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Riding the Rails

Linear Technology Makes Power Fun?

Let’s just acknowledge up front that practically nothing is as unexciting for digital designers as power supply design.  

All the way back to when we were wire-wrapping our TTL parts, there was absolutely no inspiration in the part of the project where we put that “+5V” on the board.  Who picked 5V for those things anyway?  Didn’t they know what we had available?  9V batteries – nope.  1.5V batteries – that could get us to either 4.5V or 6V – close but nope. 120V AC (in the US) – super nope!  Maybe the people designing the logic chips were in cahoots with the guys who make power converters?  At any rate, the power supply thing was just a whole bunch of no fun, and we generally slogged through it in engineering agony so we could get to the good part – making our logic stuff go.

Today, all that is different, of course.  We dream about the days that we just had to put +5V on the board and route it to one pin on each chip.  Today, the logic design people must have a full-fledged conspiracy going with the voltage converter people, because devices like FPGAs require practically a PhD in power pins just to get them to light up properly without the telltale puff of blue smoke.  Plop down a few-hundred-pin package, and a good percentage of those pins turn out to be power rails – in assorted shapes and sizes.  We need 1.2V here, and 0.9V over there.  Each type has a different current requirement.  We have noise issues – particularly when we’re powering SerDes I/O.  When we bring up the chip, we need to sequence the power rails in a millisecond waltz – like “The Flash” manning the light board in his own Broadway production.

We’ve now transformed that which used to be boring and uninspiring into that which is insanely difficult and complicated – and uninspiring.  

Do it all right, and your circuit is now “on” – woo hoo.  Do it wrong (this is the normal case) and you get everything from unexplained bad behavior to restart lockups to damaged parts and expensive re-works.  “Um, you see, boss, we need a couple of weeks to build another prototype board because, uh, we had some new ideas about how to make the design… cheaper.”  (Much cheaper, in fact, because it won’t be burning up the FPGA every time somebody powers it up.)

Linear Technology has heard our cries, and they’ve been off making power supply design both easy and fun.  

I can see that look on your face – even through the internet.  

Picture this – a “Space Invaders” style game where power rail requirements are falling down from the top of the screen, and you’re at the bottom with a laser-zapper thingy that knocks them down before they hit the bottom and mess up your design.  Then, you’re in engineering school again, but you can’t remember what class is next, and you have to get exactly 0.80 Volts on a power rail, or you’ll fail the exam… No, wait, that was a recurring engineer nightmare.  Nevermind.

Linear Technology actually has been in cahoots with the FPGA people, but in a good way.  They recognized that FPGAs had special requirements for power, and they worked with the FPGA vendors to build solutions that were simple and robust.  First, they hit us with DC/DC switching power regulators – a bit of heresy at the time, because conventional wisdom was that switching regulators could never have low-enough noise for noise-sensitive FPGA I/O requirements.  Next, they packaged these DC/DC switching regulators into tiny packages called “micromodules” – generally smaller than 15mmX15mm, and looking like a surface-mount digital device.  One example, the LTM4616, is dual-channel, supplying up to 8A per channel, and can be ganged to produce a single channel at 16A.  Input voltage can be anywhere from 2.7V to 5.5V, and output is tightly regulated to whatever you pick from 0.6V to 5V.  The device is only 15mmX15mmX2.8mm (LGA) which means it fits nicely under the “You must be at least this tall to cause clearance problems” sign.  It has overcurrent and thermal shutdown protection, and total DC output error is +/- 1.75% from… Hey, wake up!  We just started through a few regulator specs and already you’re snoozing!  What is it with you people?

OK, so maybe DC/DC switching power regulators made powering FPGAs easier, but it did not yet reach our goal of “fun”.  That’s where the new LTC2978 comes in.  Granted, your idea of “fun” may not normally be described with a title like “Octal PMBus Power Supply Monitor and Controller with EEPROM.”  Can I hear a “Yee Haa!” from anyone?  Anyone?

The LTC2978 can simultaneously manage up to eight regulators.  It monitors the output voltage of each channel plus one input voltage .  For each channel, it has an under/over voltage supervisor and a 10-bit IDAC that can ramp voltage-buffered inputs up and down during adjustments.  The device can sequence power-ups according to a precise schedule that you define, and it has a programmable fault response that can disable the faulted power supply plus additional ones you designate, as well as a “retry” option after a fault is detected.  These devices are also cascade-able, so if you have more than 8 channels of power to manage, (bless your hearts) you can have them all dancing in step.  Without putting you to sleep with specs again, the 2978 can do just about anything you want with your power rails, all in a nice QFN 64 pack.

I hear you – that’s all interesting and certainly well-engineered, but we have not yet reached the standard of “fun.” OK, maybe I was pre-mature with that. Did I mention that there is a GUI?

20090929_power_fig1.jpg
Image courtesy of Linear Technology.

Whoa! Yes. That got your attention. We skipped right over the obvious question up there, which was – “Hey, what’s the EEPROM for in that 2978, and how do I control all the things that are controllable? Are there little knobs and slider switches all over the top of that QFN 64? Do I have to program it by soldering in a bewildering array of resistors? Nope and Nope. There’s a GUI called LTPowerPlay that mind-melds with your 2978 via PMBus and lets you control and monitor everything going on with your power rails. This part is fun. You can set up target voltages for each power supply channel, configure over-under voltage margins, set up warning voltages that take action before a shutdown is required, sequence the startup of all your power supplies, and then report back a detailed list of power optimizations you need to implement in your FPGA design.

OK, that last item was fake.

You can, however, use the GUI to simulate various failure modes and give your power supply design a good wringing-out at the development board stage, preventing that subsequent walk of shame to the boss’s cubicle. LTPowerPlay is free, and it’s easy enough to use that even an editor can manage it.

The LT2978 and LTPowerPlay can be used to manage a variety of regulators – even ones not made by Linear Technology. Based on past trends, we expect Linear to follow the 2978 with an array of similar devices with various permutations of features and capabilities, adapting to the variety of target applications that require complex power management. If you’re building something with high-end FPGAs, chances are you fall into this category, so grab a game controller and get ready to ride the rails!

 

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