Tag Archives: led

Review: TI’s High-Power LED Driver Evaluation Board

On my desktop, I keep a list of miscellaneous parts I’d like to buy at some point (e.g. power resistors, laser diodes, etc).  Parts not destined for any specific project, just things that I’d like to toy with.  Well for a long time, I’ve wanted to get my hands on some high-power LEDs.  I suppose I’m just a sucker for pretty lights.  But for some reason, I’ve never gotten around to ordering any – probably because I’ve never had a good means of driving said LEDs (and I’m too busy lazy to make my own driver circuit).

Well last week Farnell (Newark in the States) came to my rescue with an offer to send me any product from their site (within a certain price limit) for free.  All they asked of me was an evaluation (this post) and a link to the product on their site.  And which product did I pick?  The TPS62260LED-338, a three-color LED driver evaluation module:

TPS62260LED-338This board hosts three 500mA LEDs (W5SM) from OSRAM.  Each LED is driven by a TPS62260 step-down DC-DC converter.  A low-cost MSP430F2131 microcontroller controls all three drivers via pulse-width modulation.

Out of the box, my first impression: these LEDs are painfully bright (especially that red one – my vision is still spotted as I type this).  They’re not kidding about protective eyewear.  But I wouldn’t want it any other way. 🙂 For most of my testing however, I simply covered the LEDs with about four sheets of paper.  That brought their intensity down to a comfortable level.

I must commend TI on making this board very easy to use and probe.  They’ve provided several nice wire-loop test points for connecting scope probes.  And they’ve even broken out the power and ground connections for people like me who don’t have the proper barrel connector power supply.  I was also pleased to see how they’d integrated heat sinks for each of the three LEDs into the PCB itself using a plethora of plated drill holes.  In operation, the board only just becomes warm to the touch.

But let’s talk about the real highlight of this board: the LED driver circuits.  Because LEDs operate within such a tight voltage range (their operating voltage is actually assumed to be about constant), they’re normally powered by some type of current controller (since the brightness of an LED is proportional to the current flowing through it).  Any yet, this board features three DC-DC voltage converters – devices which take a high input voltage and convert it to a lower output voltage.  So how is this supposed to work?

Well, each converter IC provides closed-loop control over its switching output.  In other words, the TPS62260 measures a feedback voltage and uses this to adjust its output duty cycle.  So regardless of how much current (well, up to 600mA) is being drawn from the output, the converter is able to maintain a fixed output voltage.  But here’s the tricky part: you can attach the converter’s feedback measurement input pin to anything (within reason, of course).  In this case, TI has wired each feedback pin to a 2Ω current-sensing resistor (part R9, below) connected in series with each LED.  Each converter will adjust its output in order to maintain 0.6V at its feedback pin (as 0.6V is the internal voltage reference of the converter).  Using ohm’s law, and realizing that the current will be the same in both the sense resistor and the LED, since they are in series, we can determine the LED’s current to be I = V/R = 0.6/2 = 0.3A or 300mA.

LED Driver Schematic

But wait, the current-sensing resistor is fixed, the converter’s internal voltage reference is fixed… so how do we control the current delivered to the LED?  Simply put: we don’t.  Then how can we control its brightness?  Pulse-width modulation.  Imagine flipping a light switch on and off so rapidly that you can no longer detect a flicker.  Then, adjust the ratio of the on and off times.  The longer the on time, the brighter the light will appear.  This is precisely what the MSP430 microcontroller is doing to control the brightness of the LEDs.  In fact, you can see this happening if you wave the board around rapidly while one of the LEDs is being dimmed (in this case, the blue LED):

Pulse-width modulation in action!

That image was captured with a 0.1s shutter speed.  And actually, with that knowledge, we can calculate the frequency of the PWM signal.  I count about twelve blinks of the blue LED there – so twelve blinks in 0.1s yields a frequency of 12/0.1 = 120Hz (a result I confirmed with my IOBoard oscilloscope).  If you’d like to read more about pulse-width modulation, check out my previous post on the subject.

So out of the box, the microcontroller on this evaluation board is programmed to slowly turn on and off each LED in sequence, such that one LED is always fully on while another is being ramped on or off.  This produces a very pleasing color gradient.

