Schematic Check Please [High Side PMOS Switch]

Discussion in 'The Projects Forum' started by skybox, Jan 27, 2011.

  1. skybox

    Thread Starter Active Member

    Mar 2, 2009
    68
    1
    Hi guys,

    Attached is a schematic of a high side PMOS switch. The base of the transistor will actually be controlled by a microprocessor. The BJT is used to provide enough gate voltage to keep the PMOS off. When 3.3V is applied to the base, the BJT should send the gate signal to ground, turning on the fet.

    Attached is the schematic.

    PMOS datasheet:
    http://www.fairchildsemi.com/ds/FD/FDS6681Z.pdf

    BJT datasheet:
    http://www.nxp.com/documents/data_sheet/MMBT3904.pdf

    Any suggestions would be greatly appreciated. I am more of software guy.

    Thanks!
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi,

    Turn on time will be fast, but turn of time will be much slower (~3.8mS)since only the 100K is discharging the gate charge.

    How fast do you need it to be?

    Regards,
    Ifixit
     
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  3. skybox

    Thread Starter Active Member

    Mar 2, 2009
    68
    1
    Thank you for the reply. Doesn't need to be too fast. This is for a simple push button design where if the user pushes the button for 2 seconds, the microprocessor will send a high and turn the switch on.

    Should I add a 10k resistor to ground on the base side to make sure base it at ground when PIC is outputting a 0?
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Drive the base through the 10K resistor to limit the current in to the base. When the signal is low (0V) then the MOSFET will be off, when High (3.3V) the MOSFET will be on. Put another 10K from the base to ground to keep the 3904 off when the MPU output is tri-stated, such as during a reset or whatever else may cause a tri-state condition.

    Have fun,
    Ifixit
     
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  5. skybox

    Thread Starter Active Member

    Mar 2, 2009
    68
    1
    Thank you! Makes sense!
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    With the suggestions above, you shouldn't have problems, though I'd suggest bread boarding it first (without the uC) to make sure it does exactly what you want, essentially a "soft turn on" with very short delay?
     
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