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I have a thermostat that outputs a DC power supply for a communication daughter board. When I read it with the voltmeter, it reads ~3.9v, but the zigbee module that I want to use requires 3.3v regulated.

Since its a tight squeeze inside the thermostat, I have to stick with SMD components. I looked at the the MCP1703 in the SOT-223-3 to do the job, but in the example schematic, the input voltage is 9v so I'm hesitant to use the 1uF input and output filter caps that they used.

Do I need an oscilloscope in order to do this right? Can I just use the example caps even though my input voltage is ~3.9v rather than 9v? How "clean" does my output voltage need to be for a typical zigbee module?

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The MCP1703 requires 0.625V overhead to regulate a 3.3V supply when taking 250mA. This means it needs 3.925V for it to work correctly. However if you are only taking 25mA the drop-out voltage is more like 0.1V and this means you will be OK: -

enter image description here

There are more detailed graphs of other output voltages and their respective drop-out voltages but this one is closest to +3.3V

It'll work fine with the capacitors you specified - you just need to check what current your zigbee circuit might take under worst-case conditions. If in doubt make the regulator's output capacitance 10uF or larger but i don't think you'll need larger than 10uF

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  • \$\begingroup\$ Thanks for the exact answer I was looking for! The worst case conditions for the zigbee module that I'm using is 150 mA when transmitting. Is 1uF for the regulator's output capacitance still good given this current? \$\endgroup\$ – Justin Manuel Apr 9 '13 at 19:05
  • \$\begingroup\$ Probably but it's one of those close calls that is best either using overkill and a 10uF or playing it by ear when you have it built \$\endgroup\$ – Andy aka Apr 9 '13 at 19:59

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