"John Fields" wrote in message
...
On Mon, 11 Jun 2007 17:57:48 -0600, "starfire"
wrote:

I'm posting the associated circuit and traces here, per request from Paul
Hovnarian. Thanks for the suggestion, Paul.

---
View in Courier:



+-------------------3.6V to MOSFETS
|
+----[1N5817]--+---3.3V to PIC
| |+
[BAT] [BFC]
| |C1
+---------------+
|
GND

The 1N5817 is a Schottky diode and will drop about 0.3V.
The BFC is there to keep the PIC supply from dropping too far and
resetting the PIC when the MOSFETS supply power to the load.

The value of the capacitance is:

IdT
C = -----
dv

Where C is the capacitance in Farads,

I is the current in Amperes,

dT is the length of time you require the capacitor to supply
current to the PIC and,

dV is the PIC supply voltage minus the lowest voltage the PIC
supply can fall to before the PIC resets.

So, making some assumptions:


IdT 5E-2A * 5E-6µs
C1 = ----- = ---------------- = 2.5E-6F = 2.5µF
dv 3.3V - 3.2V

Which means that if the battery voltage falls below 3.6V during the
load transient, a 2.5µF cap will keep the PIC supply above 3.2V for
5µs or longer if the PIC circuitry draws 50mA or less. Of course
you could use a larger cap, say 10µF or so and run farther away from
the hairy edge.


--
JF

I like it! For the surface mount application, though, I'll use a BAT54A (or
BAT54C) for the Schottky diode and probably bump the capacitance to about
10uF to 22uF. The BAT54A (series) can take up to about 200mA without
popping. SOT23-3 format, too.

Thanks! I'll try that out.

Dave