5v supply problems

Hi
I´m designing a device around the atmel atmega 328p and I have one problem with a can-bus driver that has operating voltage of 5v to 7v, i have a 5v power sorce (from 12v) the problem is that the voltage drop from a transistor will bring the voltage under 5v. The reason i want to cut the power is that the can bus driver eats power when not used. The problem for me is the voltage drop and that most solutions add power draw. When the device is in sleep mode the mcu only eats a few mA but the can bus driver dont have sleep mode. I dont want to add any mechanical switch as the device will be in a very cold place.
Thank you for taking time to read this.

Can you upload a copy of your schematic or the part you're discussing?

You can use a p-ch mosfet. The mosfet will be on when it's gate is low. This can be driven directly from a microprocessor. You may want to add a pull up resistor to the microprocessor output if your microprocessor output doesn't have an internal pull up. There will be virtually no drop with any reasonable mosfet. One of the specs is the on resistance, usually in milliohms, so you can calculate the drop. You can find bunches of sample circuits for high side switches using p-ch mosfets on the internet.

I've solved problems like this, in the past, by using one of the microcontroller pins to supply power to the external device. If you have a spare pin, and if an atmega output can supply sufficient current (not familiar with the atmega line), then just before putting the microcontroller to sleep, reset the output that is supplying power to the can-bus driver, thus turning it off.

Also, make sure that you don't exceed the microcontroller's max power spec (because of driving the can-bus driver).

If the output _can't_ supply enough current (and still maintain a high enough voltage to satisfy the can-bus driver's power requirements), use Stephen's PMOS solution. If the atmega is a MOS device, you could, also, consider paralleling outputs to increase the current drive, but be sure to toggle them simultaneously.