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AL5812FF-7

P1-P3P4-P6P7-P9P10-P12P13-P13
AL5812
Document number: DS35616 Rev. 4 - 2
7 of 13
www.diodes.com
March 2014
© Diodes Incorporated
A
L5812
Application Information
Description
The AL5812 is a Linear LED driver and in normal operation has the LEDs connected to the same potential as its V
CC
pin and regulates the LED
current by sinking current into to its LED pin see Figure 13.
The LED current is set by the use of an external resistor, R
SET
, connected from the R
SET
pin to GND. This resistor supplies the bias current of the
AL5812 together with current regulator to set the LED current.
The nominal LED current is determined by this equation:
SET
LED
R
5.0
*1500I
where 1500 is the current ratio between the LED pin current and R
SET
pin current.
With R
SET
= 7.5k
mA100
k5.7
5.0
*1500I
LED
The AL5812 with its 60V capability on its supply pin, V
CC
, and its LED drive pin allows it to operate from supply rails up to 60V and/or directly
drive LED chains up to 60V as shown in Figures 13 and 14. The voltage applied to the V
CC
pin can be greater or lower than the voltage applied
to the LED string. Figure 14 shows where you might control it from a 5-V rail but power the rails from a 12V rail.
Figure 13 Low Side Current LED Setting
Figure 14 Low Side Drive – Separate Supplies
AL5812
V
CC
LED
GND
R
SET
R
SET
= 4.99kΩ
V
IN
C
IN
0.1µF
V
LED
AL5812
Document number: DS35616 Rev. 4 - 2
8 of 13
www.diodes.com
March 2014
© Diodes Incorporated
A
L5812
Application Information (cont.)
High Voltage Operation
An extension of Figure 14 is to derive the power for the AL5812 from the LED chain itself see Figure 15. LED chains greater than 60V can be
driven in this manner as long PWM dimming is not utilized.
Figure 15 Low Side LED String Tapping
Figure 16 High Side Current LED String
Figure 15 shows the use of RC delay to match the power time delay between Vcc and LED pin. The AL5812 can also be used on the high side of
the LEDs, see Figure 16. This is a simple way of extending the maximum LED chain voltage, however, it does increase the minimum system input
voltage to:
V
IN(min)
= V
LED_CHAIN
+ 3.5V.
Where
V
LED_CHAIN
is the LED chain voltage.
AL5812
Document number: DS35616 Rev. 4 - 2
9 of 13
www.diodes.com
March 2014
© Diodes Incorporated
A
L5812
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
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 an 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.
T = +25°C
2 LEDs
R = 7.5k
f = 500Hz
A
SET
PWM
10 200901008040 5030 7060
DUTY CYCLE (%)
Figure 18 PWM Dimming Linearity
110
100
90
80
70
60
50
40
30
20
10
0
LED
C
U
R
R
EN
T
(m
A
)
-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
I
P
A
T
I
O
N
(W)
U-DFN3030-6
MSOP-8EP
P1-P3P4-P6P7-P9P10-P12P13-P13

AL5812FF-7

Mfr. #:
Buy AL5812FF-7
Manufacturer:
Diodes Incorporated
Description:
LED Lighting Drivers 60V 150mA ADJ Linear LED Driver
Lifecycle:
New from this manufacturer.
Delivery:
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