Tag Archives: review

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.

Picking The Right Output

The other day I ran across a publication from Allegro MicroSystems which was filled with an extensive list of terms and definitions for IC outputs. For instance, do you know the difference between a bipolar and a unipolar output? Well, a bipolar design allows the output to both sink and source current via controlled connections to power and ground. A unipolar output, however, can either sink or source current, but it cannot do both.

Now I don’t know about you, but I frequently confuse NPN and PNP outputs. I guess I just don’t work with these terms that often. Well, here’s one way to keep track of the difference: NPN outputs connect to the Negative supply while PNP outputs connect to the Positive supply. So just remember “N” for negative and “P” for positive! Take for example the following NPN sensor connected to a microcontroller (MCU):

NPN Sensor Wiring

The actual sensor circuitry is not shown here, only the output transistor. Now in this case, we have a unipolar output (the “Signal” line) which can only sink current to ground. In other words, this NPN sensor either connects the output to the negative supply (ground) or lets it float. In this configuration, however, the output does not float but gets pulled up to the positive supply line (V+) by a pull-up resistance. In this way, the input to the MCU is always either high (V+) or low (Gnd), but never in between (an undefined state).

For completeness, here’s one example of an actual PNP output used on a light curtain:

PNP Output ExampleThis may be a little tougher to interpret at first. However, there is one transistor directly connected between the positive supply rail (the topmost line) and the output (labeled OSSD). This means that the output may be connected directly to the positive supply as expected with a PNP configuration. This particular device also includes a pull-down resistor of 2.2kΩ. The difference here is that the pull-down may not always be connected to ground. That extra lower transistor can be disabled, allowing the output to float.

Anyway, check out that document and save it someplace – it might come in handy!

The $868.73 Op-Amp

Yes, you read that right; this here is an $868.73 operational amplifier, the Apex PA50A:

Apex PA50 Power Op-Amp

It doesn’t look like much, does it? Well at only ~1.5″ square, it’s not much larger than most power transistors. However, I can guarantee you it’s impressive. Most impressive. This is a power op-amp, which means it’s designed to deal with high currents and voltages. How high, you ask? Try 40 amps continuous (100A peak) at up to 130V. But you’re going to want a heat sink – this little guy may dissipate up to 400W internally.

JEDEC MO-127 HeatsinkNow sadly, these pictures aren’t mine, as I do not have a PA50 of my own. I also doubt I’d ever buy one (I just like to look at crazy-expensive parts on Digi-Key). However, this would be a fairly useful device. Normally op-amps are used at small-signal levels (from a few millivolts up to a few volts), and as such are only useful for amplifying sensors, performing filtering, etc. But with 40A and 130V to play with, you could drive multi-kilowatt speaker systems, go-kart motors, high-voltage transformers, etc (assuming stability with inductive loads). This baby would also make one serious output stage for a function generator, although its gain-bandwidth product is only 3Mhz. Interestingly, the datasheet lists “semiconductor testing” as the only potential application. Come on guys, I think a little more creativity is in order for such a product!

Now of course, you’re going to need one or two fairly beefy DC voltage sources to power the PA50, but I imagine they’ll cost you less than $800. And just what’s so special about this op-amp that drives up the cost so dramatically anyways? Frankly, I don’t know. Take a look at the following “equivalent schematic” provided in the PA50 datasheet:

PA50 Equivalent Schematic
Although this schematic doesn’t give values for individual components, we know that the final output MOSFETs (Q5,Q7,Q20,Q22) need to be rated for at least 100A (peak) and 130V. Well I’ve spec’d transistors like this in the past, and they’re not terribly expensive – perhaps $6-8 per device (double that if you want something really special). Even if all of the transistors in this package cost $10, that’s still just $220. Maybe there’s something costly about laying out all of this hardware in a single package? I’m certainly no expert in semiconductor design… Or perhaps the actual device is encased in 24k gold and diamond-studded? No, I guess they’re just trying to recoup development costs.

By the way, if you’re looking for high voltage instead of high current, try the Apex PA89, rated for ±600V output. It’s only $885.94 plus tax & shipping from Digi-Key. And don’t worry, I’m sure no one will question such a purchase. 🙂

ThinkPad T43p, T61p Power Consumption Tests

One of my biggest hobby electronics projects thus far has been the development of a solar power system, used primarily for home automation (for details see Part I and Part II). In the past, I’ve used it to partially power my five-year-old ThinkPad T43p laptop, which also acts as the brains of the system. Unfortunately, given the limited number of photovoltaic (PV) panels I possess (not to mention the lack of a good mounting location for these panels), I could only run the laptop off-grid for about six hours on a sunny day.

