Hello,
I am looking at this schematic for the SD/MMC card connector and wanted to find out why the transistor was necessary to drive the mosfet?
There are some low voltage mosfets that seem like they would allow you to go directly from the pin to the mosfet. Wouldn’t something like this
http://www.fairchildsemi.com/ds/FD/FDG313N.pdf
work in its place?
It would help if you provided a link to the schematic you are looking at.
Oops must of been an off day. Here is one example schematic:
http://freeweb.siol.net/mpavlin7/lpc2400/mem.pdf
note that whoever drew that one used a nmos diagram image when it should be a pmos.
Since I see that one of the chips can run off of 1.8V I think the BJT is there to provide sufficient drive to the FET if 1.8V is used.