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SN74CBTLV3257: Input current

Hailiang Zhang
Expert 4745 points
Part Number: SN74CBTLV3257
Other Parts Discussed in Thread: TLC555, , SN74AHC157, SN74LVC157A, SN74LVC1G3157, SN74AHC541, SN74LVC541A, SN74LVC1G97

Hi Team，

I am wondering does the TLC555's output have enough drive capability for eight inputs of your SN74CBTLV3257PWR? The driver capability of TLC555 is 10mA.

Regards,
Hailiang

  • 0
    TI__Genius 10500 points

    Hello Hailing,

    I want to understand  your schematic setup  do you have a picture?

    If you are trying to feed the current through the io pins of the mux your design and application should be fine! Because this mux is a passive device.

     This device (and all most all other devices in the analog switches and multiplexers portfolio) just steer a signal in the selected path configuration. 

    Please let me know more! Thanks!

  • 0 in reply to Kameron Hill
    TI__Expert 4745 points

    Hi Kameron，

    I do not have schematic now. Pls refer to the diagram. Thanks!

    Regards,
    Hailiang

  • 0 in reply to Hailiang Zhang
    Guru 244370 points

    Any switch, when enabled, behaves pretty much like a 5 Ω resistor. So whether the TLC555 can drive these signals depends on what loads you have connected on the other side of the switch, and how many of them you enable at the same time.

    What is connected to the xB2 and xA pins? Why are you switching the 555 output? It might be possible to use buffered logic gates instead of passive switches.

  • 0 in reply to Clemens Ladisch
    TI__Expert 4745 points

    Hi Ladisch，

    It is for a Switch project. This TLC555 would be used to drive 8 pcs of 3.3V FAN PWM signals.  I agree that may be a buffer is a good option, and checking with engineers. Meanwhile, for providing PWM signal for FAN module, SN74CBTLV3257 should work, right?

    Regards,
    Hailiang

  • 0 in reply to Hailiang Zhang
    Guru 244370 points

    What is connected to the xB2 pins, i.e., what should happen to the PWM signals when they are not driven by the 555?

  • 0 in reply to Clemens Ladisch
    TI__Expert 4745 points

    xA is connected to 8 fan modules, xB1 is connected to TLC555, TLC outputs a square wave with a fixed duty cycle to make the fan work at a certain speed; xB2 is connected to CPLD, and CPLD can adjust the duty cycle to adjust the speed. When the CPLD is not working, it will switch to TLC555. The customer is asking if there is a Multiplexer with integrated buffer, could you please comment?

  • 0 in reply to Hailiang Zhang
    Guru 244370 points

    2× SN74LVC157A, or SN74AHC157 with lower drive strength.

    You could also use a single buffered or analog multiplexer (SN74LVC1G97 or SN74LVC1G3157) to drive eight buffers (SN74LVC541A or SN74AHC541).