My ThinkPad T43p - This is one of my stock images.  If you look closely, you'll see that the lappy is actually in standby, despite what you see on the screen.Now that my PV system is (mostly) inactive, and I’ve got my Kill-A-Watt meter back, I’ve taken some time to quantify just how far I can reduce the power consumption of my T43p by disabling components. While I was at it, I ran the same tests on my more modern ThinkPad T61p. And if you’re like me, I think you’ll find the results rather interesting. 🙂

However, before I begin, I must tell you that this morning I’m mourning the loss of my ThinkPad T60. What, another laptop? Yes, admittedly, I have something of an obsession with ThinkPads. I bought the T60 as a replacement/upgrade for my T43p. I got a great deal on it from a guy on EBay, too – just $230 for a mint condition, perfectly functional PC. Even the battery still held about 80% of its design capacity. Unfortunately, after owning and operating my T60 for about six straight months, it shut down and now won’t start.

Dead, Partially-Disassembled, ThinkPad T60
I’m not really sure what’s happened; I wasn’t around when it failed. I had it running as a server in a remote location for several weeks. Then one day I just couldn’t connect. It appears the motherboard has been damaged somehow, but certainly not from any physical impact. Perhaps an electrical storm? I’ve tried everything – pressing the power button 10+ times as some forums have suggested, removing components, etc. It just won’t turn on. When plugged into AC, the battery and plug lights come on, but nothing more.

Well, at least I can still use it for parts. I’ve already swapped one of its 2GB RAM sticks into my mom’s virtually identical T60. And the battery fits my T61p, so now I can carry that along as a spare. I may look into buying a new motherboard, but it’d probably cost me another $80-100. Oh well… so far this is the only blemish on my ThinkPad experience.

Testing Power Consumption

P3 Kill-A-Watt MeterIn order to perform these tests, I’ve used a P3 Kill-A-Watt power meter. This is a super handy gadget for anyone interested in determining how quickly their devices are using electrical energy. It’ll measure up to 15A at 125VAC. In addition to current, voltage, and real power, this device also measures frequency, VA, and power factor.

I should quickly mention that while I trust the Kill-A-Watt’s real power measurements, I’m not sure it accurately computes power factor and VA for devices with non-sinusoidal current draws. And most laptop power supplies draw a very distorted current waveform due to the nature of their input rectifiers and capacitance. Here’s one example I recorded a few months back using the supply for my T61p and a nice hall-effect current probe:

ThinkPad T61p Current and Voltage Waveforms

The Kill-A-Watt claims this signal represents a power factor of 0.59. Although I haven’t crunched the numbers, my guess is that this is a little on the low side. Of course, I’m no expert in power-factor correction, particularly with such odd waveforms…

Well let’s get on to the good stuff. During this testing, I always allowed my laptops to reach a steady-state condition before recording anything. Thus, I took no data during boot-up, program launches, etc. I also ran all tests using a fully-charged battery. The following numbers would of course be somewhat higher if the battery were charging. However, just how much higher they’d be may vary depending on the battery’s state of charge. So in an effort to eliminate some variability, the battery in each laptop was first fully charged. The data I collected was then condensed into one simple graph (click to enlarge):

T43p Power Consumption Graph (Watts)Now you can look at this however you like, but I prefer to read it from left to right. So beginning on the left, in blue, you’ll see that the T43p draws a minimum or “base” power of 16W. As we move right, the numbers you see represent how much additional power is required by each specific component. For example, next in the sequence you’ll see that 1W is consumed to trickle charge the laptop’s battery. I determined this by first running the laptop without any battery connected. I then installing a charged battery pack and recorded the difference in measured power.

Moving right along the above graph, you’ll see that the T43p’s LCD backlight consumes seven watts. Next, I used the laptop’s power management software to set the CPU to its maximum clock speed, which required an additional four watts (and at this point, no additional software was calling for CPU time). Inserting the optical drive drew another watt. Surprisingly, enabling bluetooth consumed six more watts, while 802.11 wireless only called for 2W. Loading the CPU by enabling World Community Grid called for 10W. Next, running the hard drive with HD Tune required three more watts. Finally, I stressed the graphics card using the OpenGL benchmark within CineBench R10. This drew an additional six watts, bringing the T43p’s maximum power consumption to 56W. Not bad.

Next up, I performed the same tests with my main workhorse, the ThinkPad T61p:

T61p Power Consumption Graph (Watts)Interestingly, the T61p consumed slightly less base power than the older T43p – just 15W! Its backlight did draw a bit more power at 9W, but this isn’t terribly surprising since its screen is bigger. What I found particularly impressive was the ability of the CPU to throttle back, even when set to its highest clock rate. The difference between an unloaded and loaded CPU at full clock was 22W. The more modern graphics card of the T61p consumed about twice as much power as that of the T43p – fully 13W, bringing the total to 67W. Still not too bad. I should also note that with the T61p there was no noticeable difference in power consumption (e.g. from trickle charging) with the battery removed.

So there you have it! There’s quite a bit of power to be saved by cutting back your CPU speed (if your laptop allows it) as well as disabling your wireless radios and optical drive. Since I’ll be using my T43p as a server, I’ll also be able to disable itsLCD screen, which will get me very close to a base power of 16W. Not quite as good as an some netbooks, or the SheevaPlug, but still not bad given the processing power it affords me.

