Power Supply Design for AVR Microcontroller

The most commonly used voltages for AVR microcontrollers are 5V and 3.3V. This circuit uses a switch to switch between the two voltages and indicates them with a two-color diode (green light for 5V and red light for 3.3V).

Diode D1 prevents users from plugging in the wrong polarity of the power supply. D2 allows users to reverse the voltage into this circuit without damaging the 1117-ADJ.

The characteristic of 1117-ADJ is that there will be a 50uA current output at pin 1 and a 1.25V voltage at pins 1-2. Using this characteristic, the output voltage can be calculated:

When the SW switch is turned to the left, the current on R6 is 1.25/0.33 = 3.78ma. The current on R8 is 1117-ADJ 1 pin current plus the current on R6, that is, 0.05+3.78=3.83ma. It can be calculated that the voltage on R8 is 3.84V. So VCC=1.25+3.83=5.08V. The error is within 2%.

When the SW switch is turned to the right, the current on R6 is 1.25/0.62 = 2.02ma. The current on R8 is 1117-ADJ 1 pin current plus the current on R6, that is, 0.05+2.02=2.07ma. It can be calculated that the voltage on R8 is 2.07V. So VCC=1.25+2.07=3.32V. The error is within 1%.

Using 1% precision resistors, the overall output voltage error can be controlled within 3%.

Important note: In actual application, depending on the brand of 1117-ADJ used, the input voltage can be as low as 7V or even lower. (You can also use a low voltage drop diode instead of 1N4007).