Re: Rechargeable 3v battery details for microcontroller (Can't lose data!)

From: mike (spamme0_at_netscape.net)
Date: 01/17/05 

Date: Mon, 17 Jan 2005 04:00:31 -0800

Kevin wrote:
> Hi Group,
>
> Let me pick your collective brains to see if I'm on the right track
> here:
>
> The situation:
> I'm designing a microcontroller-based data logger (don't yawn...yet)
> that will interface to a piece of equipment and keep track of when
> certain events occur. These details are kept in microcontroller ram
> and must be maintained even through power loss / daily shutdown
> without corruption or loss of data.
>
> Microcontroller is the ever popular PIC16LF628A (Nanowatt/low voltage)
> using the 4 MHz Internal RC oscillator. I also plan to use a 32.768
> KHz watch crystal to drive Timer1 so that the PIC can periodically
> wake and check if power has returned and resume normal operation. I
> need to preserve data in the PIC's ram during a power failure but due
> to reliability concerns I think powering the PIC from battery during
> loss of power is the safest solution versus using EEPROM etc.
>
> I plan to power the PIC16LF628A using power from both the machine
> (tapping into 12VDC power and regulating that to 3.3V for
> power/charging) as well as a rechargeable lithium 3V battery (see
> crude ASCII schematic below):
>
> 1N5817
> IN4001 3.3V Reg Schottky Diode
> 12V *--->|---+----[LM2950-3.3]--->|-----------+------+----- To PIC VDD
> from | | | |
> equipment | | | |
> | | \ | +
> | | 390 / |
> ==== 220uF | Ohms \ ==== 33uF
> 16V | / 6V
> ==== | | ==== Electrolytic
> | | Panasonic ----- |
> | | ML1220 battery --- |
> | | 17 mAH 3V | |
> | | | |
> GND *--------+---------+----------------------+------+
> from
> equipment
>
> 1. Any recommendation for the Schottky diode? (through hole package -
> no SMD parts). I'm thinking of something with a low Vf (so far the
> best I've come up with is a 1N5817 - approx 0.4V) but are there any
> other concerns I should be aware of?
> The battery spec *** mentions the charging voltage range should be
> 2.8V to 3.2V to achieve the rated mAH capacity. (I am pretty close to
> the low side of this range as 3.3 - 0.4 = 2.8V). If I could use a
> diode with a lower Vf it would help raise the charging voltage.
>
> 2. Although it's not on the schematic above I plan to detect whether
> 12V is present via an 4n25 optoisolator & zener diode ("power fail
> signal") to determine whether power is failing/failed (and if so go to
> sleep). As stated above the PIC will periodically wake from sleep and
> check if power has returned (so it can continue monitoring). This
> sounds reasonable, right?
>
> 3. Should I be concerned about leakage from the 1N5817 diode when 12V
> is absent and the PIC is on battery power? If this leakage is a
> problem then what other charging circuit design should I use? I'd like
> to keep things simple and use only fairly common inexpensive parts
> (nothing that you couldn't get from Digikey). Similarly, would leakage
> from the 33uF cap pose a problem?
>
> 4. Another thing I'm concerned about is battery life regarding
> charge/discharge cycles. The battery specs list these as 1000 cycles.
> Now the application shouldn't even remotely come close to draining the
> battery in daily usage but I'm just wondering about this. (I
> understand that the data*** is probably on the conservative side).
> An alternative might be one of those large close-to-a-farad supercaps
> but again I don't know how long the PIC will run off of one of these.
>
> 5. Because the PIC Vdd voltage is 3.3v input PIC pins will no longer
> be to TTL specs in terms of voltage levels, correct?
>
> 6. Finally, is there anything I might have missed with regards to low
> power design? (Already checked PIClist.com & read Microchip's "Power
> Managed Tips N Tricks" app note (41200B).
>
> Thanks for your assistance.
>
> Kevin.

I'm too lazy to look up the spec. Don't all recent PICs have flash memory?
Does the thing have to log while the power is off? If not, might think
about using a super-cap.

Can you use a fet in place of the diode and run the gate off 12V?
mike 

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