Driving 7 Segment LED Display with High Efficiency - Page 1
Forum section: electronics / beginners

author: Kane
author_rank: Contributor
timestamp: June 22, 2024, 08:30:11 pm
content: Hi,I am making a battery powered digital readout which uses 2 AA's and 4 7-segment red LED displays. I would like some help on how power the segments efficiently.The LED's will be drawing minimal current, around 100 uA per segment (400 uA if multiplexing each digit) Enough to be visible indoors. Currently I have common anode 7-segment digits but I can get either if a solution works with one better than the other.My main power supply from the batteries is a boost converter to 3.3V which powers the microcontroller and another sensor.The Red LED's in the display have a forward voltage drop of around 1.8V. If I simply used a current limiting resistor nearly half the power is wasted in the resistor.I thought about adding a buck converter to drop closer to the LED's forward voltage and then using a smaller current limiting resistor but the current regulation is poor when the supply voltage changes a small amount, I'm not sure if this would be an issue since the buck converter should keep it fairly constant. I've also thought about using simple transistor current sinks and I tried a few circuits up on a breadboard but they also seemed to change a lot with temperature such as when I touch the transistor with my fingers.I would be happy with anything 80% or higher efficiency. I'm not really sure which direction to go as buck converter's aren't 100% efficient so by the time it goes through say an 80% efficient buck conversion and then if whatever I use to drive the LED's is 80% efficient, that whole system would be only 64% efficient which is almost as bad as just using a resistor.I would greatly appreciate any help,Thanks,Kane

author: Benta
author_rank: Super Contributor
timestamp: June 22, 2024, 08:44:00 pm
content: With this kind of power constraint, I'd personally use a 4-digit LCD instead. Perhaps combined with a "backlight-on-demand" if needed.

author: Kane
author_rank: Contributor
timestamp: June 22, 2024, 08:50:35 pm
content: Hi, I did consider a 4 Digit LCD I wasn't happy with the visibility, both contrast and viewing angles can be quite poor depending on the lighting. Having a backlight on all the time which would be a requirement for visibility would put it in the same realm as an LED 7 segment display I would think so I opted not to use one.

author: floobydust
author_rank: Super Contributor
timestamp: June 22, 2024, 09:02:59 pm
content: Another approach is to add a boost-converter to say 5V so you can drive higher voltage >3V (green, blue, white) LED's which are really much brighter, much more efficient than red.The payoff is some energy losses in the boost-converter but more than recovered by gains in LED brightness and the new lower current needed for them.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 22, 2024, 09:08:52 pm
content: Yes, if the driver/mcu is fine with ~2Vdd, you could take that approach.Normally converters won't be very efficient at very low loads.Still, 100uA will have very low brightness, have you considered a small oled screen? They will last a lot if not driven at full brightness 24/7.

author: Benta
author_rank: Super Contributor
timestamp: June 22, 2024, 09:10:59 pm
content: Quote from: Kane on June 22, 2024, 08:50:35 pm7-segment LCDs have excellent contrast and viewing angle. Just look at your multimeter (if you have one). Also at 3.3 V. You just need to select the right supplier instead of the cheapest Sumdingwong from PRC.I never said anything about constant backlight. I said "on-demand". For example a button or a light sensor.

author: Benta
author_rank: Super Contributor
timestamp: June 22, 2024, 09:14:24 pm
content: Quote from: floobydust on June 22, 2024, 09:02:59 pmI think you need to re-analyze that statement (and read a few data sheets). I don't completely disagree about the green LEDs, but for the rest...

author: selcuk
author_rank: Frequent Contributor
timestamp: June 22, 2024, 09:20:54 pm
content: 7-segments and 2 AA batteries are not a good idea. What is the expected operation time? You want it indoors but for example outdoor daylight performance of reflective or transreflective LCDs are generally better than LEDs.I think you should control the LEDs with current mode control instead of voltage and eliminate the resistors completely to increase efficiency. Think about 1W or 3W lighting LEDs used nowadays. They all have current controller and no resistors.

