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CD74ACT273SM96: How can I best match delays and drive-strengths of the individual FFs? (need floorplan info)

Steve Wood
Prodigy 140 points
Other Parts Discussed in Thread: TS5A23159

I wish to parallel 4 gates of the CD74ACT273SM96 and match this quad "super-gate" with another "super-gate" formed by the parallel combination of the remaining 4 gates in the package. If I knew the physical layout of the chip, I could select two groups of 4 that would provide the best matching (in RdsON, delay, rise/fall time) of the two "super-gates". Could you please provide the gross floor plan of the chip so that I can select groups of 4 that are interleaved in the physical layout?  I could also use the 74ACT374 or 74ACT574, so information on these would also be helpful.

- Thanks!

  • 0
    TI__Guru* 96591 points
    Hi Steve,

    Can I ask why you are paralleling flip-flops? I have never heard of this being done.
  • 0 in reply to Emrys Maier
    Prodigy 140 points
    To decrease on-resistance of the output to make it small compared to the precision resistor that its driving. This paralleled FF is being used as a very fast analog SPDT switch (which of course can only switch between VDD and VSS).
  • 0 in reply to Steve Wood
    TI__Guru* 96591 points

    That's an interesting application.  Just be careful to avoid going past the max Vcc/GND current of 100mA total.

    As far as sharing the layout of the silicon - I'm afraid I can't do that.  Generally speaking though, the similarity between gates on the same piece of silicon is so close that doing what you are suggesting is perfectly fine.  As a disclaimer though, I've never tried it with a device as complicated as a flip-flop.  We recommend avoiding paralleling gates across separate devices, but it's ok within the same device.

    My personal recommendation would be to just do a skew test over a few devices and choose the channels that are closest together.

    As a second recommendation - have you taken a look at the TS5A23159?  It's a dual 1 ohm analog SPDT switch.  Since the lowest output impedance this device has (in the datasheet) is ~22 ohms, even parallelling 4 channels won't get you close to 1 ohm.

  • 0 in reply to Emrys Maier
    Prodigy 140 points

    Thanks, Emrys.  I'll check out the TS5A23159.