LED visibility optimization at very low currents

[jmaja]

I tested another LED. It's a new green one, but not a bright. http://datasheet.octopart.com/LTST-S110KGKT-Lite-On-datasheet-158286.pdf

It was clearly less visible than the old red one at the same current, but seems to have much wider viewing angle. It was extremely dim at 10 uA. There are LEDs that are much brigther at the same view angle. The one I have is said to have 130 deg and 35 mcd at 20 mA 2.0 V while this one is 130 deg, 995 mcd at 20 mA 3.2 V. http://www.sunledusa.com/products/spec/XZM2DG53W-6.pdf

It shows a straight intensity vs. current graph, thus it should have the same brightness at 1/30 current. At 300 uA the LED the green one was bright enough so 10 uA should be enough. The forward voltage is high for the system powered from a LiPo and using ~3 V regulator, but already at 5 mA Vf is 2.8 V and maybe 2.5 V at 2 mA.

What would be a good way to pulse these with the cpu sleeping so that the average current would be ~10uA and 0 uA during daylight? Charge a capacitor and then let it pulse the LED while sleeping? Just a few uF ceramic and no resistors using two pins from the MCU. One for charging and the other for grounding the LED.

So then there would be a few mA pulse + uA level tail every 500 or 1000 ms.

[Zero999]

It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.

Better how ?
Unless the LED datasheet specifies output at low currents, you have no guarantee of brightness.
White LEDs in particular can show huge brightness variations below a few hundred uA.
Some OSRAM lighting LED datasheets actually state that they should not be run below 10% of their rating for this reason.

That's because the response of the phosphor in a white LED is often non-linear. It isn't just the brightness which can change but the spectral output and colour, hence manufatures are hesitant to specify their products beyond what they've themselves tested them to. In this case the user is not interested in white LEDs, so that's irrelevant. Whether or not the datasheet specifies it or not, luminous efficiency of the LED junction itself improves at lower currents.

I'm siding with mike on this, LEDs (even within the same batch) can vary wildly at low drive currents. And the luminous efficacy can drop as the current decreases..

Those graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.

Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

[Zero999]

I tested another LED. It's a new green one, but not a bright. http://datasheet.octopart.com/LTST-S110KGKT-Lite-On-datasheet-158286.pdf

It was clearly less visible than the old red one at the same current, but seems to have much wider viewing angle. It was extremely dim at 10 uA. There are LEDs that are much brigther at the same view angle. The one I have is said to have 130 deg and 35 mcd at 20 mA 2.0 V while this one is 130 deg, 995 mcd at 20 mA 3.2 V. http://www.sunledusa.com/products/spec/XZM2DG53W-6.pdf

That LED is AlInGaP based which is a very old technology. Try a modern InGaN LED which will be much more efficient.

It shows a straight intensity vs. current graph, thus it should have the same brightness at 1/30 current. At 300 uA the LED the green one was bright enough so 10 uA should be enough. The forward voltage is high for the system powered from a LiPo and using ~3 V regulator, but already at 5 mA Vf is 2.8 V and maybe 2.5 V at 2 mA.

That makes sense. If the brightness varies linearly with current, then it's more efficient at lower currents, as Vf drops and power = V*I. Unfortunately, if you're limiting the current with a resistor or linear regulator, then there will be no efficiency gain, since the extra voltage is converted to heat anyway.

What would be a good way to pulse these with the cpu sleeping so that the average current would be ~10uA and 0 uA during daylight? Charge a capacitor and then let it pulse the LED while sleeping? Just a few uF ceramic and no resistors using two pins from the MCU. One for charging and the other for grounding the LED.

So then there would be a few mA pulse + uA level tail every 500 or 1000 ms.

For ultimate efficiency, don't PWM the LED, use a buck converter, that way the output current can be lower than input current. It should be fairly trivial to implement with an MCU and since it's low power, no additional switching transistors are necessary.

[Someone]

Those graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.

Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

If you wish to bring the data into question why can't you provide the data to show its wrong? Phosphors have well known saturation and temperature limitations at the high end, but I can't find any papers discussing non-linear effects towards zero. I'm not sure what you think luminous efficacy is, but its the well defined method of assessing visual brightness:
https://en.wikipedia.org/wiki/Luminous_efficacy
Yes, it no longer applies in a scotopic vision environment but then you wouldn't be distinguishing colours clearly which is not what the OP is discussing. You raise a lot of questions without backing it up with data or references or even explanations to help people here. Sitting on your high horse and being pompous isn't of much value to the OP when you could be adding information to the discussion rather than trying to discredit it all without alternative facts to back it up.

[Zero999]

Those graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.

Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

If you wish to bring the data into question why can't you provide the data to show its wrong? Phosphors have well known saturation and temperature limitations at the high end, but I can't find any papers discussing non-linear effects towards zero. I'm not sure what you think luminous efficacy is, but its the well defined method of assessing visual brightness:
https://en.wikipedia.org/wiki/Luminous_efficacy
Yes, it no longer applies in a scotopic vision environment but then you wouldn't be distinguishing colours clearly which is not what the OP is discussing. You raise a lot of questions without backing it up with data or references or even explanations to help people here. Sitting on your high horse and being pompous isn't of much value to the OP when you could be adding information to the discussion rather than trying to discredit it all without alternative facts to back it up.