author: floobydust
author_rank: Super Contributor
timestamp: June 22, 2024, 09:25:25 pm
content: Quote from: Benta on June 22, 2024, 09:14:24 pmCan you elaborate? Low voltage red LED's (AlInGaP 2V) are still at the bottom for efficiency and brightness compared to the 3V club green, blue.InGaN phosphor-converted amber, red LED's have taken over when you need highest efficiency and brightness but they are basically blue >3V.This is why I suggest using a boost-converter, as well as for getting more life out of the AA batteries.

author: Kane
author_rank: Contributor
timestamp: June 22, 2024, 09:59:14 pm
content: Thanks for all the replies.I have tried Kingbright red, yellow, green and Innolux red and blue 7-segment displays. I found for a given current that the reds/blues/greens were very similar brightness with red having much lower forward voltage which is why I picked red.I considered OLED but was worried about burn in as it's displaying digits all the time and most datasheets specify many milliamps to operate.I have a fluke 87V and a brymen 786 multimeter and I find myself reaching for the backlight button often, it's just the nature of LCD's without a backlight in poor lighting conditions which can often be the case indoors. I haven't ruled it out but for now I will try make the LED 7-segment display work.I don't have a hard target for operation time, currently it would probably last around 6 months being used 8 hours a day 5 days a weeks. I would be okay with that length of time as a minimum but I know it can be much better. Software optimisations such only lighting up non 0 digits and an efficient drive system for the LED's I can see being able to hit 12 months or 6 months with double the current for better visibility.Thanks again

author: floobydust
author_rank: Super Contributor
timestamp: June 22, 2024, 10:35:57 pm
content: You can also add a tinted display window for more contrast in bright ambient light.I seeKingbrightoffers red 7-seg displays that are10xbrighter than the other reds. Hyper Red AlGaInP on GaAs substrate 1.85-2.35V at 10mA.But then I see you have to consider the human eye's sensitivity also factors into it. A bright LED (high luminous intensity rating) does not mean it appears bright.https://www.slideserve.com/ofira/phosphor-converted-white-led

author: Benta
author_rank: Super Contributor
timestamp: June 22, 2024, 10:43:11 pm
content: Quote from: floobydust on June 22, 2024, 09:25:25 pmYou're talking lighting LEDs, which is complete different story, running at 350 or 700 mA or even higher.For low-power LEDs, red or green are the most efficient, due to low forward voltage. Red: ~1.8 V, green ~2.0 V, as well as current/light conversion ratio. Red leads here.Example:https://www.vishay.com/docs/83180/tdcx10x0m.pdfGreen has an advantage in "eye efiiciency". The human eye is most sensitive to green light, followed by red (and at the bottom: blue).Hope this clarifies my opinion.

author: Peabody
author_rank: Super Contributor
timestamp: June 22, 2024, 11:03:36 pm
content: I've never used one, but it appears there are buck converters that would operate on the two AAs and output nominally 1.8V. Of course there would be losses, but it might be significantly more efficient than using resistors with 3.3V.Traditionally, with IR LEDs and such, the solution is to power two LEDs in series, which doubles the output power on the same current, with a tiny series resistor. But I don't know of a way to do that with 7-segment displays where you never know which segments will be lit. But I wonder if there might be 7-segment displays which have two red LEDs in each segment. Then you could feed them 3.6V, but at half the current, and get the same brightness. Actually, Adafruit sells a 7-segment display with two LEDs per segment, but it's 1.2 inch.

author: Kane
author_rank: Contributor
timestamp: June 22, 2024, 11:21:45 pm
content: Yea it's typically the larger 7-segment displays that have 2 per segment Kingbright's smallest one with 2 LED's per segment is 1"At the moment I have the single digit 0.56" SA56-11SURKWA with a typical rating of 27 mcd at 10 mA. Running those at 100 uA per segment is okay but a little dim but I also have some smaller 7-segment displays and those appear brighter, I assume because the light is concentrated in a smaller area.Inolux also have INND-TS56GXX which are green which have a rating of 218 mcd at 10 mA but I can't find them available anywhere to test them.

