Al5811, New prod uc t al5811, Application information – Diodes AL5811 User Manual
Page 6
AL5811
Document number: DS35255 Rev. 1 - 2
6 of 13
January 2013
© Diodes Incorporated
NEW PROD
UC
T
AL5811
Application Information
Description
The AL5811 is a Linear LED driver and in normal operation has the LEDs connected to the same potential as its VCC pin and regulates the LED
current by sinking current into to its LED pin. 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 AL5811 together with current regulator to set the LED current.
The 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
= 15k
mA
50
k
15
5
.
0
*
1500
I
LED
=
=
The AL5811 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 1 and 2. The voltage applied to the V
CC
pin can be greater or lower than the voltage applied to the LED
string. Figure 2 shows where you might power the AL5811 from a 5V rail and power the LED string from a 12V rail.
AL5811
V
CC
LED
GND
R
SET
R
SET
10kΩ
V
IN
C
IN
0.1µF
Figure 1. Low Side Current LED Setting
Figure 2. Low Side Drive – Separate Supplies
High Voltage Operation
An extension of Figure 2 is to derive the power for the AL5811 from the LED chain itself see Figure 3. LED chains greater than 60V can be driven in
this manner as long PWM dimming is not utilized.
Figure 3. Low Side LED String Tapping
Figure 4. High Side Current LED String
The AL5811 can also be used on the high side of the LEDs, see Figure 4. 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.
AL5811
V
CC
LED
GND
R
SET
R
SET
= 10k
Ω
V
IN
C
IN
0.1µF
V
LED
V
IN
C
IN
0.1µF
AL5811
V
CC
LED
GND
R
SET
R
SET
AL5811
V
CC
LED
GND
R
SET
R
SET
V
IN
C
IN
0.1µF