Digital Current Control for LED array

It's easy to get under $4.50 an LED and have efficiencies significantly higher than the 30% you propose (10V supply, 3V load). The MOSFETs are about $0.50 each, the 4-channel opamp is under $2 ($0.50 per channel), the discrete resistors are negligible, and the cheap pots are down to $0.25 each. It isn't just a nest of wires but the supply can be made to accommodate a very small dropout voltage for very high efficiencies. The solution is elegant rather than brute force. I'd hate to throw away 50W of power for 21W of light.

Most folks designing with the high-power Luxeon LumiLEDs are interested in efficiency above convenience; otherwise incandescent lamps would be fine.

If you apply 15 posts toward an effort you have no interest in in order to bait someone, I'd respectfully recommend putting the word "Professional" in your signature in quotes from now on.

No pies in my sky: it's engineering.

It's true that using 7 scalable voltage-controlled current sources as series pass elements is wasteful but the waste is minor. It would be best to have 7 voltage controlled switching current regulators but the loss with the simpler series regulators is easier to implement with very little additional loss. Calibration would be done at one point since this will be a first-order match of brightness with the proportional currents providing a close approximation to proportional luminance values; the differences in the current/luminance curves at that point are second-order mismatches.

The DC supply can be tuned to a voltage slightly higher than the largest Vf LED (and series regulator) would require at 100% rated current. Done properly, the source is a switching regulator that self-adjusts to have only one gate drive near (rather than at) the positive rail, the rest at lower drive voltages. If you increased the supply voltage, the gate drive levels would simply reduce and the Vds values increase by the same amount the supply increases. Nowhere was it specified that the supply had to be a dumb, fixed voltage but even then a 5V supply would produce good results in all cases.

If a relatively large current sense resistor (as current sense resistors go) of 100 milliOhm were used (100mW is hotter than I like) the voltage drop lost to sensing would be 100 mV. The MOSFETs in series with the worst-case Vf LED should have a drop of about 200 mV at the gate-source voltage provided by a rail-to-rail output op amp. The worst case for

*loss* would have all LEDs at 100% current but one of the LEDs at the highest Vf, the rest at the lowest. But in any case the overall *power* dissipated in each LED/regulator combination would be less than or equal to 100% rated current at the worst Vf of the 7 LEDs plus 300 mV for the pass elements. The only way to improve on the loss in the "more efficient" lower Vf LEDs would be to have individual switchers for each LED, still had for less than $4.50 each.

This is not brute force. This is engineering tradeoff. You can have a power draw for 7 LEDs that's 7 times the power draw of the largest Vf device with simple pass regulators (1 MOSFET, 1 OpAmp channel, a current sense resistor and 2 set resistors for the OpAmp) presenting one voltage that controls 7 channels. Or you can use 1A switchers per-channel for

90% efficiency overall; these still need to servo to fixed percentages. In most cases where the Vf is similar for the multiple devices, the efficiencies of the two approaches will differ by only a few percent.

When you suggest 30% efficiency rather than 85%-95%, you can see where "brute force" easily loses out. The brute force approach you recommended would produce extremely poor matching results in a system where some LEDs have a Vf of 3.03V and others are 4.47V, the range of Vf at 700 mA as published in the Lumileds datasheet; the current ratios would no longer match as the system is dimmed and brightened because of the large disparity.

--- Actually, I think the brightness tracking would be OK because of the initial "calibration". That is, with my "brute force" approach, the differences in Vf would wash because the LEDs would all be forced to the same brightness initially and their individual delta Vf's, from bright to dim would, I suspect, vary little from bright to dim. They are, after all, only diodes in conduction. If you doubt it, build an array using my "brute force" way and report back with what you find, OK?

In any case, as I've already stated, the system I described was to give what's-his-face a little food for thought. Much like describing running into a brick wall going a hundred miles an hour isn't advocating doing it.

But, after that crack you made about my sig, I suspect you're trying to pick a fight, so you're not going to be happy with anything I say.

Right?

-- John Fields Professional Circuit Designer

--
What you\'re saying is that in order for you to be happy, you want
everyone to conform to what _you_ consider to be professional
behavior.  Forget it, it isn\'t going to happen.

I won't be happy with unprofessional behavior. I don't pick fights though I sometimes register my distaste for those who make the public forum ugly. This is a "newsgroup" where people can pick fights somewhat anonymously and get joy out of the experience while degrading the quality of the information that *should* be exchanged in forums like this. No posts are removed here, though by the quality of postings that I've seen I have a greater respect for moderated information exchange forums.

As for the LEDs: what happens at 20% brightness? if a 10V supply provided 100% current to the 3.03V and 4.47V Vf LEDs for matched brightness, 20% to the 4.47V LED would be 4.47V+(5.53V/5)=5.58V which would give the other LED (5.58V-3.03V)/6.97V or 37% brightness. If the lower Vf LED were reduced to 20%, the system voltage would be

3.03V+(6.97V/5)=4.42V. which means the other LED has zero current.

The reality is the Vf varies with forward current but it varies for all LEDs. While the high Vf LED may not be "off" at 4.42V, it may be off at the drive voltage required to get the low Vf LED to 20%. There will be a first-order mismatch in the LEDs with the high-loss brute-force method you presented to "give what's-his-face a little food for thought."

As someone who actually would like to help a few people when they have a question that 1) I have interest in and 2) I can provide sound help with reasonable expertise, I would like to see the discussions here be about supplying answers to questions.

My original idea was to use only passive components.

What current regulator are you using in your setup? If you use potentiometers isn't the setup getting too complicated? If you already have a current regulator why use a MOSFET? I would use MOSFETs to switch to different resistances but that would limit and complicate my brightness control.

If you are really set on connecting the LEDs in parallel I'd use the MAX1916 . I'm sure its a cheaper option. It has a current match of 0.3%. and you can vary the brightness by changing the duty cycle of PWM signal on the enable pin. Let me know what you think.

theJackal

Passive components alone won't give good tracking of current with one master control if efficiency is an issue.

The current regulator I will use is a combination of a MOSFET and an OpAmp with current sensed through a small-value resistor (under 100 milli-ohm). Current regulators (at the 1A level) are not something I'm aware you can buy readily as a low-dropout device; simple 3-terminal variable voltage regulators have a dropout of at least 1.25V (the reference voltage) when wired up as a constant current source.

The 7 LEDs need to be configured for a "best match" at a nominal master current level; this can be done through simple open-frame potentiometers, swapping resistor values in and out in the OpAmp feedback, or through voltage control that is otherwise made proportional to the master level control such as an 8-channel multiplying DAC.

The best control over the LED intensity is direct control over the current. Making these currents track each other based on one master intensity control is the design goal. Different LEDs won't have an exact track of intensity versus control voltage but will be close enough for most purposes.

I suggested the MAX6958/6959 as one example of LED control early in this thread. The device uses PWM but is designed for regular LEDs. These high-power LEDs would have to be driven with a MOSFET to get the milliamps of current to change to an Amp of control per LED.

I figure making a separate regulator for each channel that can deliver a low-dropout voltage-controlled current source is the best combination of efficiency and cost. I haven't bought the parts yet to wire up my own 3W Lumileds but I expect to have them lit with something other than a lab supply within the month. I may look into per-LED switchers as an alternative but I'd expect those to be more costly and more prone to EMI troubles for something "slapped together" rather than detailed on well laid out PC boards.