Sunday, June 20, 2010

DIY DCDC Stepup with Max756

Well, after talking about ladyada’s minty boost, I wanted to check out the datasheet for the max756 chip. I wanted to see how she went about designing this circuit, and then wanted to let my readers know the process as well. I figured this would be a good chip to talk about. If you read ladyada’s post.

max756 pinout

From the datasheet, I copied this above pinout diagram of the Max-756 chip. You can see that there are 8 pins on this chip. The above pinout gives a name to each. Now this can be intimidating, but I am going to attempt to discuss each pin, and show you how you can go through this design process.

Pin 1 is a shutdown pin. This is a pin that will power down the chip. When you see a line over the name of a pin it means that it is active when grounded. So we want to give +5v to this to turn the chip on.

Pin 2 is a selector pin. This chip will either do 3.3v output, or 5v output. We want 5v output, so you put this pin low, or grounded. Once again, pay attention to the line above the name. It tells to ground this pin for 5v.

Pin 3 is a reverence voltage pin. I don’t really know the purpose of this, but we don’t need it. Just follow the command, and the schematic. We will put this to ground through a capacitor.

Pin 4 is a low battery detector pin. I don’t care about it, so we can leave it disconnected. You could feasibly use this pin to connect to an LED to show when power is low.

Pin 5 is the low battery input pin. This would be connected to the battery source. When the voltage gets lower than 1.25v, it will tell the LBO to sink low. We aren’t using this because we are using 2 batteries in series to make 3v. You would probably have to use a voltage divider to connect to this. It is not necessary.

Pin 6 is the output pin. It has to be connected like the schematic in the datasheet. This is all pretty much unchanged.

Pin 7 is ground for the circuits inside the chip. It is simply connected to ground.

Pin 8 is LX. It is connected as in the schematic as well.

The above schematic is straight out of the datasheet. The schematic below is from Ladyada. You can see that she basically followed the recommendations with a few changes. She uses the same inductor and zener diode. Ladyada left the LBI unplugged, because she is not using this feature.

Anyway, if you want to do a project, but are afraid of it, then don’t be. If you know of a device or chip that you want to use, then simply go to the datasheet. It will explain all of the pins, and give you a sample schematic. Most of the time, the schematic they give you can be used, or slightly modified. Now Ladyada actually made a circuit board, and etched it herself. You don’t have to do this. You can simply use a breadboard for testing to get your project to work. You can use a simple hole board and solder wires to make up the connection. Not as pretty, but just as functional.

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