power consumption

championx · Joined: 28 Feb 2006 Posts: 151

Hi! I'm working on a battery powered device with 18F252. Now I want to try if I can get a minimum current consumption.

The circuit is a simple pic circuit:

LCD 16x2
i2c eeprom
i2c rtc
LDO 5v regulator
20mhz xtal

Now, the circuit drops 8mA... but I'm sure that I can get less...

any suggestions?

(sorry for my english)

The other question is: When I use the SLEEP instruction, the circuits drops around 1mA... A LOT! What do I need to do on the program to get a few uA??

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

If you stop the crystal oscillator in sleep, the uP current consumption drops down to uA, rtc and eeprom shouldn't consume considerable current, if operated correctly.

The LDO current is strongly type dependant, the LCD display probably won't go below 0.5 or even 1 mA. You have to turn it off or use a low power display.

Guest · Posted: Wed Oct 29, 2008 11:21 am

Thanks for your answer! How do I turn off the LCD?

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

The simplest is to connect the LCD power supply to some port on the PIC, the LCD is on when you drive the port high, and off when you drive it low.
If your LCD consumes below 1-2 mA the PIC's port is well enough to supply it. Be sure not to drive high any data line of the LCD when it is turned off.

dbotkin · Joined: 08 Sep 2003 Posts: 197 Location: Omaha NE USA

One thing to think about is reducing the *average* current, even if you can't reduce the max current. For example, if you just need to update the LCD once per second, the PIC can be in SLEEP mode most of the time. The RTC can probably generate a 1 second pulse to wake up the PIC, spend a few milliseconds active, then go back to sleep.

Also look at the quiescent current of your LDO. If you're concerned about current consumption, it seems you're likely running on battery power. If you're on battery power, why not select a battery voltage appropriate for the PIC and other parts? 3 alkalines will give you roughly 4.5V, 3 NiMH will give you between 3.5 & 4V, and so on. Save the regulator, save current and money.

I have one product that uses a 12F683. It runs on a 3V 220 mAh CR2032 lithium coin cell. Operating current is 1-3 mA, sleep current is low enough that there is no power switch. As near as I could measure, it looks like about 4 nA in SLEEP - I had to measure voltage drop across a resistor to figure it out. The battery will last a few years while the thing is sleeping.