Lpc2000

I am wondering what voltage is considered low on VBAT pin. I found 
information, that VCC below 2.6V should be avoided. But I haven't found 
anything about VBAT limits.


I am wondering because on OLIMEX dev board, there is diode in series 
with batterry. Batteries on the market are available only as 3V, so the 
voltage I can count on should be 2.5V.

robert

rseku wrote:

>I am wondering what voltage is considered low on VBAT pin. I found 
>information, that VCC below 2.6V should be avoided. But I haven't found 
>anything about VBAT limits.
>
>
>I am wondering because on OLIMEX dev board, there is diode in series 
>with batterry. Batteries on the market are available only as 3V, so the 
>voltage I can count on should be 2.5V.
>
>  
>
At low current levels, a silicon diode would only give approximately 
0.1v of drop.  The diode would be needed to isolate the battery from the 
main supply in the case of a power failure.  The main suppy under a 
power failure / off condition would otherwise cause the battery to 
attempt to power the other electronics of the board.  There should also 
be a series resistor connected between the main supply and the battery, 
this is a charging current limiting resistor.

You would want to use a lithium cell for your battery, they exhibit a 
nearly flat voltage discharge curve.  A lithium cell will discharge at a 
fairly flat voltage output until it is nearly exhausted, then the 
voltage drops very quickly.


TomW


>robert
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-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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