dc/dc converterpower supply
I would like to build simple and small power supply for my projects with microcontrollers. This supply should be powered by two Ni-MH batteries and output should be 5V (maximum current 500mA).
I know there are plenty of them on the Intenet (MintyBoost from Adafruit and cheap ones from China on eBay) but I want to build one by myself.
The LTC3400 looks very good but not cheap. Do you know any other step-up converter (Vin 1.8V and less, output current 500mA and more)? By cheap, I mean circa $1.
For current to flow backwards, against the voltage generated by the DC/DC converter, you need some other energy source. If you have no such energy sources, then it can't happen. Possible sources:
If you have any connectors at all, it's good practice to assume someone will connect them to something bad at least once. Putting diodes on them to clamp the maximum voltages seen here to the supply rails is pretty common, but then if these diodes start shunting excess voltage into the supply rails, where does it go?
simulate this circuit – Schematic created using CircuitLab
It's even common for CMOS ICs to have diodes like this internally on all their inputs and outputs for ESD protection. The microcontroller in your Arduino certainly has them. Are any of those pins accessible from external connectors that might be abused?
When excess voltage is encountered on these, it's dumped into the supply rails. If that current has nowhere to go, the supply voltage increases and things go boom.
The robust solution is some over-voltage protection, usually in the form of a crowbar circuit. Adding a diode to isolate the DC/DC converter from everything else will just protect the converter, leaving the overvoltage fault to destroy everything else.
The datasheet does say 300 mA, but only when Vin > 2.4V. Take a look at the efficiency vs. output current curves for Vo = 3.3V. You will see different curves for Vin = 2.4V, 1.8V, and 0.9V. Note that the 1.8V efficiency curve stops at 130 mA (at 78%). In other words, if you have only 1.5V, forget about outputting 130 mA. You need more input voltage or a beefier converter.
I'm not sure why your battery had 150 mA going through it, but it sounds like your boost converter didn't have the capacity to operate at the load you were driving it at. And why are you trying loads as low as 47 ohms? Most circuits are designed with high Rin so as not to load the previous stage. I suggest figuring out the Rin of the circuit you are connecting this boost converter to and inserting a buffer if it's too low.
I recommend watching this EEVblog video where Dave goes through the process of selecting a DC-DC boost converter. Dave goes through what factors you need to consider when selecting a boost converter and how you should read a converter datasheet.
Best Answer
Olin's right about Microchip doing some nice switching regulators - I use the MCP1603 parts quite regularly. However these are buck regulators and don't have a 5V fixed option anyway. I just had a quick look on Farnell and didn't notice any MC parts that fit for this requirement. It may be worth a look on their website though.
The cheapest 5V boost regulator I am aware of that can work down below 1.8V at 500mA is the NCP1450 from ON Semi. It's £0.63 in qty 1, can source up to 1A, and can startup with low as 0.8V with no load. It comes in a nice and small 5-pin SOT-25 type package.
I haven't used it, but I saved it whilst searching for another regulator as it looked like a nice part. ON Semi make a lot of decent cheap parts in this category.