I want to multiplex a 5-digit, 7-segment display, each segment made up of an array of low-power red LEDs. The few LED datasheets I've seen rate them at around 20mA continuous, while peak ratings range from 50 to 70mA even at low duty cycles (10% or less).
Does this mean that low thermal inertia and/or the danger of local hotspots do not permit operating the LEDs at higher peak currents to get a bright display? (I'm referring, not so much to the effects of persistence of vision as to maintaining the
I am using an LED in a product, right now, that is spec'd at 20mA continuous and 100mA at 10% duty. That's a current factor of 5, not the 10 you might expect from the duty cycle/average calculation. It's not unusual.
I haven't spent any more time thinking about it than just now, but part of the reason may be due to the higher voltage required when pulsing larger currents and the associated dissipation that resutls from the difference. For example, the old 1980's style red LED had a simplified (linearized) model that looks about like Vf= 1.55V + 21*I. Power is then Vf*I, so P=1.55*I+21*I^2. At 20mA, this is about 40mW. At 100mA, this is 365mW... and divided back down by 10, this isn't far from the 40mW continuous figure. Anyway, that's about how I see it.
There are some 2mA per segment 7-seg displays to be had. You might see if their specs may give you a little more margin.
That seems to explain it. Better to stay within specs then. Thanks for the reply.
I'm not working with small standard displays using a single LED for each segment. I need a large display 1-2 ft high and since ready-made ones in that size are not easily available from where I live, I'm going to have to build them - two 5-digit units.
I need a bright display because it's to be used outdoors where it won't be possible to keep it in the shade all the time. Pity about not being able to use the continuous rating as a multiplexed average. It's probably wise to reduce the peak current even further because the black mounting board will absorb heat from sunlight.
Since you are dealing with the outside environment, contrast becomes a whole new ballpark. You really need to carefully think the optical side through. You can waste a LOT of power and expense where it may not be needed, ignoring optical contrast issues along with choosing color well. The whole field is called "contrast enhancement" and it's worth looking into, I suspect.
You want to maximize the contrast between ON and OFF conditions. You can help this by reducing the reflected ambient (I gather that's the reasoning for your choice of a black background) and by concentrating on maximizing the light reaching the intended eye.
The means aren't entirely about making a black background, though. For example, circular polarizing filters (linear polarizer + quarter-wave) can be placed in front to extingish reflections from a specular surface. Doesn't do so much for non-specular ones, since that kind of surface just messes up the electric field angle all over again. Also, at steep angles, all polarizations of sunlight reflect but at shallow angles horizontally polarized light reflects better, so you might consider using an appropriately aligned linear polarizer (to extingish those horizontal polarizations) depending on likely angles. There are anti-reflection coatings; louvered filters (like the window coverings, in a fashion); cross hatch versions of the louvered filters; wavelength filters can help, too, but some of that depends on what wavelength you are considering and the kind of days there are (clouds filter out a lot of the red, whereas broad daylight is pretty much "flat".)
Can you talk a little about your planning on this aspect? I don't know a lot, but I might be able to suggest some things to try out or consider. My guess is that you will do a LOT more for yourself worrying about this part of the problem than worrying about just how many milliamps you can pump into the segments. That part is pretty much set for you by the better devices you decide to use. So consider that a 'done deal' and worry a lot more now about the optics side and do some experiments before committing to a final design.
I held off discussing the more general issues with ICs, since this was an LED. I didn't imagine an LED was complicated by different trace widths, local densities of power dissipation, differential ion migration from heating. But yes, in general.
If the displays are that big, the extra cost of using direct drive will be pretty trivial in comparison, so this is probably the best option as you can guarantee operating in spec, possibly with headroom.
For electromigration, generally 1ms on-time is the limit where you can play duty cycle games.[1ms on, 3ms off , 4x current, etc.) Obviously you would need to contact the factory to get their opinion. The number can be relaxed a bit. Anyway, that implies the strobing has to be neighborhood of a few hundred Hz on the low end.
The easy way to check for an abused LED is to measure the reverse leakage.
I think you will have a hard time running a large board at a decent refresh rate. You don't want to run into overvoltage conditions due to a large L*di/dt. I'm inclined to say you should avoid multiplexing. I am assuming the segment will be a string of LEDs, so you will have high voltage (relatively speaking) driver circuits to deal with. Remember, you need dead time when muxing to avoid ghosting. I think it's going to be a lot of work, and certainly will be generating a lof of RFI.
Many of these LED traffic lights are RFI sources since they flash the LEDs a bit in the attempt to get higher perceived brightness. The reality of brightness perception has been argued back and forth on this newsgroup, never to a decent conclusion in my opinion, but I think the pro-DC crowed argued better than the muxed and boosted crowd.
Complex ICs cannot be readily designed to equally distribute watts across the surface, I'd imagine. There will be a number of "weak" points and the weakest link determines it. Much like the longest combinatorial pathway determines the shortest cycle time for a particular micro design.
LEDs aren't entirely uniform blobs, granted. But the calculations I gave seem to work well enough, just using a linear approach. So I don't think a lot of extra worry about nth order effects achieves much more.