author: Kane
author_rank: Contributor
timestamp: June 22, 2024, 11:25:49 pm
content: Now that I check, they do have whites in stock with a rating of 130 mcd at 10 mA and with a forward voltage of 3.2 V at 20 mA. I may have to get some to tryEdit: Upon further inspection, it seems that the mcd rating of these displays are done two different ways. Kingbrights has a higher and lower rating, the lower value is specified as "Luminous intensity value is traceable to CIE127-2007 standards" while the higher value is specified as "Luminous intensity / luminous Flux: +/-15% "The SA56-11SURKWA that I have is 97 mcd but 27 mcd when measured in accordance to CIE127-2007 standards.I think Inolux is mixing these measurements in a single table as their amber, white and especially green seem absurdly high.

author: Kane
author_rank: Contributor
timestamp: June 26, 2024, 12:51:03 pm
content: I have decided to try powering the 7 segment digits directly from the the battery with a constant current source using the op amp and dac built into the microcontroller. I'm also multiplexing per segment rather than per digit so I can use a single current sink.My thought on powering the segments directly from the battery is that as the battery voltage decreases, the efficiency increases. Using red 7 segments with a forward voltage of 1.8V will ensure it will work across the entire battery range until they are dead. Initially it would be around 56% efficiency (1.8 Vf / 3.2V Vbatt) but would increase toward say 95% efficiency (1.8 Vf / 1.9 Vbatt). Assuming a fairly linear voltage decrease over time which from the discharge curves appears mostly to be the case then it should have an average efficiency of 75% or so I would think?I've attached a schematic that I tried on the breadboard and seemed to work really well.Any thoughts?

author: Peabody
author_rank: Super Contributor
timestamp: June 26, 2024, 04:50:57 pm
content: At low battery voltages, I question whether the NFETs will turn on. In effect, you have two N-channels in series, and need to get above the threshold voltage of both of them, and that's over and above the drops across the series resistor and the LEDs. Do you have specific part numbers in mind for those mosfets?

author: floobydust
author_rank: Super Contributor
timestamp: June 26, 2024, 04:54:14 pm
content: The strategy can work but the brightness will change depending on how many segments are on, because you are controlling total display current. So a "1" is brighter than an "8". Also the extra (voltage) energy is wasted as heat in Q10, R12 operating in a linear mode. You could use Q1 to do the work for fewer parts.I am not sure if you are multiplexing the displays or not. I would try using narrower pulses to drive the LED display dimmer, to save energy. The human eye will integrate the pulsed LED light and it appears to be steady on. So a narrower pulse is less energy that a wide pulse, for the same voltage. Basically PWM.

author: Kane
author_rank: Contributor
timestamp: June 26, 2024, 05:13:40 pm
content: The N channel FETs were just some 2N7000's I had laying around to test the idea. I am multiplexing per segment, I didn't draw the other 3 digits on the example schematic but I would be turning on a specific digit with the PMOS at the top of the schematic and selecting which segment with the NMOS's. The brightness doesn't vary as only one segment is on at any given time. I have tested it all the way down to 1.8V where the current then reduces as it goes under the forward voltage of the LED.Are there any constant current sink IC's out there that have many channels that also consume microamps of quiescent current not counting the sink current flowing through it?To clarify regarding the threshold voltages, a microcontroller is driving the MOSFETS at 3.3V from a boost converter. I just didn't want to constantly run the LED's from 3.3V due to the poor efficiency. The current setting resistor R12 will be set to have a maximum 0.1V drop across it so the N-FETS should have no problem turning on. Q10 will always be trying to sink 100 uA or whatever I end up settling on so the sources of the N-FET's connected to the segments should be at 0V if all segments are switched off or 0.1V when the 100 uA is flowing through the 1k resistor.Edit: Would it actually be 0.1V + the voltage dropped by Q10? This would be about 1.4V dropped with fresh batteries and approach 0V as the battery voltage meets the forward voltage of the LED's. This would mean 1.8V Gate to source on fresh batteries. It seemed to work fine though.

author: BennoG
author_rank: Frequent Contributor
timestamp: June 26, 2024, 06:28:53 pm
content: have you considered dumping the series resistor and use short pulses e.g. 5% PWMThen you don't have the power loss in the resistor.Benno

