>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 <
.....engineerKILLspam
.....cotse.net> wrote:
>
>
>>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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