Zik Saleeba wrote:

>I'm designing a handheld PIC-based device (a handheld GPS / nav.
>unit). It'll be using a PIC18LF2620 and will be battery powered. I'd
>like to give it a software power-on/off feature so I can use one of
>the control buttons for power on/off rather than having a big ugly
>power switch. I was wondering what kind of circuit people would
>suggest for the power on/off feature?
>
>Just to complicate the issue some parts of the circuit operate at 5V
>and others need 3.3V. The 18LF2620 can operate at down to 2.0V but
>when the circuit's in full-power mode it needs to be running at 5V to
>talk to some of its peripherals. My plan was to use a 6V Ni-MH battery
>pack with MAX883 and MAX884 regulators. These have a low dropout
>voltage and provide a shutdown mode. I was thinking I could power the
>PIC through a schottky diode off the 5V normally and then have an
>"off" mode where it's powered on ~3.6V (three Ni-MH cells) through a
>different schottky diode. The "off" mode would power only the PIC
>rather than the whole circuit. The PIC would be in sleep mode while
>the device is "off". A button press on one of the interrupt-on-change
>lines would wake the CPU and it would raise an output line to tell the
>regulators to power up again.
>
>Does this sound line a feasible plan? Is there an easier way of doing this?
>
>Thanks,
>Zik
>(Apologies if this appears multiple times - I've been having some
>problems with the mailing list)
>
>  
>
I ran into one of those power up / power down problems before. I used an 
SC70-sized (approx SOT23-6)
set of flipflops to actuate the PIC and other peripherals, switching at 
the battery level. The power to the
PIC and the other stuff is then supplied thru a switched FET.

Now, the battery system...

.. I am not trying to be critical here... the diode drops etc... but I 
think you are missing a lot about the way
batteries work. NiMH  cells are nominally 1.2V.  When you say 6V, do you 
really mean  5 cells  (5*1.2=6V)?
That's a hefty, barely manageable pack, now you are adding another pack 
just for the OFF mode?

 If it were me, I'd design the system around a pack operating with two 
Li-Ion 3.6V cells in series at 7.2V. I
say this because there are some reasonably-priced HIGH QUALITY battery 
charger chips made for two
Li-Ion cell packs.  Then you design a simple switching regulator that 
will supply 4.75V reliably with minimal losses
FROM THE 7.2V supply. That means that your power from the pack is 
efficiently converted to 4.75V, with a
further pass 3.6V regulation during ON mode. Then, in the OFF mode, you 
can simply ADJUST THE
SWITCHER to deliver 3.6V during the off mode. Look how it would work.... 
You now can get FLAWLESS
power out of a pack that can fall as flat as 2.6V per cell, yet the 
switcher will STILL deliver 5V as long as
5V is at the input. The advantages are: EXTREMELY long battery charge 
life. Dramatic  reduction in size
and weight. Charges up quickly... Li-Ion cells can be charged in a few 
minutes to a useable amount.

Now, to carry it through the last step... Linear Technology sells a 
switching battery charger chip. You can
use a wallwart or automobile as a charging source, yet the chip 
generates almost NO heat. That being
the case, you can design your product with the battery charger INSIDE 
YOUR PRODUCT. Very
convenient. The wallwart can be ANYTHING from 9V to 19V, and a cigarette 
lighter plug is inside that
range perfectly....

That entire system I just described can fit into a 1.5" square, the 
downswitcher and the downswitching
charger.

Did that help?

--Bob
 
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