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 26, 2024, 07:39:33 pm
content: You may get better efficiency (visible with lower current) with a white LED display, or deep-green if they make them.Maybe consider using individual LEDs to make up the segments, as optical efficiency may be better

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 26, 2024, 07:52:54 pm
content: What I once did to reduce overall power consumption on a display was to use a PIR sensor to enable the display only when there was someone in the vicinity.You may want to implement a light sensor to vary the display’s brightness depending on the actual illumination level. At night, you can reduce the current to 1/5 or less and still have a pretty readable display.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 26, 2024, 08:23:34 pm
content: That running off 2 AA's requirement has been a stiucking point in a few projects I've worked on. These days, some phosphor coated (blue) LED's can be remarkably efficient, being very visiable in low light running off <0.2mA. But... as has been stated, they need > 3.2V to run, 3,3V is pushing it. The 2-3V from AA's is fine for micros and even analogue, but it does making powering indicators annoying.Boost converters have their place, and can get to >90% even for mA level output, if they use pulse skipping, but they generate noise (especially when hicupping to save power) which is pretty bad for RF or sensitive analogue. By the time you've tested and designed a reasonably efficient converter to power the blue/white/phosophor LED's, any efficiency gain those LED's have has gone. As you already have one there, I'd try white segment displays.Re: LCDs. The old STN types do indeed have crappy viewing angles - or rather, they're fine for a narrow range, at the right position - but terrible from the opposite side. But modern VA displays are good up to about 160. They also have a much better contrast, with on being pretty black and let a lot more light through when "off" making the backlight brighter for a given current.Segment LCD displays, even the older types (TN) - the kind used on most multimeters can easily run below 100uA.It's hard to get VATN digit displays, but the old HD44780 character displays are all over Aliexpress and the like. They're about 1mA Icc, with 0.5mA on the backlight being more than good enough to see in low light. However... AFAIK they are all 4.5V min. Sure many character displays can run on 3V, but they have a negative voltage generator on board for the glass that will increase current consuption.Like all engineering, sounds like you gotta compromise if you want to reduce power consumption further.I do agree that LED segmentt displays and "low power" don't really work. I also agree with your numbersEdit: Just noticed Mike mentioned the white displays already, sorry

