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Pulling up/Pulling down v/s directly connecting to VCC/GND

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  • Pulling up/Pulling down v/s directly connecting to VCC/GND

    The questions are related to "Learn Altium Second Edition Course" from Fedevel Academy.

    1) In the reference design, when we need to connect VCC to some pins(EN1 and EN_LDO for example), it is connected directly but when we started designing our own version of the board, we have connected it through a Pull Up Resistor. There should be some advantage of introducing a Pull Up Resistor(otherwise it will become extra components for each board). Could you please clarify why we pull up/pull down certain pins instead of connecting them directly to VCC/GND?

    2) What is the purpose of Dual N-Channel MOSFETS and LGATE/UGATE Pins in the circuit. Why can't we have a Single Channel MOSFET? Could you please explain or share some pointers to resources so that I can read more on these?

  • #2
    1) You can connect them directly. However I like to use Pull ups/downs especially for board testing and debugging. If you directly connect that pin to GND or VCC, usually that connection is very short or it may be even under the chip and for testing or debugging it may be very hard to cut that connection to control the pin if needed (also if connected directly you will not see if there is something wrong on the signals). So, it is not really a question of electrical connection, more like it is practical in some cases.

    2) You can. I have done it on number of boards where I needed higher currents. Using dual mosfet saves some space.


    • #3
      What about the current consideration when we connect those pins directly(instead of connecting it through a Pull Up/Down resistor). Will it cause extra current to flow through the input pin in the absence of a resistor?


      • #4
        Usually you can see in datasheet how much current will flow inside of an input pin. Usually it is not much.