Oh and if you’re curious, here are my CineBench R10 results for both machines:

  • T43p Rendering (Single CPU – Pentium M 760 at 2.0GHz): 1694 CB-CPU
  • T43p Shading (OpenGL – 128MB ATI Mobility Fire V3200): 1015 CB-GFX
  • T61p Rendering (Single CPU – Intel T9300 at 2.5 GHz): 3061 CB-CPU
  • T61p Rendering (Multiple CPU – Intel T9300 at 2.5 GHz): 5757 CB-CPU
  • T61p Shading (OpenGL – 256MB NVIDIA Quadro FX 570M): 4212 CB-GFX

Scotty, I need more power!

Today is Monday, October 11, 2010. What does this mean for you? I’m not sure really… how are you? For me, it means entering my seventh week of job hunting. Actually I’ve been looking for longer than that, but I’ve only been out of school and intensely searching for about seven weeks. Now if you’ve ever been in this situation, you know it gets pretty boring pretty fast. So to help break up the monotony, I’ve been blogging quite a bit and, recently, thinking about all the cool stuff I can buy once I’ve got income again. 😛

You know what I’d really like? A quality benchtop power supply. So far I’ve been working with a simple linear supply which I inherited about four years ago. I think it actually came from a DIY kit. It’s got one fixed 5V, 2A output and two adjustable ±1.5V to ±15V, 1A outputs. Being a linear supply, it’s fairly inefficient. It also can’t do current control, although it does provide over-current and short-circuit protection.

My Current Power Supply (Art Supplied by Author)Now don’t get me wrong, I love this supply. It’s performed with gusto and with fervor, and I certainly couldn’t have accomplished much without it. But as I said, it lacks the ability to perform current control – something I’ve found very useful. It would also be nice to have digital readouts showing voltage and current (since those adjustable knobs aren’t terribly accurate). Now I know I could modify it to perform most of these tasks, but I’d rather use a power supply than build one (despite all the street cred that would get me).

With that in mind, here’s what I’m looking for in my next benchtop power supply:

  • Voltage range of 0-40VDC (or greater)
  • Current supply rated to 5A (or greater)
  • Adjustable current control
  • Digital displays of voltage and current
  • Single output
  • Switching supply preferred for efficiency (but linear is alright)
  • Cost should be under $500

I’ve already located a few suitable candidates; first up is the B&K Precision Model 9110:

B&K Precision 9110

This supply meets all of my requirements, and costs just $275 at Digi-Key. The only exception is that it does not supply its full current (5A) at all output voltages. The diagram above illustrates how this supply is power-limited instead of fully current-limited. So if I want full voltage output (60V), I’ll only be able to draw about 1.7A. This I could probably live with. I also find it interesting that this claims to be a “mixed-mode” supply. So in some ranges it operates as a linear supply, but in others as a switching supply.

Next up is another B&K Precision supply, the new Model 1550 (now with USB charger!):

B&K Precision 1550

This $139 switching supply is good for just 36V and 3A, so it’s a little shy on power. But I had to mention it because of its iPod-like front panel. That new touch pad design and glossy finish make it look remarkably like the iPod Nano (which, conveniently, you could charge with this supply’s USB port). Somehow I doubt I’d use that feature though.

Now here’s yet another B&K Precision PSU, the Model 1743B – a little pricey at $539:

B&K Precision 1743B

This supply provides up to 6A throughout its 35V range and includes two four-digit LED displays. It’s a pretty nice supply, albeit expensive. I’m not sure whether this is a linear or a switching supply, but I’d guess it’s probably linear. That 6A output sure sounds nice…

How about something non-B&K for a change? Here’s the PWS2326 from Tektronix:

Tektronix PWS2326While its voltage rating is a little lower than I’d like (32V), I do appreciate one thing: buttons. No need to fiddle with knobs to get the right voltage and current settings – just type them in! Plus it’ll source 6A, and for $449 it certainly beats the B&K 1743B. It also allows you to return to sixteen different preset voltage and current settings. Again, I kindof doubt I’d ever use that feature, but maybe someday.

One final supply worth mentioning is this beefy Protek model 75005M:

Protek 75005M

This supply certainly meets all of my specs and then some. The prices I’ve found online tend to vary a bit though , from $530 to $630 – either way, it’s definitely on the high side. Plus, I can’t say I’ve ever heard of Protek anywhere. Anyone out there use Protek?

Summary

Comparison TableWell right now I’m learning towards either the B&K 9110 or the Tektronix PWS2326. I really like the buttons provided on the Tektronix, but I also appreciate the higher voltage provided by the B&K as well as its substantially lower cost. Of course, until I find work, they’re all out of my price range.

If anyone out there has experience with any of these supplies or would like to suggest another, please leave a comment! Thanks!

Probably just one output would do fine