author: mariush
author_rank: Super Contributor
timestamp: June 26, 2024, 09:15:17 pm
content: edit :Alternative to boost regulators would be charge pump regulators that can double the input voltage with up to 97% efficiency ... so for example 2 x 1.2v-1.35v = 2.4v-2.7v for rechargeable -> 4.8v-5.4v which is fine.The problem is when user adds two fresh alkaline batteries that may be up to 1.65v each but even if they're not fresh you double 2x1.5v = 3v to 6v and you have issues because most microcontrollers support maximum 5.5vend editYeah, if you want lowest power consumption, a LCD screen would be lowest power and a lcd segment driver can be very cheap.You could also use an actual led driver that would allow you to set the maximum current and then multiplex the digits and give you brightness control so you could reduce the current as much as you want.For example, this 14 segment 8 digit lcd display is only 3$ at Digikey and it's transflective, meaning it will be quite bright with regular light :https://www.digikey.com/en/products/detail/varitronix/VIM-878-DP-FC-S-LV/1118603The reflective version is also cheap at 4.6$ if you get only 1 :https://www.digikey.com/en/products/detail/varitronix/VIM-878-DP-RC-S-LV/1118605They get cheaper at 10pcs, around 1$ off each price.Here's an example of a reflective display, with 4 digits, which allows you to have a couple leds on the sides and provide backlight if needed :https://www.digikey.com/en/products/detail/lumex-opto-components-inc/LCD-S401C52TR/469783Both of these work with minimum 3v but you produce 3.3v anyway with your boost regulator.Some yellow/green/amber leds would run direct from battery (2.4v to 3v) or from 3.3v and you could simply have a button user could press to turn on light for 10-30s and then auto shut down to save power.The 8 digit displays seem to be 32 anodes x 4 cathodes , the 4 digit one naturally lessHere's loads of other segment lcd displays:https://www.digikey.com/short/qvtfj58qexample of lcd digit drivers32 x 4 :Holtek-Semicon-HT1621Bhttps://www.lcsc.com/product-detail/LCD-Drivers_Holtek-Semicon-HT1621B_C7873.htmlorTitan-Micro-Elec-TM1621Bhttps://www.lcsc.com/product-detail/LCD-Drivers_TM-Shenzhen-Titan-Micro-Elec-TM1621B_C2980111.htmlGN-Semic-GN1621CBhttps://www.lcsc.com/product-detail/LCD-Drivers_GN-Semic-GN1621CB_C265477.html36 x 4 :Titan-Micro-Elec-TM1726https://www.lcsc.com/product-detail/LCD-Drivers_TM-Shenzhen-Titan-Micro-Elec-TM1726_C2937431.html32 x 8 :GN-Semic-GN1622Ahttps://www.lcsc.com/product-detail/LCD-Drivers_GN-Semic-GN1622A_C2758336.htmland loads of others here:https://www.lcsc.com/products/LCD-Drivers_584.htmlexample lcd drivers on digikey :https://www.digikey.com/short/p9mq7885If you want LED based digits, you could use proper dot matrix drivers to limit the maximum current, and then have the driver adjust brightness per led.. the problem is the quiescent current of some of these drivers is quite high at 1-2mAFor exampleLumissil IS31FL3738https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3738-ZLS4-TR/14308389Easy to use TSSOP package , 8 sources, 6 sinks , so ideal for 8 digits with 6 segments or 4 digits with 8-12 segments, 2.2 mA quiescent current, works down to 2.7vLumissil IS31FL3746B -https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3746B-QFLS4-TR/12675543-It does 4 sources (anodes) x 18 sinks (up to 72 leds), so you could have 8 digits each with 9 leds or 4 digits with 14 leds or something in between. Quiescent current is average at ~ 2.3mA and can run down to 2.7vLumissil IS31FL3728 :https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3728-QFLS2-TR/5319755It can do 8x8 , 7×9, 6×10 or 5×11 ... quiescent current is high at 4mA, but shutdown current is uA worth of current, and can run with as little as 2.7v

author: PlainName
author_rank: Super Contributor
timestamp: June 26, 2024, 09:20:14 pm
content: How fast does the display have to change. If it's slow then perhaps eink might be a solution: basically zero power between updates and easily visible in normal lighting.Check out your local Aldi to see the displays they use for price labels - if you didn't know you'd swear they were printed paper things.

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2024, 09:26:09 pm
content: Quote from: Benta on June 22, 2024, 10:43:11 pmActually no, green LEDs, especially the type with the low voltage drop have pitiful efficiency, even taking into account the human eye's increased sensitivity to green, which they also miss, because they're yellowish-green.Decent, modern red LEDs are more efficient than blue ones, in terms of power in vs power out, but many 7 segment displays use old, inefficient red LEDs.Modern pure green LEDs, with a higher voltage drop are more efficient, than the old yellow-green LEDs, but still not as much as red and blue, although they do look brighter, because the eye is more sensitive to green.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2024, 07:01:35 am
content: Ignore all advice regarding how LEDs made in some specific colors are more efficient than others; just check the actual numbers. Which will take us directly to the point:LED choice is by far the most important! Look at as many distributors and datasheets as you can find, sort for the best luminous intensity. There might be easily 50x difference between the worst and best LEDs, in such case it is totally secondary if you can increase your driver efficiency from 55% (resistive dropper) to say 90% (very well designed mini-buck per segment).Now youcanignore some colors due to the sensitivity curve of the eye, but this is wasted effortbecause eye sensitivity curve is already baked in the lumen and candela numbers!Following wrong forum advice you might end up ignoring a really efficient 625nm red LED just because it's "red", so that eye sensitivity curve gets taken into account twice by accident, first by the manufacturer then by you again.Also, is there really a reason to run the microcontroller at 3.3V? Most low-power MCUs run at lower than this, and while doing so, also consume less power. 2 AA cells range from 2V..3.4V; many MCUs can run directly off this supply. With average battery voltage being a tad below 3.0V or so, you would already gain extra efficiency in the simple resistor solution; some 20% improvement from lower drop over the resistors, and another 20% from skipping the boost . Low-end 2.0V is still enough for red LEDs with Vf around 1.6V; while multiplexing anyway, you could increase the duty cycle (on-time proportion) based on measured battery voltage; which you likely measure anyway with the MCU - which again would be easier if you did not have an unnecessary boost converter, as most MCUs can easily measure their own supply voltage.If the sensor requires 3.3V to operate, I would look for a replacement part which operates over the 2V..3.4V range, getting rid of the boost converter would be a huge help. If this is impossible, then running only the sensor off the boosted supply, and level-shifting the communication to sensor (if necessary) is possible.TLDR; invest your design time in:* Finding highest luminous intensity LED display,* Finding an MCU which operates directly of 2xAA* Finding a replacement sensor which does the same, if unable, limit the boosted supply to sensor only and level-shift communication

