Saturday, April 19, 2008

AA to USB, part II

This is the continuation of my little project to build a device to power a USB port off a pair of AAs. Click here for part I.

So, I started casting about for components. Thankfully, the proliferation of USB devices provides plenty of possibilities and reference designs. I found a nice chip designed for this exact application from Texas Instruments, the TPS61092. It's a tiny bugger (more on that later), but it satisfies the two things I wanted to improve upon. It can supply up to 500 mA at 5 V, and would be upwards of 96% efficient in converting the power, which means that more of the energy in the AA batteries is delivered to the device, and less is lost to heat. It also has two features I haven't found in most other DIY ipod chargers - an input to turn the thing on and off (most designs are on all the time unless the batteries are pulled), and an output that lights up when the batteries get low.

I wanted my design to be compact, which implies implementing it on a printed circuit board and using surface mount components. In order to do a printed circuit board properly, one needs to first lay it out on a computer. I used a free program for this, popular in the Make and open-source communities, called Eagle. It looks and operates like most other electronics CAD program I've ever used, but the controls take some getting used to. It's a bonus that it works on my Mac laptop - most are windows only.

If I were posting this on Instructables or Make, I would go through some of the process of explaining the design. Instead, for a more lay audience, I'll just throw up some pretty pictures.

This is how the layout looks in Eagle. The white outline at the top is the USB port, which should put the size of things into some perspective. The whole board measures 1" x 1.5". I am pretty sure I can eventually cut that in half. All the red are the copper tracings. This board only has one layer of such tracings, which makes it easy to prototype. I had a coworker take this design an cut it out on something called a board router. A board router is like a dremel tool with a tiny cutter tip that is attached to an X-Y stage. What it's routing is a sheet of fiberboard that has copper adhered to the to and bottom. The cutter tip cuts the unwanted copper away, leaving the traces you want behind. This is a relatively quick way to prototype a circuit board, but usually only in one or two layers. A computer or cellphone has a circuit board with 4-8 layers - very complex - and gets made with a different process.

I said earlier that the chip, the TPS61092, was a tiny bugger. is. The chip measures 4 mm to a side, and has 16 miniscule contacts around the perimeter. Most surface mount chips have leads - little bits of metal that extend out from the package a bit so that you can get some solder onto them. This chip doesn't have leads that extend out, they only are on the edge, and they're 0.3 mm wide. What is more, the underside of the chip is itself one relatively large contact - for heat dissipation. Being on the underside means that I can actually get a soldering iron onto it to heat it up. But, with some a little help and advice from another coworker who does this kind of thing for a living, I was able to get the chip soldered on.

Here's another view, from an angle, so that you can see the small leads soldered on. In doing this delicate hand soldering, it was quite helpful to have a device somewhere between a microscope and microfiche viewer. It magnifies the view and puts a lot of light on it, so you can see what you're doing and inspect it later. It also magnifies every tremor in your hand, amking you believe you'll never be able to hold anything steady, even if you've skipped the coffee. Looking into the scope at the magnified work area, tiny tweezers shaking in one hand, soldering iron shaking in the other, I was put in mind of how a neurosurgeon must feel anastomosing a 2 mm blood vessel in the brain. Thankfully my life doesn't depend on the outcome of this fine work - just my ego.

Next time - populating the rest of the board, and trying to get a pulse...

No comments: