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74ACT output drive capability.

Michael Sirkis
Prodigy 90 points
Other Parts Discussed in Thread: SN74ACT244, CD74ACT541

I am working on a project that utilizes many 74HC165 and 74HC595 (operating at 5volts as I/O expanders (utilizing SPI from the processor).

I want to distribute six identical 10MHz SPI clocks. I am considering a 74ACT device to interface between the 3.3 volt processor and the 5 volt '165 and '595 devices.  The board will be controlled impedance and the termination at the end of each line will be thevenin with two 150 ohm resistors. I believe the drive from each  74ACT output will  be 24ma.

If I run six outputs from one device, am I exceeding the devices ratings?

Looking at two data sheets:

1. SN74ACT244 the maximum supply or ground current is shown as +/-200ma.

2. CD74ACT541 the maximum supply or ground current is shown as +/-100ma *

For the CD74ACT541 the data sheet (*) indicates add +/_25ma for each additional output..

Will either of these devices safely provide the six outputs that I require?

If not, should I consider using more 74ACT packages and only using 2 or 3 outputs per device, or perhaps something completely different?

Thanks in advance.

  • 0
    TI__Guru* 97106 points
    Can you provide a schematic for the application? I think I understand, but I don't want to give you bad advise based on my assumed understanding.
  • 0 in reply to Emrys Maier
    TI__Guru* 97106 points

    Michael,

    Thanks for the schematic - that made things much clearer for me.  The resistor values indicate that the maximum current for each output will be +/- 33.333 mA (and actually it looks like that will be the sustained output as well).  The total max current is 33.333 * 8 = ~267 mA.  Unfortunately, I don't know of any parts that will be able to handle this level of sustained current, on a single channel or total current.  Our ACT series of parts would definitely be damaged by this use.

    If you must have this much current, there are two solutions I can think of.

    1) Tie together two outputs to get a stronger drive current.  Always do this with adjacent channels on the same device and ONLY if the inputs are also tied together.  Otherwise timing issues could cause damage to the part.  Also, make sure not to violate the maximum current through Vcc/GND for the part.  You will likely need to use multiple devices.

    2) Use external transistors to drive the load for each channel.  In this case, you can select any 8 channel driver as your splitting stage, then use either a BJT or FET based push-pull amplifier to provide the required current.

  • 0 in reply to Emrys Maier
    Prodigy 90 points
    Thanks for the answer. I was using www.interfacebus.com/Design_Termination.html
    as a guide. In the section "Thevenin Trace Termination" it shows that driver current is Vo/R1 or Vo/150 ohms.
    I was led to believe (from a web search) that the minimum Ro of the 74ACT541 is 16 ohms, which would make each output at 27.5ma and six acive outputs => 6* = 165ma.