author: Kane
author_rank: Contributor
timestamp: June 28, 2024, 04:45:39 pm
content: Thanks, Siwastaja, Great advice.Since starting the post I did find a sensor without the 3.3V requirement which as you said allowed me to eliminate the boost converter from my design and run directly off batteries.I am quite happy with where the designs at. Everything running direct off batteries, Red 7 segments with low forward voltage and high brightness which while using a constant current sink will run across the whole batteries voltage range from 3.2V fresh down to the forward voltage of 1.8V.I'm currently looking into making my own 7 segment display from discrete LED's as a lot of manufacturers have extremely bright 7 segment displays but are not available anywhere and the modern discrete LED's often have luminous intensity's 10x higher than the 7 segment displays available.

author: Peabody
author_rank: Super Contributor
timestamp: June 28, 2024, 05:26:53 pm
content: Or you might find a really efficient single digit, and get four of them. I'm curious how the Vishay TDSR1350 and 1360 compare to your current display if you can get them in the "K" Luminous Instensity Factor classification. The datasheet says they produce 1800-3300 ucd at 1mA. I did a video back in the day explaining multiplexing by segment, and used these displays. Of course I was working much brighter than you will be, but they seemed to be quite bright on little current. The average current to display "88" was 4mA at 3.3V, which was a lot better than the girls I had dated up to then. Discussion of the displays is at about 10:40 into the video.

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2024, 06:24:38 pm
content: IT also depends on the light level. In the dark, the peak response of the human eye shifts towards the blue end of the spectrum, making cyan LEDs the most efficient choice, purely on the response of the human eye.

author: bsdphk
author_rank: Regular Contributor
timestamp: June 28, 2024, 06:46:25 pm
content: When HP made the first pocket calculator, LED display power drain was a major issue for them.There is a quite detailed article about it somewhere, the gist of which is that our eyes register the peak intensity rather than the average, so their solution was to drive the display at very low duty-cycle something like 1µs 100 times per second (= 1:10000), but at a current which was only something like 10-100 times higher, thus saving a LOT of power.The limiting factor is the temperature of the LED dies in the display.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2024, 06:48:23 pm
content: Quote from: Zero999 on June 28, 2024, 06:24:38 pmThis shift isn't massive. It would come into play if you had to choose e.g. between yellow LED rated at 1000 mcd and cyan-green LED rated at 900 mcd - AFAIK lm and cd are defined per the day vision sensitivity curves, and the difference you mention would swap the order of their perceived brightness. With that being said: in extreme enough darkness for the shift you mention does happen, it doesn't matter because a ridiculously low amount of light would suffice anyway, say just tens of uA. If your device ever has to be operated within anything more than complete darkness (even just room lighting), so that the day sensitivity curve applies, then you are obviously going to choose the most efficient LEDs for the day vision curve; which is already baked in the lumen and candela ratings as I said, so no need to overthink it.And any lens angle is baked into the candela rating so it's probably the best metric available.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2024, 06:55:07 pm
content: Quote from: bsdphk on June 28, 2024, 06:46:25 pmIMHO this is a total myth. I have at least tried to test it myself and don't see it working, unless you go slow enough that it starts really hurting your eyes. Make your product blink brightly at, say, 30Hz in darkness, and sure it won't go unnoticed - but I mean in a bad way.The real reason for fast blinking is just multiplexing - or that some ancient LED technology getting better efficiency at higher current.One thing is sure: on all modern high-brightness LEDs, driving short pulses at large currentisgoing to decrease efficiency compared to driving at the same average current, DC. Two things happen when current increases: fewer photons are generated per coulomb, and Vf rises, both lowering the efficiency.This is the opposite of some ancient LED technologies which would have sweet spot for current. LEDs have changed a lot during early 2000's, a lot of old advice (such as efficiency estimates based on color like "green is bad"), or this pulsing thing, simply do not apply today.