I apologise if I came across as pompous.

I agree with you about phosphors but that's not relevant here because the original poster is not using an LED with a phosphor. What I'm referring to is efficiency droop, i.e. there's no point in driving at high current with a low duty cycle. The LED should be run at the current, which gives the highest efficiency. Now I agree, if that's higher, than the OP's power budget then PWM may be a wise choice. In any case, a switch mode supply should be used for optimum efficiency.

[Someone]

Those graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.

Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

If you wish to bring the data into question why can't you provide the data to show its wrong? Phosphors have well known saturation and temperature limitations at the high end, but I can't find any papers discussing non-linear effects towards zero. I'm not sure what you think luminous efficacy is, but its the well defined method of assessing visual brightness:
https://en.wikipedia.org/wiki/Luminous_efficacy
Yes, it no longer applies in a scotopic vision environment but then you wouldn't be distinguishing colours clearly which is not what the OP is discussing. You raise a lot of questions without backing it up with data or references or even explanations to help people here. Sitting on your high horse and being pompous isn't of much value to the OP when you could be adding information to the discussion rather than trying to discredit it all without alternative facts to back it up.

I apologise if I came across as pompous.

I agree with you about phosphors but that's not relevant here because the original poster is not using an LED with a phosphor. What I'm referring to is efficiency droop, i.e. there's no point in driving at high current with a low duty cycle. The LED should be run at the current, which gives the highest efficiency. Now I agree, if that's higher, than the OP's power budget then PWM may be a wise choice. In any case, a switch mode supply should be used for optimum efficiency.

Again you're thinking upwards into the well known thermal/saturation limits of efficiency in LEDs, even the links from your lazy google search show that the peak efficiency is at a single forward current point with either too high or too low drive dropping the efficiency (or efficacy):
http://www.semiconductor-today.com/news_items/2016/feb/postech_030216.shtml
http://spie.org/newsroom/6402-demonstration-of-novel-high-efficiency-blue-leds-on-silicon-substrates
Which is the region the OP is considering driving the LEDs at. Mike had already summed it up neatly in his first post:

For maximum visibility, (Cool) white, small size, narrow angle if your viewing angle . match angle to required viewing angle. If you want to stand out from other lights, maybe deep green, as this is at the peak or the eye's sensitivity.
Where you want "noticeability" for emergency type applications, strobing can help as the eye is more sensitive to movement.
In principle, for a steady brightness, PWMing doesn't give any advantage in terms of perceived brightness, but efficiency at low currents (< about 5% of nominal current) can vary a lot, particularly with white, so if only for consistent appearance, a higher current pulsed at a low duty cycle can be beneficial.

The OP was very clear on their operating point:

The device is targeted to consume less than 50 uA and it will have four LEDs than can possible be all on. The user can only see two at the time (red and green).

Is it possible to reach this goal with max 10 uA/LED? Probably the LEDs could be blinked at ~1 Hz with 25-50% on time, which would help saving current at the same visibility.

Is it better to use PWM or a constant current drive for better visibility at the same average current? PWM may be a problem, since the CPU needs to sleep.

Does the size of the LED change the visibility? Is higher lm/W always better? lm/w is given at much higher current typically. Narrow viewing angle would help, but rather wide must be used.

Which I'll continue to suggest is usually well below the ideal operating current of an LED if you are trying to extract maximum efficiency from it, so the PWM concept at a peak current within the manufacturers specifications is a much better design.

[Zero999]

Those graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.

Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

If you wish to bring the data into question why can't you provide the data to show its wrong? Phosphors have well known saturation and temperature limitations at the high end, but I can't find any papers discussing non-linear effects towards zero. I'm not sure what you think luminous efficacy is, but its the well defined method of assessing visual brightness:
https://en.wikipedia.org/wiki/Luminous_efficacy
Yes, it no longer applies in a scotopic vision environment but then you wouldn't be distinguishing colours clearly which is not what the OP is discussing. You raise a lot of questions without backing it up with data or references or even explanations to help people here. Sitting on your high horse and being pompous isn't of much value to the OP when you could be adding information to the discussion rather than trying to discredit it all without alternative facts to back it up.

I apologise if I came across as pompous.

I agree with you about phosphors but that's not relevant here because the original poster is not using an LED with a phosphor. What I'm referring to is efficiency droop, i.e. there's no point in driving at high current with a low duty cycle. The LED should be run at the current, which gives the highest efficiency. Now I agree, if that's higher, than the OP's power budget then PWM may be a wise choice. In any case, a switch mode supply should be used for optimum efficiency.

