Parts has arrived yesterday and I started test-building my USB Host Shield prototype PCB to check functionality and buildability. Since design is not final, I’m not publishing Eagle files yet – they will be online as soon as I get all errors fixed. In the meantime, the PDF of the schematic diagram is available.
One of the goals of this design is to make it compatible with both 5V and 3.3V Arduinos. Since MAX3421E is 3.3V part, I added some extra circuitry to provide 5V compatibility.
I started my testing by building 5V to 3.3V converter piece to check parts values and circuit layout. The circuit is located in the upper-left corner of the schematic and in the lower-right corner of the board. Please also take a look at the title picture if you haven’t already to see the layout. This converter comes in handy if, for example, you are using 5V Arduino and/or need to provide Vbus power for bus-powered peripheral and want to use this supply to power the shield/Arduino also. Converter is rated for 600ma, enough to power both MAX3421E and Arduino, if necessary.
I am big fan of Linear Technology products so I designed this converter around LTC3406. The first build uses LTC3406AB-2 variant with switching frequency of 2.25MHz. LTC3406 comes in A, B, AB, and B-2 variants, I’m going to test them all to see if they work in this circuit and post the results.
Testing reveals that converter works well with very little noise and ripple – ceramic capacitors make a whole lot of difference here. I loaded 3.3V output with 5ohm resistor giving 660ma load current. The power supply ammeter shows 480ma at 5V, which gives slightly more than 90% efficiency. I don’t put much trust to my power supply instruments so more realistic efficiency range would be 85-95%. Not bad at all, plus the circuit runs cold. When loaded to 660ma (which is a bit more than rated 600 ), the inductor is slightly warm to the touch, around 40 degrees Celsius. The rest of the circuit is at room temperature. In addition to all that, circuit footprint is smaller than that of linear regulator in TO-220 package.
Now, the word of caution. The maximum specified input voltage to LTC3406 is 5.5V. It means that 5V power supply has to be stabilized somehow – wall warts won’t work here. If you are going to use the shield with 5V Arduino, take stabilized 5V from Arduino. If you want to power your circuit from Vbus power supply, by spec this supply has to be stabilized anyway.
The final conclusion of this test is that the converter works quite nicely; the only change necessary is position of output contact – it sits too close to the inductor, so I moved it up and further left to be close to board’s 3.3V contact point.
The next step is to make this board working with 5V Arduino, which will arrive next week. I will be testing 5V to 3.3V converter on real circuit as well as level shifters and MAX3421E layout.