Now, according to the manual that came with the board, you’re also supposed to be able to turn the knob on the board in order to manually adjust the color balance.  Unfortunately, this feature did not work for me.  When I turn the knob on my board, the automatic sequence stops and the LEDs hold their current brightness states.  However, they do not change brightness when the knob is turned further.  I’ve probed the knob (which is actually a digital encoder) and believe it to be working properly.  My guess is that somebody just botched up the software.  It happens.

This brings me to my final point of discussion: reprogramming.  The TPS62260LED-338 provides a JTAG header for the traditional four-wire JTAG programmer.  Unfortunately, I do not possess such a programmer.  I was hoping instead to use the MSP430 programmer which is integrated into my LaunchPad development board.  Sadly, I never checked into the details: the LaunchPad programs via the two-wire SpyBiWire (SBW) interface, not the standard JTAG interface.  And of course, the MSP430F2131 does not support SBW.  So for now, there will be no reprogramming.  Of course, thanks to all of the convenient test points, it’s fairly easy for me to just put the micro into reset and drive the LEDs using my own PWM waveforms.  If anyone out there has any tricks for reprogramming though, please let me know!

So in conclusion, I’d say the TPS62260LED-338 is a product worth checking out.  For just over $20, it’s a pretty good deal.  If they’d given it the USB programming interface of the LaunchPad, I’d probably be happier, but then they would’ve needed to lower the current draw of the LEDs, which would’ve been no fun, or required a separate power supply, which wouldn’t have been such a big deal.

No no, you’ve got it backwards.

A lot of things in this world just aren’t easily reversible. And no, I’m not referring to the strict definition of thermodynamic reversible processes. What I mean is that many conversions (energy, chemical, etc.) and systems cannot be readily reversed. Your hairdryer likely can’t turn heat into electricity. You can’t very well make oranges out of orange juice. And I’m pretty sure your car won’t turn carbon dioxide back into gasoline.

Converting OJ Into Oranges: It Just Can't Be Done

Motors and Generators

Now of course, some processes can be reversed. For instance, many people know that DC electric motors can also be used as generators. Such motors work because of the forces generated through the interaction of two magnetic fields. One of these fields is brought about by the flow of current through coils of wire; the other is created by permanent magnets attached within the motor housing. See this HowStuffWorks article for more details. This same DC motor can also be used as an electrical generator.

NRC Steam Turbine Driven Electrical Generator
Generators work because of Faraday’s Law of Induction, which states that “The induced electromotive force (emf) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.” In other words, a changing magnetic flux (e.g. a moving permanent magnet) will induce an electromotive force (a voltage) in a nearby electrical circuit (e.g. the motor’s windings). This Wikipedia entry on electrical generators is quite an interesting read and includes some history of generators such as the one pictured above.

Light-Emitting Diodes (LEDs)

Optek High-Intensity LEDsNow here’s something a bit more curious. I’m sure you’ve heard of or at least seen an LED. They’re everywhere: from alarm clocks to cell phones to outdoor lighting. As their name says, they emit light. But did you know that LEDs can also be used as light sensors? It’s true! While LEDs are optimized to emit light, they are, physically, not that different from photodiodes. Thus, instead of operating them in a forward bias, you can reverse bias them just as you would a photodiode. The following schematic comes from an Altera white paper and illustrates how the same the same LED can be used as both a sensor and an emitter:

Altera LED Emitter/Detector Design SchematicThis white paper also describes how, with two pins on a CPLD, microcontroller, or FPGA, the same LED can be used as both a sensor and an emitter without rewiring. This type of dual-purpose use could result in significant cost savings for devices produced on a large scale. In addition, it’s also possible to transmit and receive data using single LEDs. At RPI and other universities, research is proceeding on high-speed data transmission using ambient LED lighting – “Smart Lighting,” it’s called. In the future, when incandescent and fluorescent lights are replaced by high-intensity LEDs, your laptop might actually connect to networks via the lights in a room. They’ll be doing double duty: lighting the room and transmitting data via high-frequency modulation. Pretty cool, right?