As far as PWM goes, it seems to me that if the junction to ambient is truly a constant (it may not be) independent of the junction temperature, then it's merely about staying under some peak temperature. I think the expoxies have a glassification temperature point where it would be best to stay underneath. (I think that point occurs well below any worry about metal or dopant migrations.) This would explain some of the derating (in my calculations, earlier, I showed a
40mW average and with a 10% duty a 36.5mW average -- which I think comes from such a derating. If the PWM frequency is very fast compared to the thermal times, then one could probably hold closer to that 40mW average. If the PWM frequency is low enough, the peak is worrisome and derating is needed. I believe specs usually include a frequency at which they have rated their 10% figures, which makes sense here.
It's not hard to test. I've some software available up on the MSP430 web site that can be used with a $10 device from TI to test things. Everything is already in place for those wanting to play.
The first-order approximation is that humans perceive average intensity when the pulse rate exceeds the critical fusion frequency (CFF.) This is the Talbot-Plateau law. For 100% modulation (on/off control), which is the usual case folks talk about here, and assuming we are talking about photopic vision and fairly bright illumination then somewhere between
45Hz and 70Hz is where things seem to fuse. Since the contrast won't be perfect so as to make it appear to be 100% modulation, that CFF may be lower in this case.
There is a Broca-Sulzer effect that isn't entirely understood (at least, not before when I last read about it) which holds that there is a modest increase in the appearances of how bright a source looks when the ON pulse width is on the order of 75ms! But that pretty much leaves out the usual PWM case. You'd have to keep the ON pulse at about 75ms and vary the OFF time and I don't think that works well for displays. (Not at all well.) So it's not an effect that can be much used to advantage.
If anyone is interested, it's not hard to set up a test station to play with Broca-Sulzer and, as I mentioned, I've already developed an educational set that includes docs and all the necessary software to test out Talbot-Plateau for $10 US. (TI gets the money, not me.)
Bottom line is -- forget it. If using PWM, there's few free lunches to be had. Plan on average and realize that the eye sees things on a log scale so a 50% duty cycle is NOT the same as half-brightness.
Oh, almost forgot. The CFF also varies a bit depending on the size/area of the emitter. With LEDs on signs seen a ways out, this probably isn't an issue. But for significant areas, it turns out that the extrafoveal retina does a better job at recognizing flicker (probably because of the need for all mammals to see something coming from the periphery of their vision) than the foveal retina does. So wider areas require faster flicker rates.
Why not use those flip-segment displays - they have a little coil that you pulse for each segment; painted in a flourescent green, on edge they almost disappear.
Thirty years ago, or so, we had a problem with a semi-custom contactor driver. The short story was that they were getting clobbered by the inductive kick back, not of the contactor, but the 20' of wire from the control board to the contactor (the free-wheeling diodes were at the contactors). TI's devices were crap, Motorola's not so much, and Sprague never had a failure. TI basically took a logic device and added a oversized 200mA transistor on the output, where Sprague took a power transistor and added a little logic to the front end. No wonder the Sprague was a better power device (and they did have good power devices). The problem was secondary breakdown (localized heating).
But at some frequency it won't be. There will *always* be a temperature gradient.
Absolutely backwards. If the PWM frequency is above the thermal time constant there *will* be hot spots. The heat doesn't have a chance to average out over the chip (and spreader). This has nothing to do with dopant migrations. It's a very short term effect (milliseconds or even microseconds)
It depends on the "on" *period*. If that's less than the thermal time constant you will have localized heating.
There exist SPICE thermal models of some packages (more transistors such as SOT-23 than LEDs). To model the actual effects there are several time constants in series. Unless you're worried about how even the current distribution is across the die (which would also be a problem at DC in many cases, so perhaps it is built into the ratings), one could step the current down to a relatively small fixed current in the off times (maybe just parallel the switching device with a resistor or an active current sink) and measure the actual (more or less average) die temperature by looking at Vf. Easily characterized in an environmental chamber at DC, or check it ast some fixed temperatures such as 20C/100C and interpolate/extrapolate.
Best regards, Spehro Pefhany
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Once you realize eye-response is log, the answer is obvious. Use geometric progression. I wrote a series of tutorials (most of which will be way below your interest, as they were targeted largely for beginners) which culminate in using PWM to drive an LED brightness according to an apparently linear brightness curve. It includes some things you can toy with and try out, as well. It's a series of 12 projects and a PDF documentation file.
If you haven't already purchased one, buy one of these:
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They are $10 if you can find and use a discount code. Last one I tried was this: MCU2009-03 but there is a change it won't work, now. Yesterday it worked, though.
Then I can send you a zip file of materials I wrote or else you can just download them from this folder:
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The direct file address for the zip file is given as:
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Which is __way__ long. Tiny URL gives me this from it:
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But I don't know if you need to be logged into Yahoo to get it, or not. So that may be a problem.
You don't need to buy the device, obviously. If you read Project 12 in the series, that may be enough for you.
I think that's what I was saying. The duration of the ON time is key. Did I say something that disagreed with this? I wouldn't have, had I realized it.
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