how to keep a MOSFET off during powerup?

Discussion in 'The Projects Forum' started by badbaud, Oct 28, 2013.

  1. badbaud

    Thread Starter Member

    Mar 5, 2009
    26
    0
    Using a 89C4051 MCU with a DS1833-10 reset chip. The chip puts out a 350 ms positive going reset pulse at power up.

    After power up a N-Channel, 5V gate MOSFET is turned off by the MCU.

    Before the reset pulse has finished the MOSFET turns on briefly.

    How do I keep the MOSFET turned off during power up long enough for the MCU to take over?

    The MOSFET gate circuit is described as a 10 Ohm resistor tied to the gate, on the other side of the 10 Ohm there is a 10K resistor tied to ground and a 1K Ohm resistor tied to Vcc. This 3 resistor junction connects to a port pin on the MCU.

    The MCU port pin can sink more current then it can source so I am using this feature to turn the MOSFET off and the 1K pullup resistor as the means of turning the MOSFET on.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,233
    You could get rid of the 1k. ;)

    The fact that it turns on must mean the micro is tristated when in reset. If that's the case you could add a little NPN driven by the reset pulse to hold down the gate voltage.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    With a 1K and a 10K resistor you are driving the gate with 91% of Vcc during power up. No wonder it glitches. Loose that 1K as rony correctly suggests.

    BTW, just how much current is the gate of this MOSFET taking anyway?
     
  4. badbaud

    Thread Starter Member

    Mar 5, 2009
    26
    0
    Attached is the data sheet for the MOSFET.
     
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