Hello!

I'm working on a project that basically includes an avr microcontroller
and a few ics around it : including a digital variable resistor e.t.c.

Some of my peripherial ICs have a shutdown option which puts them into a low-power mode, some don't. The ICs that do, sometimes still consume too much current for my taste.

Just to give you an idea : the avr mcu i am using consumes 400 nAmps in deep sleep mode, so when I have a digital pot that consumes 0.5 mA in low power mode, you can see how that can become the bottleneck.

I've been trying to use power-mosfets to control the power supply to those ics, but not sure if i am reinventing the wheel here: some of the ICs have multiple ground/vcc connections, each one is connected using a capacitor to the ground.

My question is: what would be the best practice in controlling the power supply from an mcu to a peripherial IC using a transistor: do i hook a p-channel at the vcc end, or a n-channel at the ground end, or perhaps is it something else completely.

Would appreciate your feedback.

 

Either approach is doable, but you need to be sure that the impact on the part that you are removing power from is acceptable. Parts that have memory may lose it. The behavior at the I/O pins may not be acceptable given what they are hooked up to. The parts may have unacceptable behavior as they are powered back up until they settle.

Lots of things to consider.

 

I've seen issues with parts when you disconect ground. I would stick with using a P-channel and disconnecting power.

Be careful as WBahn said. If you have an interconnected set of parts, some powered and some not powered, you're asking for trouble. SOme parts will actually partially "power up" through powered I/O pins when then have no VCC.

If you really need to do this, I would consider something like this for your lowest power condition...

• Use separate power rails for your micro and the rest of the board.
• In sleep state, power off the entire board (if possible in your application) and leave only the mico running.
• Set all the micro outputs to "safe" states.
• Put the micro to sleep.

 

thanks jeNNic, hurold, I think i will proceed in the direction jeNNic has outlined.
I just need to find a power mosfet that doesn't bleed too much current , i think
that the higher they go in power rating the higher their Id bleeding becomes.

 

Once when making a source switch out of 2 transistors, I noticed I could buy a complete low-voltage dropout regulator with an enable input for less then the 2 transistors.

So if you need a linear regulator anyway you may want to consider breaking it up into 2 or more units, one for "always on" and others for microcontroller switching.