AND8109 Source Scheme Datasheet

AND8109 Datasheet, PDF, Equivalent


Part Number

AND8109

Description

LED Constant Current Source Scheme

Manufacture

On Semiconductor

Total Page 2 Pages
Datasheet
Download AND8109 Datasheet


AND8109
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 VOUT pin
and the Vadj 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 Vadj 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)
VIN
LM317L
VOUT
IOUT
Rsense
Vadj
Rload
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?
VIN VOUT
LM317L
Rsense
IOUT
LED1 LED4 LED7
Vadj
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
Rload with an LED string, as shown below.
VIN LM317L VOUT
IOUT
Rsense
LED1
Vadj 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
January, 2003 – Rev. 0
1
Publication Order Number:
http://www.Datasheet4U.com
AND8109/D

AND8109
VIN
LM317L
VOUT
Vadj
AND8109/D
R4 Q1
Rsense1
MPS2222
LED1
LED2
LED3
R5 Q2
Rsense2
MPS2222
LED4
LED5
LED6
R6 Q3
Rsense3
MPS2222
LED7
LED8
LED9
Figure 4.
When the circuit operates properly and all the LEDs are
running, the three sense resistors have about 1.25 V across
them, which turns the transistor switches ‘on’. This connects
all three sense resistors back to the Vadj pin allowing the
proper current to go through each leg. If one string opens up,
the sense resistor for that leg won’t have any voltage across
it, turning ‘off’ the transistor and disconnecting its sense
resistor from the Vadj pin. Therefore, the other two LED
strings are unaffected by the fault. This same scheme can be
expanded to accommodate as many LED strings as needed.
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2
AND8109/D


Features AND8109/D LED Constant Current Source Sc heme Theory of Operation Prepared by: M ark Scholten Senior Applications Engine er http://onsemi.com The LM317L wants to see 1.25 V between its VOUT pin and the Vadj pin, and it will do whatever i t can to keep that voltage differential between them. So if a resistor is put in series with the output, and the Vadj pin is connected to the load side of t he sense resistor, a current source is set up which follows Equation 1: 1.25 I OUT + (Approximately) Rsense (eq. 1) B ut what if a series/parallel combinatio n of LEDs is wanted? The following circ uit works fine, assuming that there are not large variations in the forward vo ltage drop of the LEDs. There could be a problem however if one of the strings opens up for some reason. The LEDs tha t remain would have 50% more current fl owing through them, which could cause t hem to be driven too hard and fail. So how does one get around this problem? V IN LM317L VOUT Rsense IOUT LED1 Vadj LED4 LED7 VIN LM317L VOU.
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