The Speakers are Listening

A Typical SpeakerFinally, one of the most interesting and practical reversible technologies is the speaker. Just the ordinary, magnet-and-coil cone speaker. Did you know that a speaker can be used as a microphone? Interestingly, the reason this works is precisely the same reason that DC motors can be used as generators. Normally, speakers produce sound through vibrations created using coils and magnets. Just as with the motor discussed earlier, a magnetic field is created by passing current (the audio signal) through a coil of wire. This field will cause the coil to either be pulled towards or pushed away from the permanent magnet. This motion, when done very rapidly, results in sound. Check out this HowStuffWorks article for a brilliant animated illustration of this effect as well as further details on speaker operation.

Now, the great thing about the way in which speakers operate is that it’s entirely reversible. Instead of passing a current through the coil and causing it to move, we move the coil using sound and then amplify the resulting current. Again, this works because of Faraday’s Law of Induction (see above). But how does this work practically? Well, not too bad actually. Some people say that an average speaker may sound better than a cheap microphone. If you’d like to give it a try, just find an old speaker, then wire it into the microphone jack on your stereo, laptop, etc. To test, just speak into the speaker!

So You Want to Use PWM, Eh?

PWM Waveform Captured on an OscilloscopePulse-width modulation. It probably sounds a little confusing if you’re new to electronics. Kindof a word mashup, really. What do pulses, width, and modulation have to do with each other anyway? I remember first learning about PWM during my freshman year of college at RPI. I was in a pilot course called “Foundations of Engineering” under the excellent instruction of Professor Kevin Craig (whom I later worked for). I remember thinking later, “Hey, this PWM stuff is pretty clever!” So let’s take a look at PWM and see what we can learn. (If you’re already familiar with the basics of PWM, skip down a few paragraphs for more advanced topics and experiments!)

Say you’ve got a light-emitting diode (LED) and a battery. If you connect the two directly, the LED should produce a lot of light (assuming the voltage of the battery isn’t too high for the LED). But what if you wanted to reduce the amount of light that LED produces? Well, you could add a resistor in series with the LED to reduce the amount of current supplied by the battery. However, this won’t allow for easily adjustable brightness and may waste a bit of energy. That loss may not matter for a single LED, but what if you’re driving several high-power LEDs or light bulbs? This is where pulse-width modulation comes into play.

PWM Graph - 30% Duty CycleImagine you could connect and disconnect the LED and battery multiple times per second, causing the LED to flash or pulse (see graph above). If this ON-OFF cycle is fast enough, you won’t even notice the blinking. In fact, the LED will appear to be continuously lit, but reduced in brightness. In addition, its brightness will be proportional to the ratio of the on and off times. In other words, if the LED is connected for 30% of a pulse cycle, it will appear to be producing about 30% of its full brightness continuously, even though it’s actually turning completely on and off. So to adjust the brightness of the LED, all we need to do is adjust, or modulate, that ON-OFF ratio, also known as the pulse width – hence the name! The ratio between the on and off time is also commonly called the duty cycle.

Now in case you’re imagining yourself frantically flipping switches on and off, or tapping wires against battery terminals, you can stop. Just put a transistor in series with your LED! It can act as a switch which can be controlled by a microcontroller or some type of oscillator circuit (see links below).

Hobby Servo (Commanded via PWM)So what’s PWM good for, anyways? Well, dimming LEDs and other lights is just one of a number of applications (example). You’ll also find PWM used in motor controllers. You can make a very simple DC speed control using a PWM generator and a single transistor (examples – notice the extra diodes in use here to prevent damaging inductive spikes). In addition, PWM is very important for some types of power supplies; specifically the aptly-named “switched-mode” PSUs. This technique can also be used to create a digital to analog converter (DAC) by low-pass filtering the square wave. Finally, pulse-width modulation is sometimes used as a means of digital communication. For example, to command the position of a hobby servo.

Now you may be wondering why I’m writing about PWM all of a sudden. Well, there’s actually a point to all of this background information. By now, you’ve probably seen a car or two with these new-fangled LED tail lights. They’re pretty easy to spot since you can typically make out the individual LEDs within the whole tail light assembly:

Ford LED Tail Light Upgrade - Ain't that a Fancy Photo?
But have you ever noticed that on some cars (e.g. Cadillacs), these lights tend to flicker? You may not see it if you’re looking straight ahead, but if you quickly move your eyes from left to right, you may catch a glimpse of the flicker created by a low-frequency PWM controller. Now, call me strange, but I find this really annoying and distracting. Maybe I just have fast eyes or something, but I hate flicker. Back in the days of CRT monitors I could usually tell the difference between 60Hz and 70Hz refresh rates. But in the case of these tail lights, it sounds like there’s danger for people with photosensitive epilepsy. According to the Epilepsy Foundation, flashing lights in the 5 to 30Hz range can trigger seizures. Obviously, having a seizure while driving would not be a good thing for anyone.

