Issue
129 April 2001
Have
You Seen the Light?
by
Ed Nisley
Current
Drive
LEDs
are, first and foremost, diodes, with their exponential
current-versus-voltage characteristic. In general, you
must regulate the current through an LED and let the LED
set its terminal voltage. Imposing a constant terminal
voltage) is a recipe for disaster.
As
you see in Figure 1, the LT6750 connects the anode ends
of the diode strings to a common (as in shared, not ground)
terminal that connects to a positive supply voltage. The
green LED strings require about 20 V to turn on and the
red LED threshold is 13 V. You must supply enough voltage
to not only turn on the LEDs, but also account for drops
across the switches and current limiters. Because the
two LED strings have different current ratings, you must
use two limiters, not one in the common lead.
You
probably used resistors to set LED currents in your circuits.
Subtract the LED forward drop from the supply voltage,
divide by the desired LED current, and you get the limiting
resistor in ohms.
The
LT6750 spec sheet shows typical and maximum voltages for
each string. Those voltages differ by 1.5 V, and the datasheet
mentions neither the minimum LED voltage nor its temperature
coefficient. Suppose you pick 80 mA for the green LEDs
with a 24-V supply voltage and the maximum LED voltage
of 20.5 V. The resistor sees 3.5 V, so 80 mA means 44
Ohm.
The
reason your resistors worked so well is that the LED forward
drop is usually much lower than the supply voltage, making
typical variations small compared with the nominal voltage
across the resistor. In this case, I don’t want to
produce 40 V just to drop half of it across a resistor!
The
solution requires a resistor that adapts to changing voltage
while maintaining a constant current. The familiar LM317
can serve as a current limiter, although most folks don’t
think of it in that role. IC3 and IC4 in Figure 2 show
how it’s done.
A
single resistor sets the current limit according to the
formula:
So,
to get 100 mA, you’d use a 12-Ohm resistor. The LM317
regulates the current within about 1% and protects itself
against output shorts.
The
current-setting resistor, however, must carry the entire
output current across a voltage drop of 1.2 V. At the
100 mA I chose for the green LEDs, that amounts to 120
mW, uncomfortably close to the 125-mW rating for 0805
(0.08² × 0.05²) surface-mount resistors.
I
laid out the circuit board with two parallel, 0.25-W,
through-hole resistors for each LM317. Carbon film resistors
have a higher power limit, they’re easier to install
and replace, and you can hand-select two cheap 5% resistors
to precisely set the current. I used one 12-ohm resistor
to get 100 mA and a 24-ohm resistor for 50 mA, with no
trimming required.
The
series resistor ahead of each LM317 drops 1 or 2 V, reducing
the regulator’s dissipation. R16 is a small resistor
that’s handy for measuring the total output current,
but I replaced it with a jumper after I saw that the LM317
regulators work exactly as expected.
You
should maintain at least 3 V across the LM317, however,
to ensure that it regulates correctly. The green string
runs under that limit; it seems my LM317s have a lower
drop. You can use a low-dropout regulator if your minimum
supply voltage gets closer to the maximum drive voltage.
With
a MAX629 supplying voltage and LM317s setting the current,
all that’s left is some on/off control to make the
LEDs blink. I used a pair of NPN switches driven by an
external pulse generator to make things simple. The bias
resistors hold them on with no external input, just to
make for easy setup and testing.
Blinking
those LEDs at 10 Hz with a 30% duty cycle gets my attention.
I wonder how it works on the road.
