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

V_IN
LM317L
V_OUT
V_adj
AND8109/D
R4
Q1
R_sense1
MPS2222
LED1
LED2
LED3
R5 ^Q2
R_sense2
MPS2222
LED4
LED5
LED6
R6 ^Q3
R_sense3
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 V_adj 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 V_adj 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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AND8109/D