AND8109 Datasheet, PDF (1/2 Pages) ON Semiconductor – LED CONSTANT CURRENT SOURCE SCHEME

AND8109/D
LED Constant Current
Source Scheme
Theory of Operation
Prepared by: Mark Scholten
Senior Applications Engineer
http://onsemi.com
The LM317L wants to see 1.25 V between its V_OUT pin
and the V_adj pin, and it will do whatever it can to keep that
voltage differential between them. So if a resistor is put in
series with the output, and the V_adj pin is connected to the
load side of the sense resistor, a current source is set up
which follows Equation 1:
IOUT
+
1.25
Rsense
(Approximately)
(eq. 1)
V_IN
V_OUT
LM317L
I_OUT
R_sense
V_adj
R_load
But what if a series/parallel combination of LEDs is
wanted? The following circuit works fine, assuming that
there are not large variations in the forward voltage drop of
the LEDs. There could be a problem however if one of the
strings opens up for some reason. The LEDs that remain
would have 50% more current flowing through them, which
could cause them to be driven too hard and fail. So how does
one get around this problem?
V_IN
V_OUT
LM317L
R_sense
I_OUT
LED1 LED4 LED7
V_adj
LED2 LED5 LED8
LED3 LED6 LED9
Figure 1.
In order to set up a constant current source for an LED
string, the same circuit can be used by simply substituting
R_load with an LED string, as shown below.
V_IN
LM317L ^VOUT
I_OUT
R_sense
LED1
V_adj
LED2
Figure 3.
The following circuit takes care of this issue by splitting
up the current sense resistor into three resistors, one for each
leg of the LED string. The current through any one of the
LED strings is set by Equation 2:
IOUT
+
1.25
V)
R1
Vsat
(Approximately)
(eq. 2)
LED3
Figure 2.
 Semiconductor Components Industries, LLC, 2003
1
January, 2003 – Rev. 0
Publication Order Number:
AND8109/D