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LM317LBDR2-LM317LZRA-MPS2222ARLRM
Small Signal General Purpose NPN
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Theory of Operation
Prepared by: Mark ScholtenSenior Applications Engineer
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
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.
LED1
LED2
LED3
Figure 2. VIN
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
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� 1.25V� Vsat (Approximately) (eq. 2)
