Re: LED array current regulation
- From: JosephKK <quiettechblue@xxxxxxxxx>
- Date: Sat, 31 May 2008 18:28:39 -0700
On Fri, 30 May 2008 19:09:24 +0200, Fred Bartoli <" "> wrote:
Paul Mathews a écrit :
DJ Delorie <dj@xxxxxxxxxxx> wrote in news:xnod6ok0xs.fsf@xxxxxxxxxxx:
I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". The application is PCB photolithography.
So far, I've looked into a few options:
1. Resistors. 10 LEDs in a string, 175 ohm resistor. Cheap and easy,
but slight variations on Vf would cause big variations in If.
Bigger resistors and shorter strings perhaps? 470 ohm w/ 8 LED
strings? Still only 10% regulation, not sure if that's OK for LEDs
(I don't have much info on the specific LEDs I got, other than what
I've listed above).
2. Infineon BCR402UE6182. This is a discrete single-channel current
regulator, but doesn't seem to guarantee a low enough If.
3. MAX6971 (or 2xMAX6970). Only needs a resistor and clock source to
drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.
4. MAX6957. In stock at Digikey, but requires an MCU to program it
and common-base transistors to isolate it from the high voltages.
5. Redoing the power supply for 18VDC to accomodate some other
solution.
Suggestions? Alternatives?
How I'd do it if you really want identical current: all LEDs in series
and use boost regulator. Of course, the switcher will develop over 300
volts, so this approach has its disadvantages. You could also consider
using, say, 4 strings of 25 LEDs and 4 boost regulators. There are lots
of control chips designed to provide constant current boost output.
OTOH, if you reduce the number of emitters in a series string so that
there is more voltage across the resistor, you'll find that resistive
current limiting actually works pretty well. Emitter follower drive,
with a current-setting resistor in the emitter, also provides good
current regulation for low cost if you can drive with a constant
voltage.
Paul Mathews
Or use this to share the boost current between the leds strings:
.-----------.
| BOOST |
| |-----+--------+--------.
| | | | |
| | V -> V -> V ->
'-----------' - - -
^ | | |
| . . . Leds strings
| . . .
| | | |
| V -> V -> V ->
| - - - R1
| | | | ___
| | | +--|___|-.
| | | | |
| | | | |
| |/ |/ |/ |
| +--------+--------+----------+
| |> |> |> |
| | | | |
'-----------+ | | |
.-. .-. .-. .-.
| | | | | | | | R2
| | | | | | | |
'-' '-' '-' '-'
| | | |
=== === === ===
GND GND GND GND
That is slick. I will remember this one.
.
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