMPERATURE (C)
Figure 19 Power Dissipation Derating
1.6
1.4
0.2
0.0
0.6
0.4
1.0
0.8
1.2
P
O
W
E
R
DI
S
S
IP
A
T
IO
N
(W
)
U-DFN3030-6
MSOP-8EP