CMOS switch help

Discussion in 'General Electronics Chat' started by MrJojo, Jan 23, 2013.

  1. MrJojo

    Thread Starter Member

    Jan 23, 2013
    45
    0
    Hello,

    I am using a MAX4623CSE chip (http://pdf1.alldatasheet.net/datasheet-pdf/view/73426/MAXIM/MAX4623CSE.html) and having trouble understanding how it works. I have Pins 13 and 14 to ground, Pins 12 and 11 to +5VDC, Pin 1(COM1) to a power supply, and Pin 16 (NO) to a simple LED circuit (current limiting resistor then LED connect to ground all in series). From my understanding, when Pin 15 is brought high, +5VDC, it will close the switch and allow current through. Then, when it is brought to ground, it will not let current through. Finally, if there is nothing at Pin 15, it will remain open not letting current through

    The first 2 thoughts working for me, meaning when the pin is high, it allows current through and vice versa. However, when there is nothing in Pin 15, it seems to remain high. I've even tried to force it low then remove connection from Pin 15 to see if it blocks current, yet it allows current through

    Any thoughts?

    Thanks in advance,
    Matt
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Floating inputs on CMOS is a no-no. The resulting output will be unpredictable. An input needs to always be connected to either logic 0 or logic 1, even if it is not being used.

    What is the voltage on pin 1?
     
  3. MrJojo

    Thread Starter Member

    Jan 23, 2013
    45
    0
    The Voltage on Pin 1 is 5 VDC. I have a constant voltage source on pins 1, 11, and 12.

    Long story short, when I power my actual circuit (same set up, but with 4 high power LEDs and a temp sensor) there will be times when the LEDs don't light up. After reviewing the users manual for the MAX4623, it gives a specific order to apply voltages. I am trying to determine if just applying power through the circuit at once causes the error or if I designed my logic incorrectly.

    Matt
     
    Last edited: Jan 23, 2013
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I'n not sure what your problem is, but there are cheaper and more efficient (lower resistance) ways of switching LEDs, if that's all you are trying to do.
    I would use a logic-level p-channel MOSFET. You can get them with a few tens of milliohms of ON resistance (Rds(on)), as opposed to 5 ohms for the MAX4623.

    EDIT: You can also use a logic level n-channel MOSFET if you can switch the low side of the LEDs. N-channels are more common.
     
    Last edited: Jan 23, 2013
Loading...