By the way, if you’re ever trying to determine the frequency of a blinking light, just snap a couple pictures while moving your camera (or the light). The one catch is that you need to be able to specify a known shutter speed. Then you just have to count the blinks and divide by the shutter speed (in seconds) to find frequency. Here’s an example:

LED PWM Frequency Comparison

This method can also give you a pretty good indication of duty cycle – in this case it looks to be about 60%. Here’s a second shot I took while on the road one night. You can tell the streetlights are running on 60Hz AC (although they’re not LEDs so they never go completely dark during a cycle), while the green stoplight is likely getting DC:

Pulsing Streetlights

I’m thinking this long-exposure shot might also pass as modern art in some circles.

The Advanced Stuff

So what’s the deal with these awful low-frequency PWM tail lights? Well, one reason you might choose a lower frequency is to save on energy lost during switching. Both LEDs and the transistors used to drive them have parasitic capacitance. In other words, they store a very very small amount of energy (think nanojoules) each time you turn them on. This energy is consumed in addition to the steady-state power drawn by the LED to provide illumination. Furthermore, this stored energy is rapidly dissipated (and thus not recovered) each time the device turns off. Now if you’re turning an LED on and off fifty times per second, it’s probably no big deal. But what if you wanted to eliminate any possibility of flicker by driving the frequency up into the kilohertz range? Would this introduce substantial power loss? I was curious, so setup a simple experiment to find out.

Test Setup
The heart of this test circuit is fairly simple – two bright red LEDs (Model OVLBR4C7) along with 92Ω current-limiting resistors controlled by a BS170 MOSFET. To measure the power consumed by this circuit, I’ve taken a non-traditional approach. Because I was worried that the cheap ammeters I have available would be thrown off by varying PWM frequencies, I decided to measure power consumption based on the discharge time of a supercapacitor. And who doesn’t love supercaps, anyways?

The theory is pretty simple. The energy stored in a capacitor is equal to ½*C*V² (Joules). So all I had to do was charge up the cap, measure its voltage, let the circuit discharge it over a fixed period of time, then measure the final cap voltage. For my 2.5F capacitor (from NessCap), I chose ~60 seconds as my discharge period. Here’s a screenshot of the voltage logging application I used to collect my test data:

IOBoard Test Program
The white line in the graph above plots the capacitor voltage during discharge. The red line indicates the voltage measured across a phototransistor (L14C1). This was used to quantify the amount of light produced by the LEDs at each test point. To get a better measurement I covered the LEDs and phototransistor with an opaque plastic cup, then covered the whole setup with a shoebox and turned off the lights. I was trying to see if, for some reason, the intensity of the LEDs was non-linear with respect to duty cycle or was affected by PWM frequency. Unfortunately this data turned out to be rather boring, but I’ve still included it in my summary spreadsheet which you can download below.

Now before I go on, you’re probably wondering what sort of data acquisition hardware I’m using. Well I doubt you’ve heard of it as it hasn’t yet been commercially released. Right now it’s being called the RPI IOboard. It’s a pretty impressive piece of hardware with dual 12-bit, 1.5MSPS ADCs, dual 14-bit, 1.4MSPS DACs, and a host of digital I/O all powered by a 400Mhz Blackfin processor. For the past few years it’s been developed at RPI and tested at a number of schools across the country. However since the project’s lead professor, Don Millard, left RPI last year, I’m not exactly sure what will become of the board. The screenshot you see above is actually one of several executable VIs I developed as examples for use with the board. Further information on the hardware can be found here.

Test Setup Closeup
So back to the experiment at hand. For my first round of testing, I utilized the IOBoard to generate varying PWM signals for the MOSFET. Thus, the current required to drive the BS170 was not included in my first measurements. I varied both frequency and duty cycle for three pairs of LEDs: white (C513A-WSN), red (OVLBG4C7), and green (OVLBR4C7).

