battery operation

Question about power
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Joined: Fri Oct 19, 2012 5:11 am

battery operation

Post by YahooArchive »

>from the help line
>I emailed a few days ago, concerned about power consumption. The super pro
board consumes 150 mw, or 30 milliamps at 5 volts. But the board has an external
32KHZ crystal on board, as well as an internal high frequency crystal. Can I
switch to a low power mode by using the 32KH crystal as the master, until some
processing is required, and then switch to the high frequency crystal?
>I am hoping to run on 2 AA batteries, with a little switching supply to develop
5 volts.

Rather than trying to use the 32KHz crystal it's probably better to go into one
of the sleep modes and then wake up using an interrupt. Details on that in the
NXP User Manual.

We originally designed the board with the idea of using the 32 KHz oscillator as
the main clock source with the PLL multiplying it up to 100 MHz. But the 32 KHz
is slow to start up and we never switched the design over to that (it can take a
few seconds).

As for power from batteries, the LPC17xx can run off as little as 2.4V. So you
could run directly off the batteries. No real point in using a switcher to
generate 5V just to have 2 linear regulators drop it down to 5V then 3.3V. The
issue would be if you want to use the A/Ds you want a regulated reference. When
operating like this, remove the 5V regulator so it doesn't load the batteries,
and replace the LP2985-3.3 with an LP2985-2.4. We will do these modifications
for bulk orders of boards.

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