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 28, 2024, 07:01:45 pm
content: Quote from: bsdphk on June 28, 2024, 06:46:25 pmUnfortunately this "amplification" of LED brightness only works for frequencies that are at the threshold of flickering and it´s also limited to signalling applications - its not working for lighting.With 100Hz or more the eye effectively averages any modulation in the perceived brightness.But if the author of this thread does not mind a slight flickering sensation, he can use e.g. 50Hz with 5%PWM. Then the display looks brighter than it is.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2024, 02:53:50 pm
content: Quote from: Phil1977 on June 28, 2024, 07:01:45 pmYeah, more specifically the "eyes register the peak intensity rather than the average" part I replied to is, IMO, incorrect. Within integration period of the eye (which is of course a huge simplification, in reality it's flexible and not like in a camera, but this simplification is good enough for this discussion), eye pretty much just counts photons like a camera sensor does. This is: average.Now to point out that there's no magic, consider this:obviouslyif you shine a bright light for a minute and then keep it off for nine minutes, you are able to see well during that minute, and have reduced average power consumption to one tenth. You can keep shortening this period, and as long as the time of bright period is within the eye's integration period, you will see it as a brighter light,because it is. But you will also be seeing the off-time.Once you bring up the frequency to the point of minimal or no flicker, your eyes are just averaging the light.

author: Kane
author_rank: Contributor
timestamp: June 30, 2024, 12:22:25 am
content: Anybody know of any IC's that contain 8 basic current sinks? I've looked for a while but all the LED driver sinks are waaay overcomplicated for my needs and often consume many milliamps. I just want something akin to the circuit below but 8 of them in a single chip, each channel can have the same set current, they don't need to be independently controllable but it would be ideal if each sink had it's own enable.It must also work across the supply voltage from 1.8 to 3.2V, as soon as I put this filter in the Current sinks categories of Digikey nearly all of the results disappear so I'm not sure if I'm searching for the right thing or in the right place.

author: mariush
author_rank: Super Contributor
timestamp: June 30, 2024, 10:42:50 am
content: Have you considered ICs like ULN2003V12 / ULN2003F12 - it's mosfet versions of classic ULN2003AThe ULN2003F12 has 4 channel, ULN2003V12 has 7 channels ... minimum 1.65v required to turn on a channel, each channel does 100mA with 3.3v on the io pin, less at lower voltages... 500mA in total on all channels.ULN2003F12 :https://www.digikey.com/short/rv7n32cvULN2003V12 :https://www.digikey.com/short/v1jtw4hhYou'd have to add resistors on each channel to limit the current, but you can get resistor networks that use very little space (ex 7 resistors in a 1206 footprint)Toshiba makes some chips with 8 channels but they usually need at least 3v .. they have some p-channel based ones for high-side switching that go down to 2vIf you can relax your input voltage requirements a bit, there's drivers like IS31FL3242 that can control 12 leds and consumes around 0.5mA :IS31FL3242https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3242-QFLS4-TR/14640385Works with min 2.7v to 5.5v - maybe a basic charge pump could boost 1.8v to 3.6v just for the driver - and no resistors to set max current, each channel can be set between 0 and 48mA digitally through commands (256 steps for 0 ..8 ma, 256 steps for 8..48mA) and then there's pwm 4096 steps for each channel.The leds can be powered directly from battery, ex 2v+ for red leds... the chip needs around 0.25v (max 0.3v) above led forward voltage to properly limit the current on that channel.