TABLE 1: Data for power consumption tests without gate-drive losses:

Frequency/Duty Cycle (WHITE LED) 30% 60% 90%
50 Hz
36.15 mW 62.08 mW 84.89 mW
300 Hz
36.26 mW 63.50 mW 85.12 mW
10 kHz
38.75 mW 64.25 mW 86.14 mW
100 kHz
38.52 mW 62.80 mW 86.59 mW
Frequency/Duty Cycle (RED LED) 30% 60% 90%
50 Hz
54.70 mW 93.82 mW 123.75 mW
300 Hz
57.76 mW 93.81 mW 125.35 mW
10 kHz
56.99 mW 94.00 mW 126.08 mW
100 kHz
56.61 mW 95.11 mW 125.47 mW
Frequency/Duty Cycle (GREEN LED) 30% 60% 90%
50 Hz
41.49 mW 71.29 mW 91.65 mW
300 Hz
41.93 mW 70.29 mW 91.69 mW
10 kHz
41.90 mW 69.96 mW 93.36 mW
100 kHz
42.57 mW 69.71 mW 93.58 mW

So if you look through the data above, you’ll notice that there is, on average, a slight positive correlation between power consumption and frequency. In other words, the higher the switching frequency, the greater the power consumption. This is just what we would expect. Again, this data does not include losses due to transistor gate capacitance, only losses due to the LEDs’ capacitance and the MOSFET’s output capacitance.

For my next test, I wanted to see what losses might be incurred in driving the MOSFET’s gate. Thus, I called on my trusted 8-bit AVR microcontroller (ATMega644P). I wrote a very simple program (which may be downloaded below) to produce a varying PWM output from one of the MCU’s timer/counter outputs. I then measured the power consumption of the entire circuit, AVR included. For this test I only used a 60% duty cycle:

TABLE 2: Data for the ATMega644 driving a BS170 and two green LEDs:

Test Frequency Total Average Power (mW) Calculated Switching
Losses (mW)
50 Hz
91.741 0.000
300 Hz
92.708 0.000
10 kHz
92.622 0.016
100 kHz
92.978 0.157
1 Mhz 95.789 1.568

TABLE 3: Data for the ATMega644 driving a FDP8860 and two green LEDs:

Test Frequency Total Average Power (mW) Calculated Switching
Losses (mW)
50 Hz
93.475 0.004
300 Hz
95.809 0.021
10 kHz
98.238 0.710
100 kHz
114.526 6.848
1 Mhz 161.657 60.914

TABLE 4: Data for the ATMega644 directly driving two green LEDs:

Test Frequency Total Average Power (mW) Calculated Switching
Losses (mW)
50 Hz
69.278 0.000
300 Hz
67.926 0.000
10 kHz
68.778 0.015
100 kHz
68.534 0.147
1 Mhz 70.708 1.467

Discussion of Results

In Tables 2-4, we’re starting to see a much clearer positive correlation between frequency and power consumption. For these tests I also added a fifth data point not gathered with the IOBoard: a frequency of 1Mhz. This should in theory increase our maximum losses by 10x. The results seem to support with this prediction.

The tables above also include a rudimentary calculation for switching losses based on capacitances. I measured the capacitance of my green LEDs to be about 120pF (this value was not mentioned in the datasheet). The gate capacitance of the BS170 is given in its datasheet as 24pF. Finally, the input capacitance of the FDP8860 (a much beefier power MOSFET) is typically listed as 9200pF. To determine switching losses I again applied the formula for a capacitor’s stored energy (½*C*V²). At each switching interval, the parasitic capacitances in the circuit store and then dissipate this much energy. So to determine how much power is lost, we simply multiply this lost energy by the switching frequency (since 1 watt = 1 joule/sec). It appears that these calculated figures match the measurements fairly well. Isn’t it nice when math agrees with reality? Gives me a fuzzy feeling, that.

Now we can essentially think of the 50Hz test point as a baseline with zero switching loss. For the data in Table 4, the 50Hz power consumption is about 69.3mW. The calculation predicts that at 1Mhz, we’ll lose 1.5mW to parasitic capacitance for a total consumption of 69.3 + 1.5 = 70.8mW. This isn’t that far from our measured 70.7mW.

