Lpc2000

I don't understand the idea of VBAT in LPC2138. 

I want to have RTC running in case of main power supply failure. 

I didn't find any information about a battery backup switch in LPC. So
if I connect a battery to VBAT, RTC is powered from the battery all the
time. I'm afraid that the life of my battery will not be long. 

I can add an external battery switch (one of ADM.. or MAX..) but
Li-Ion batteries are 3.0V or 3.6V. In case of a 3.0V battery the
voltage drop on the switch causes that VBAT is outside of
specification. In case of a 3.6V battery the switch doesn't work
(battery voltage is higher then main power voltage!). 

Have anybody solved this problem?

Darek

--- In lpc2000@yahoogroups.com, "drybak2000" <d.rybak@j...> wrote:
> 
> I don't understand the idea of VBAT in LPC2138. 
> 
> I want to have RTC running in case of main power supply failure. 
> 
> I didn't find any information about a battery backup switch in 
LPC. So
> if I connect a battery to VBAT, RTC is powered from the battery 
all the
> time. I'm afraid that the life of my battery will not be long. 
> 
> I can add an external battery switch (one of ADM.. or MAX..) but
> Li-Ion batteries are 3.0V or 3.6V. In case of a 3.0V battery the
> voltage drop on the switch causes that VBAT is outside of
> specification. In case of a 3.6V battery the switch doesn't work
> (battery voltage is higher then main power voltage!). 
> 
> Have anybody solved this problem?
> 
> Darek

Use a simple 1.2V battery with a low quiescent current 3.3V step-up 
from Maxim or other..?

OK that's fine, but it doesn't solve the problem - RTC will be powered
from battery all the time (even if main power supply is OK). I don't
think that a small battery will last long (I can't find any
information about current on VBAT).

Darek

--- In lpc2000@yahoogroups.com, "valdef78" <valdef@s...> wrote:

> 
> --- In lpc2000@yahoogroups.com, "drybak2000" <d.rybak@j...> wrote:
> > 
> > I don't understand the idea of VBAT in LPC2138. 
> > 
> > I want to have RTC running in case of main power supply failure. 
> > 
> > I didn't find any information about a battery backup switch in 
> LPC. So
> > if I connect a battery to VBAT, RTC is powered from the battery 
> all the
> > time. I'm afraid that the life of my battery will not be long. 
> > 
> > I can add an external battery switch (one of ADM.. or MAX..) but
> > Li-Ion batteries are 3.0V or 3.6V. In case of a 3.0V battery the
> > voltage drop on the switch causes that VBAT is outside of
> > specification. In case of a 3.6V battery the switch doesn't work
> > (battery voltage is higher then main power voltage!). 
> > 
> > Have anybody solved this problem?
> > 
> > Darek
> 
> Use a simple 1.2V battery with a low quiescent current 3.3V step-up 
> from Maxim or other..?

At 01:00 PM 4/20/05 +0000, drybak2000 wrote:
>I want to have RTC running in case of main power supply failure.
>
>I didn't find any information about a battery backup switch in LPC. So
>if I connect a battery to VBAT, RTC is powered from the battery all the
>time. I'm afraid that the life of my battery will not be long.
>
>I can add an external battery switch (one of ADM.. or MAX..) but
>Li-Ion batteries are 3.0V or 3.6V. In case of a 3.0V battery the
>voltage drop on the switch causes that VBAT is outside of
>specification. In case of a 3.6V battery the switch doesn't work
>(battery voltage is higher then main power voltage!).
>
>Have anybody solved this problem?

Have you considered a supercap?  A couple of Farads may be all you need 
depending on how long a power outage you want to withstand.  A simple 
resistor diode combination will then keep it charged when the power is on.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/

--- In lpc2000@yahoogroups.com, "drybak2000" <d.rybak@j...> wrote:
> 
> OK that's fine, but it doesn't solve the problem - RTC will be 
powered
> from battery all the time (even if main power supply is OK). I don't
> think that a small battery will last long (I can't find any
> information about current on VBAT).
> 
> Darek
> 

you could charge the battery when in normal power mode..
but the supercap answer from Robert could also be a good solution. we 
had to know the Vbat current to see what we can do. maybe someone with 
a dev board could have a look at it..?

Robert,
I've found in the older posts that the current on VBAT is 15-20uA
(very, very much! Standalone RTCs consumes 250-500nA in broad range of
voltage). 
OK, I estimated the life time of the RTC using 1F cap. It is about 5
hours only. And it should be let's say a couple of weeks! Such long
breaks are normal in my application. My customers wouldn't be happy if
I make them to set the time after any power-up.

I can see that the best solution is using the external RTC chip. 
Darek


--- In lpc2000@yahoogroups.com, Robert Adsett <subscriptions@a...>
wrote:
> 
> Have you considered a supercap?  A couple of Farads may be all you
need 
> depending on how long a power outage you want to withstand.  A
simple 
> resistor diode combination will then keep it charged when the power
is on.
> 
> Robert
> 
> " 'Freedom' has no meaning of itself.  There are always
restrictions,   be 
> they legal, genetic, or physical.  If you don't believe me, try to
chew a 
> radio signal. "  -- Kelvin Throop, III
> http://www.aeolusdevelopment.com/