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CD14538B: looking for a sample schematic

Michael Johnston90
Prodigy 60 points
Part Number: CD14538B
Other Parts Discussed in Thread: CSD13381F4, , SN74LVC1G123

I've been having a hard time getting this one shot pulse to function the way I would like it to. Any chance there is a sample schematic that I could look at to help me get the logic down. I need to figure out what I'm pinning out wrong. Thanks!!!

  • 0
    TI__Guru* 96957 points

    Hi Michael,

    The first page of the datasheet gives this info:

    You can only use one of the input triggers per channel - so either the +TR or the -TR should be terminated as explained above.

    Other than that, this is the complete schematic:

    I could draw it up with +TR tied to Vss and one with a connected to the signal source, but I don't think that would be much help for you.

    Can you share your schematic?  What are you trying to accomplish?  What trouble are you having?

  • 0 in reply to Emrys Maier
    Prodigy 60 points
    Let me read the data sheet and take a picture of this portion of the schematic with labeling here in a little while. Thanks for the quick reply!! By the way I'm trying to open a valve for .2 seconds. I'm having trouble with what to connect to TR+/-, Q and Qnot. I feel like I just don't have a handle on the logic, let me get there.
  • 0 in reply to Michael Johnston90
    TI__Guru* 96957 points
    What kind of voltage/current is required to drive your valve? I have a feeling you might need an external driver as well.

    The TR+ and TR- are positive edge trigger and negative edge trigger inputs (respectively). They take a signal in, and when the appropriate edge is 'seen' at the input, the device produces an output pulse (determined by the RC values). The Q output will go HIGH for that amount of time (it stays normally LOW), and the Q\ output will go LOW for that amount of time (and it stays normally HIGH).

    Does this help at all?
  • 0 in reply to Emrys Maier
    Prodigy 60 points

    It's 12V/22W so around 1.8A draw. I'm going to try the CSD13381F4 Mosfet its rated for 2.1A and 12V. Hopefully it will work out. Do you think that will work as a driver? I'd like to find a N-channel mosfet thats through hole instead of SMT. Thanks for your advice!

  • 0 in reply to Michael Johnston90
    TI__Guru* 96957 points
    I'd probably go with a FET rated to a higher voltage - since BV_DSS(min) is exactly 12V, the FemtoFET might fail in your application. I'm certainly no expert on those devices though. If you want to be sure, you might make a new post with that part number to get the TI FemtoFET apps engineers to comment. Like most of our devices, the datasheet is probably a bit on the conservative side to protect TI.

    As for driving the gate of the device with CD14538B, I would probably recommend a stronger driver. The outputs of this device are fairly weak - rated at 13.5V/6.8 mA of current for 15V typical operation. That makes for output drivers with about 220 ohm drive strength. With a 200pF gate capacitance on the FET, that will produce a relatively slow edge and with 2A of current could cause thermal damage to the device. You generally want a very fast turn-on when driving a power MOSFET.

    If it were me, I'd use a 3.3V control logic with an SN74LVC1G123. That device has an output drive strength around 10 ohms (typical). It will give you an output edge rate about 20 times faster than the CD4k device.
  • 0 in reply to Michael Johnston90
    Prodigy 60 points

    I was able to get the circuit working with a valve using a N-channel mosfet 2sk727. I'm going to convert this to a PCB now, Thanks for your advice.

  • 0 in reply to Michael Johnston90
    TI__Guru* 96957 points
    Always happy to help. If you can mark this 'resolved' I would greatly appreciate it.