Power Supply Prototype
My parts order came in Monday, so I got a chance to test out the switching power supply.
The chip I’m using is a Linear Technologies LT1615. This is the sample schematic provided by the manufacturer:
I used the same circuit except that I use a value of 220k ohms for R2 -- this gives about 12 volts out instead of 20.
The trickiest part of this circuit design is that I’ve got to meet two competing requirements: I’ve got to be able to put out a lot of current in case the display is fully lit, and the power supply chip must not take a lot of power itself. Typically, the power supplies that are capable of outputting more current draw more power themselves.
Since I decided to use a part with a built-in FET switch (for size reasons), there aren’t many ways I can modify this basic circuit. I picked a low-loss diode, a small package-size inductor, and a low-power IC that fit my needs.
I tested the circuit and it works fine at both 1mA output (about what pong takes) and 40 mA (the display fully lit).
I still have to check out the efficiency to make sure that I’ll get the battery life I’m expecting. And, I want to try it with the battery and actual display -- just in case there are any problems. The battery might not be able to provide the instantaneous current the power supply needs, and the display might require too much peak current.
Here’s another picture of the circuit for a size comparison. The two big resistors on the left will be replaced with smaller parts -- it was just easier to use the bigger ones for the prototype.
The technique I used to build the circuit is called “Dead Bugging”. When IC’s in DIP packages were popular, people would add small circuits by gluing them upside down to a board -- the pins pointed up in to the air and the whole thing looked like a dead bug.
The leads on this chip (the black part in the middle) are much smaller, so they don’t look as buggish. The two yellow parts are capacitors, and the diode is in the top right. The largest part is the inductor, and that’s buried below and to the right of the chip; the big capacitor and the diode are also soldered to it, so it’s hidden -- look for it in the picture at the top of this page.
Wednesday, October 4, 2006