Lpc2000

The BAT54C's that I see ($0.12 qty 100) all have horrible forward voltage 
drops (some are as bad as 1V at 50mA current!) and I'll still need to drop 
the voltage further with a resistor, which drops the efficiency even 
further.  This device can also run off of a battery for main power, and 
this circuit needs to power SRAM, so the peak will be around 50mA.

Do you know of any BAT54C's that have a much lower voltage drop?  I see 
Philips has BAT54CW which is 420mV at 50mA current, that's the best I can 
find (yes, these are schottky).

The FET is the same price but will give me 0.2V drop without needing the 
extra resistor, and because this disconnects the lithium there is no drop 
at all when on main power.  Is there any reason this wouldn't work?

-- Sean

At 00:17 1/31/2006, you wrote:
>Use a BAT54C. Inside this component you have the 2 diodes (schotky ones
>that have only 0,3V dropout) with their cathodes connected.
>
>
>
>
>Sean wrote:
> > Tom:
> >
> > Thanks for the code!
> >
> > Is it *necessary* to have the switching in place (as opposed to use
> > leaving
> > the lithium to power the RTC all the time)?  Just curious.
> >
> > Instead of using the batt->res->diode what if I used a FET instead to
> > provide switching between Vcc and Vbat?  This would also extend the
> > battery
> > life (since it's also powering external SRAM as well).  From Digikey a
> > FET
> > is the same price as a single diode, so that would actually be cheaper (?)
> >
> > -- Sean
> >
> > At 22:24 1/30/2006, you wrote:
> > >Sean wrote:
> > >
> > > >Hello everyone,
> > > >
> > > >I've got a bit of a strange problem with the RTC in my setup.  I have a
> > > >dedicated lithium battery going to the Vbat pin to power the RTC,
> > and it
> > > >usually works fine, however periodically the clock changes to an
> > invalid
> > > >value and stops running.  Usually this value is something like
> > "Year:129
> > > >Month:00 Day:16 Hour:02 Min:09 Sec:08", and unless I reset the RTC to a
> > > >valid value it stops running.  This will usually happen after
> > several hours
> > > >of Vcc absent (i.e. device powered off) or after a day or two running
> > > >constantly.  Nothing else is disrupted.  I have a 3.6V lithium running
> > > >through a diode which drops the voltage at the pin down to 3.165V
> > when Vcc
> > > >present and to 3.232V when Vcc absent (implying that more current
> > is drawn
> > > >when the micro is powered on??)
> > >
> > >Diode switching is needed to maintain the Vbat when Vdd (3.3v main) is
> > >absent and to switch over to the battery when Vdd disappears.  It sounds
> > >like you have that working?  It will take two diodes (1N4148A) and a
> > >resistor (560ohm) to do this properly.  Cathodes of both diodes go to
> > >Vbat line, anode of one diode goes to Vdd, anode of other diode is
> > >series with 560ohm to positive terminal of Lithium cell.
> > >
> > >This is half the solution, the hardware half.  I found that my clock
> > >would also "explode" on occasion and seemingly at random.  The solution
> > >I took was a bit more aggressive in the software.  Study my clock
> > >routines, especially awakenClock() and sleepClock().  Essentially, when
> > >the clock registers are not needed, they are "disconnected".