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Circuit Performance
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2 Circuit Performance
The LM3406 circuit and BOM that come pre-installed on the evaluation board are optimized to run from an
input voltage of 24V, but the circuit can operate from a wide input voltage range of 6.0V to 42V. The
current output ranges from 0.35A to 1.5A. Figure 2 shows the program jumper settings used to program
currents of 0.35A, 0.7A, 1A, and 1.5A.
Figure 2. Setting Output Current, J2
The LM3406 is a step-down regulator with an output voltage range extending from a V
O-MIN
of 200 mV (the
reference voltage) to a V
O-MAX
determined by the ratio of the minimum off time (typically 230 ns) to the
switching frequency. The regulator can maintain the output current through any number of LEDs as long
as the combined forward voltage of the array does not exceed V
O-MAX
. V
O-MAX
can be calculated with the
following formula:
V
O-MAX
= V
IN-MIN
x (1 - f
SW
x t
OFF-MIN
) (1)
For example, if V
IN
is 24V ±10%, then V
IN-MIN
is 21.6V. For a switching frequency of 500 kHz the maximum
output voltage for the converter is 21.6 x [1 - (5 x 10
5
) x (230 x 10
-9
) = 19.1V. Output voltage is calculated
with the following formula:
V
O
= n x V
F
+ 0.2V (2)
Where 'n' is the number of series-connected LEDs, V
F
is the forward voltage of each LED, and 0.2V
represents the voltage across the current sense resistor. For InGaN LEDs (white, blue, blue-green) V
F
is
typically 3.5V, and with a limit of (19.1 - 0.2) = 18.9V the LM3406 could drive as many as five in series.
For AlInGaP LEDs (red, orange, amber) V
F
is typically 2.5V, so a V
O-MAX
of 18.9V would allow as many as
seven to be driven in series.
3 Connecting the LED Array
The LM3406 Evaluation Board includes a female 6-pin SIP connector, J1, as well as two standard 94mil
turrets for the cathode and anode connections of the LED array. Figure 3 shows the pinout of J1. Solid 18
or 20 gauge wire with about 1 cm of insulation stripped away makes a convenient, solderless connection
to J1.
2
AN-1750 LM3406 Evaluation Board SNVA300A–September 2008–Revised May 2013
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