>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Jinx
>Sent: 17 January 2007 03:17
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] B+ disconnect
>
>
>> What kind of current drain are you expecting from the 12v 
>SLA battery?  
>> P channels MOSFETs can be had with pretty reasonable specs 
>these days, 
>> and it makes things a simpler if you can avoid voltage doublers
>
>About 25W user power from the switchers, which are ~85% 
>efficient in theory -> 30W (2.2A) input @ 13.5V. The 
>IRF5800TR, a HEXFET/MOSFET, from RS caught my eye, which has a 
>4A rating and is very cheap too, just NZ$0.68 (price not a 
>major consideration, although this is for a prototype and has 
>to be "a" consideration), several 4A - 6A under NZ$3 and 10A 
>for a $-ish more
>
>http://www.datasheetcatalog.com/datasheets_pdf/I/R/F/5/IRF5800.shtml

The IRF5800TR looks a reasonable choice.  It needs a high Vgs to switch it on hard at higher currents, but you have the voltage available.  The gate can also survive up to 20v.  From the datasheet it looks like 12 volts on the gate will give you a bit less than 0.15v drop at 2.2Amps (@25C), which is about 0.7watts.

Note that you can parallel MOSFETs used in slow switching applications like this without the problems of current hogging you get from bipolars, so two cheapies may be a better buy than one posh device...

Regards

Mike

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