Lpc2000

From the Philips Datasheet

Price : 0,08 / 100 pcs @ Digikey
240mV @ 0,1mA (that's far more than the RTC uses)

With do you need this resistor ? And even if you need it, let's see:
Using a 1K resistor, and a RTC current of 20uA gives you a dropout of  
20mV ....

I use this a lot and I'm sure it works very well.

But I can't see how do you want to use a fet in this application.

Mauricio




Sean wrote:
> 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".
>
>
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