Tuesday, April 6, 2010

Crossing Power Domains to Reduce Sleep Current. Why didn't you hook up the A/D power pin?

Them: "I can't get my sleep current below 50 uA."

Me, while looking at their schematic: "Why is there no connection to the AVcc pin on the micro?"

"I was not using the A/D, so I did not hook it up, to save power. I have it turned off in my software."

"What does that do to the part internally? Doesn't that make a lot of odd impedances and sneak-paths? The data sheet says AVcc must be with 0.3V of Vcc."

"I don't know. [Shrugs.]"

A board revision later...

Them: "To get the sleep current down to nothing, I added this NAND Set/Reset Flip-Flop driving the regulator's Enable input, but it is not working."

Me: "What does 'not working' mean? [Don't you just hate it when people are vague with problems?]"

Them: "Regulator shuts down, but then the system boots up again right away."

Me, while looking at their schematic: "You have the Set input of your Flip-Flop hooked up to one of the Micro's pins?"

"I have that pin programmed as an input. I use the button to change the menu after the unit turns on."

"You are turning the power to the Micro on when you drive that pin low."

"Yes, that is what I want it to do."

"What happens to the impedance of your input pin on the Micro when you remove the Micro power?"

"I don't know. [Shrugs.]"

The high impedance input is no longer high when power is removed.

Moral of the stories: Always hook up all of the ground and power pins, as the data sheet tells you to hook them up. Even if you are not using a function block of the part. Don't create feed back paths from unpowered pins. It is impossible to know what kind of impedance an unpowered circuit is going to present to the powered section of the circuit.

The other thing to be on the look out is feeding power to an I/O pin on a device that has no power, or feeding a voltage that is higher than the Vcc voltage. At best this wastes power, at worse parts are destroyed.

Analog Devices has a related application note that you should read if you are interested in saving power by turning things off: Protecting Off-Amps by John Ardizzoni.