It’s also interesting to note the substantially higher losses incurred when using the FDP8860. This is largely due to its (relatively) enormous input capacitance of 9200pF. This is nearly 400x the capacitance of the tiny BS170. That’s the price you pay for the ability to sustain larger currents without overheating. For more information on power MOSFETs have a look at this IRF document called “Power MOSFET Basics.”

Summary

Well after all that, I’m going to say that whoever manufactures these tail lights can’t really use efficiency as an excuse for choosing a low switching frequency. Unless they need huge FETs to drive huge currents, switching losses really aren’t so much of an issue. I’m guessing that somehow it was just cheaper to go with a low frequency. I’m pretty sure the components themselves aren’t any cheaper, but perhaps the assembly was less expensive. It may be that some automakers already had a low-frequency module in place to drive old incandescent bulbs and then when LEDs came along they just kept using that same module. Anybody out there care to comment on this?

So my advice to those making LED dimmers: pick a frequency of about 300-500Hz to eliminate flicker while keeping switching loss low. Then find yourself a sufficiently large transistor with low capacitance and low on-resistance. And if you’re working on motor controls or power supplies, things get a lot more interesting, but as a start, try a frequency in the 20+ kHz range to avoid audible whine. Good luck!

  • For further reading on LED losses, try this NI article: Light Emitting Diodes.
  • For more accurate MOSFET swithing loss formulae, try this MAXIM article.
  • Test code for the ATMega644P is available here.
  • A complete spreadsheet containing all data can be downloaded here.

Update (9/22/2010): In the comments below, Jas Strong pointed out that in my switching loss calculations, I’d also neglected the power lost in the MOSFET during turn-on. Jas is absolutely correct about that; I should have mentioned this previously. Essentially, while the gate capacitance of the MOSFET is charging, the resistance between drain and source will pass from very high to very low resistance as the conduction channel is formed. This time period, although short, includes a region of, shall we say, “moderate” resistance which briefly dissipates additional power.

Now, in the case of my two-LED test setup, I neglected the effects of resistive switching loss because they’re quite small. Let’s take a quick look at the numbers. First, we need to know how long it takes Vgs to reach the threshold voltage. For simplicity, I’m going to assume that my AVR drives the gate with a constant current of 40mA (the maximum an AVR will provide per I/O pin). Our worst-case turn-on time will occur with the FDP8860, which has a gate capacitance of 9200pF and a typical threshold voltage of 1.6V. Using the formula ic = C*(dv/dt), I find dv/dt = 4,347,826 which means we reach Vth in 1.6/4,347,826 = 368ns. At a switching frequency of 1Mhz, this represents about 37% of a switching cycle. However, we need to double this since we lose power durning turn-on and turn-off. Thus, we’re losing energy in the MOSFET’s resistance over 74% of a single cycle at 1Mhz. That sounds like a lot, but just how much energy is actually lost?

To determine this loss, I’m going to make a big assumption and say that the MOSFET ramps linearly from 20kΩ down to 0Ω during turn-on. I’m also going to assume the voltage of the diode is constant at 3V and the power supply is constant at 4.2V. Remembering that I have 92Ω resistors in series with the LEDs, the instantaneous power dissipation in the FET becomes 2*Rmos*[(4.2-3)/(92+Rmos)]^2 (based on the fact that I have two LEDs and using the formula P = RI^2 and ohms law, I = V/R). Now I need to integrate to determine an average power dissipation over this interval. If my math is correct (feel free to check me), I get a loss of 0.632mW. Since this occurs during 74% of a cycle, the total loss at 1Mhz will be about 0.468mW. Not too serious in my opinion.

Now of course, the power required by my two-LED setup is piddly in comparison with that drawn by a couple brake lights. Once you start sinking more current into your LEDs, this resistive switching loss, as well as the on-resistance of your MOSFET, is going to start to make a bigger difference. So thanks very much Jas for pointing this out!

Frequency Duty Cycle Start Cap Voltage Start Phototransistor Voltage
50 0.3 4.248407 1.464428967
300 0.3 4.246836767 1.4911225
10000 0.3 4.2389857 1.4911225
100000 0.3 4.243696367 1.538228733