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PICList Thread
'[EE] Software power-on'
2006\04\09@194849 by Zik Saleeba

face picon face
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 circuits people would suggest?

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. 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 which would be in sleep mode. 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

2006\04\09@234636 by Zik Saleeba

picon face
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)

2006\04\10@025642 by Bob Axtell

face picon face
Zik Saleeba wrote:

{Quote hidden}

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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Note: To protect our network,
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2006\04\10@034616 by Zik Saleeba

picon face
Thanks Bob - that's a fantastic help. It's great to have advice from
someone with experience in this.

My plan wasn't to have two battery packs, but rather to tap the five
cell pack at the three cell mark for the lower voltage. The choice of
batteries was more due to the fact that I already have them rather
than anything else. Component availability is an issue for me - this
is a hobbyist project rather than intended for production. I'm also
avoiding SMT where I can since the li'l buggers are so hard to solder.

I love your switching supply suggestion - I'll definitely look into
trying an adjustable switching regulator and battery charger like
that.

You said you used a FET to switch the circuit. Sorry to appear stupid
here but did you use a p-channel FET on the positive side or an
n-channel FET switching the ground? Or something else? The reason I
ask is that I tried a design using an n-channel FET to switch the
positive side and ran into issues relating to the floating voltage
level. I wasn't too keen on switching the ground but now I think about
it that might be ok. What's the preferred way of doing this?

Cheers,
Zik

On 10/04/06, Bob Axtell <.....engineerKILLspamspam@spam@cotse.net> wrote:
{Quote hidden}

> -

2006\04\10@045816 by Bob Axtell

face picon face
Ah, I thought it might be acommercil  product.

Actually I used a p-channel that was switched by the flip flops. The
flip flops are at battery level, so you have no problems switching since
you are "right there".

The problem with the tap is that the batteries will discharge unevenly,
as a result when being charged, the batteries will quickly be damaged
by the uneven useage. You just can't win, can you? <G>

--Bob


Zik Saleeba wrote:

{Quote hidden}

Switching supplies are almost a religious experience for me. They work
SO well.

{Quote hidden}

>>-

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