Again you're thinking upwards into the well known thermal/saturation limits of efficiency in LEDs, even the links from your lazy google search show that the peak efficiency is at a single forward current point with either too high or too low drive dropping the efficiency (or efficacy):
http://www.semiconductor-today.com/news_items/2016/feb/postech_030216.shtml
http://spie.org/newsroom/6402-demonstration-of-novel-high-efficiency-blue-leds-on-silicon-substrates
Which is the region the OP is considering driving the LEDs at. Mike had already summed it up neatly in his first post:

For maximum visibility, (Cool) white, small size, narrow angle if your viewing angle . match angle to required viewing angle. If you want to stand out from other lights, maybe deep green, as this is at the peak or the eye's sensitivity.
Where you want "noticeability" for emergency type applications, strobing can help as the eye is more sensitive to movement.
In principle, for a steady brightness, PWMing doesn't give any advantage in terms of perceived brightness, but efficiency at low currents (< about 5% of nominal current) can vary a lot, particularly with white, so if only for consistent appearance, a higher current pulsed at a low duty cycle can be beneficial.

The OP was very clear on their operating point:

The device is targeted to consume less than 50 uA and it will have four LEDs than can possible be all on. The user can only see two at the time (red and green).

Is it possible to reach this goal with max 10 uA/LED? Probably the LEDs could be blinked at ~1 Hz with 25-50% on time, which would help saving current at the same visibility.

Is it better to use PWM or a constant current drive for better visibility at the same average current? PWM may be a problem, since the CPU needs to sleep.

Does the size of the LED change the visibility? Is higher lm/W always better? lm/w is given at much higher current typically. Narrow viewing angle would help, but rather wide must be used.

Which I'll continue to suggest is usually well below the ideal operating current of an LED if you are trying to extract maximum efficiency from it, so the PWM concept at a peak current within the manufacturers specifications is a much better design.

I apologise, you're right. After doing some more reading, I've realised efficiency droop is more of a problem at higher currents, than at extremely low power, where efficiency also deteriorates. LED efficiency does improve at lower currents but this doesn't extend down to the 50µA and this is nothing to do with the phosphor I was so obsessed with. I think Mike was right all along! Sorry I was too pig headed to see that.

Running around 10% of the LED's recommended forward current seems to be the best way to go (unless the datasheet suggests otherwise) and that's quite likely too high for the OP's power budget. For example, assuming a resistor is used to limit the current and the LED's recommended I_F is 10mA, then run it with 1mA pulses at 5% duty cycle to give an average of 50µA. A buck converter, rather than a resistor, could be used to either reduce the current consumption or provide longer pulses, therefore brighter light for less power.

[PointyOintment]

This project to automatedly characterize LEDs and laser diodes might be interesting to everyone in this thread. His results show that the LEDs he tested could be more efficient at low currents.

[Someone]

This project to automatedly characterize LEDs and laser diodes might be interesting to everyone in this thread. His results show that the LEDs he tested could be more efficient at low currents.

Where? They plot the same singular peak efficacy point:
https://hackaday.io/project/12874-automated-ledlaser-diode-analysis-and-modeling/log/44613-this-is-why-i-am-building-the-analyzer
Their curve matches very well to the data I have shared above showing that efficiency drops as the drive current is reduced. Its exceptionally close even though they have a much more limited linear range in their detector.

[mark03]

[Sort of?] back to the original question.  If I want to drive an LED with ~ 1 mA pulsed current, at perhaps 0.1% to 1% duty cycle, so 1-10 uA average, what is the most efficient circuit one could design for this task?  Assume battery voltage > Vf.

I am wondering if it is possible without resistors, or at least without resistors in the main current path of the LED.  E.g. a simple buck converter with low-side NMOS.  You would just need a nanopower source of low-duty-cycle ~ 1 us pulses.  Is that feasible?

[mark03]

Right, I figure this could be reduced to a constant-duty-cycle oscillator/pulser.  Not much point in a feedback loop.  So, basically, 10-kHz gate drive with 1-us on time in < 1 uA average current.  If that is possible.

[GeorgeOfTheJungle]

Anything with no R that uses only L and C will in theory be.

hp did that in their calculators:

"The image above shows the calculator with the back removed. The two sets of 4 passive components on the far left are coils used in the inductive circuit to drive the LEDs. This is a different approach where the current driving the LEDs is limited by the time it takes to build up in the inductor, rather than through a resistor in conventional LED displays. This strobed inductive drive circuit is more efficient and was invented and patented by HP for use in calculators. In the worst case, display power consumption is about 110 milliwatts (thirteen 8’s and two minus signs). The LED display is covered later in more detail."

http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=1263

google.com/search?q=hp+calculator+display+inductors

[Someone]

[Sort of?] back to the original question.  If I want to drive an LED with ~ 1 mA pulsed current, at perhaps 0.1% to 1% duty cycle, so 1-10 uA average, what is the most efficient circuit one could design for this task?  Assume battery voltage > Vf.

I am wondering if it is possible without resistors, or at least without resistors in the main current path of the LED.  E.g. a simple buck converter with low-side NMOS.  You would just need a nanopower source of low-duty-cycle ~ 1 us pulses.  Is that feasible?

Yep, thats the way to do it and calculate the peak current from the inductor and pulse width. But be careful of the reverse voltage on the LEDs some are less robust than others. The dirty circuit is a boost/inverting design where you tie the diode back to the +input rather than ground, minimal leakage through just the FET switch, and at these low powers it can be switched directly from logic or a microcontroller pin.