Hello,
I was going to design a general microcontroller board using an ATmega controller and some of the usual interfaces such as RS232, RS485, LCD and keypad. I have an ordinary 7805 voltage regulator onboard, and would like to have the possibility to feed the circuit with an external stable 5V supply from a switched power supply and bypass the 7805. I would like to be able to have an overvoltage protection built in on this 5v supply using a MAX4866 or similar. These devices switch a MOSFET on/off depending on the voltage trip level that they are prefabricated with. My plan was to use either of these devices in order to switch off the power supply if something else than 5v was fed into the circuit.
But the problem is the current limiting resistor to the LCD backlight. It's only at 3.5 ohms according to the datasheet and would make the rest of the circuit see only this low impedance. Right?
So the ON resistance of the MOSFET would contribute to a voltage drop from 5V to around 4.7V with a ON resistance of 0.2V max. Calculating with lower onresistance doesn't help much, so I guess some other solution must be used in stead. I can't figure out how to solve this I'm afraid, but there's probably a better way of doing this.
Does anyone have any suggestions for changes or another solution?
I was going to design a general microcontroller board using an ATmega controller and some of the usual interfaces such as RS232, RS485, LCD and keypad. I have an ordinary 7805 voltage regulator onboard, and would like to have the possibility to feed the circuit with an external stable 5V supply from a switched power supply and bypass the 7805. I would like to be able to have an overvoltage protection built in on this 5v supply using a MAX4866 or similar. These devices switch a MOSFET on/off depending on the voltage trip level that they are prefabricated with. My plan was to use either of these devices in order to switch off the power supply if something else than 5v was fed into the circuit.
But the problem is the current limiting resistor to the LCD backlight. It's only at 3.5 ohms according to the datasheet and would make the rest of the circuit see only this low impedance. Right?
So the ON resistance of the MOSFET would contribute to a voltage drop from 5V to around 4.7V with a ON resistance of 0.2V max. Calculating with lower onresistance doesn't help much, so I guess some other solution must be used in stead. I can't figure out how to solve this I'm afraid, but there's probably a better way of doing this.
Does anyone have any suggestions for changes or another solution?