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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:
1.25 IOUT + (Approximately) Rsense
(eq. 1)
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 LM317L VOUT Rsense IOUT LED1 Vadj LED4 LED7
VIN LM317L
VOUT Rsense
IOUT
Vadj
Rload
LED2
LED5
LED8
LED3
LED6
LED9
Figure 1.
Figure 3.
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 Rsense LED1 Vadj IOUT
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 ) Vsat (Approximately) R1
(eq. 2)
LED2
LED3
Figure 2.
Semiconductor Components Industries, LLC, 2003
1
January, 2003 - Rev. 0
Publication Order Number: AND8109/D
AND8109/D
VIN LM317L Vadj R4 Rsense1 Q1 R5 MPS2222 Rsense2 Q2 R6 MPS2222 Rsense3 Q3 MPS2222 VOUT
LED1
LED4
LED7
LED2 LED3
LED5 LED6
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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