AVR P-28 supply voltage

Development Boards MicroMod
1

Is the P-28 supposed to be able to run on a 5V supply?

When I give it 5V, the input of the regulator is 4.5V, the output is at 3V (the 3.3V jumper is not installed), the voltage across Vcc and gnd on my AVR is 2.34V, and the LED will not turn on. I have to use a supply voltage of 8V before the LED will turn on, with 5.7V on the output of the regulator (shouldn’t the output be 5V?) and 5V across Vcc and gnd.

For some reason, my adjustable wall wart skips past the 7V range, so I can’t tell what happens between ~6.5 and 8V, but at 6V I still can’t turn on the LED or get 5V across Vcc and gnd.

However, with 5V supply, I can still write to my AVR.

Do you typically have to use a larger supply voltage to compensate for the drops across the diodes and small resistances?

2

most regulators need 3 volts or so of headroom. Add 0.7 if there’s a single protection diode on the DC input jack.

Low-dropout regulators need about 1 volt of headroom.

Try a 9V battery to convince yourself.