Diodes AL5812 - Manual (Page 6)

AL5812
Document number: DS35616 Rev. 2 - 2
6 of 11
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November 2013
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AL5812
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
* 1500 I = where 1500 is the current ratio between the LED pin current and R
SET
pin current.
With R
SET
= 7.5k
mA 100
k 5 . 7
5 . 0
* 1500 I
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
AL5812
V
CC
LED
GND R
SET
R
SET
= 4.99kΩ
V
IN
C
IN
0.1µF
V
LED
Figure 14 Low Side Drive – Separate Supplies
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
V
IN
C
IN
0.1µF
AL5812
V
CC
LED
GND R
SET
R
SET
= 4.99kΩ
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.