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SN74AXC8T245: SN74AXC8T245 as buffer questions

Robert Alfred Kecs
Prodigy 110 points
Part Number: SN74AXC8T245
Other Parts Discussed in Thread: SN74LVC541A

Tool/software:

Hi,

I want to use the SN74AXC8T245 as a buffer, here is the situation

1. the signals will first pass through an ISO67/ ISO77 digital isolator , 25MHz signals max

2. After the isolators there will be long traces that connect to a lot of chips in parallel so i am thinking of using the SN74AXC8T245 as a buffer after the isolator and before i connect to the chips

( I had some problems before with fewer chips then it comes to the rise time , in design my driver output was 16mA max like the isolator chips so this makes me heavily consider using a buffer since now there are even more chips)

3. All signals are 3v3 and some are default high and some default low

Do you think the SN74AXC8T245 would be a good fit for this application? 

I was also considering adding some pull up pull down resistors AFTER the SN74AXC8T245 so that when i put the SN74AXC8T245 in HiZ mode there is a known logic lvl. 

This seems ok to me but i want a second opinion, also i am not sure of the pull up value should i go for 10k or 100k? In terms of current there should be no steady state problems BUT i 

Am afraid regarding the rise fall times, there is no SPICE model so i cant simulate these transitions. Is there a simplified model that i can use, like a perfect switch with some parasitics ?

Best regards,

Robert

  • +1
    Guru 316760 points

    When you want a buffer, an actual buffer like the SN74LVC541A would be cheaper and simpler to use.

    The pull-up/-down resistors do not affect the rise/fall times when the buffer is active.

  • 0 in reply to Clemens Ladisch
    Prodigy 110 points

    Yea i was intending to use the pull ups so that i dont have floating inputs while OE pin is high ( HiZ ).

    This chip seems to have 288kOHM built in pull down that helps a bit on some signals.

    I was considering the SN74AXC8T245 since i already use a few in my design and it was for BOM consolidation reasons. 

    According to the datasheet if i need a pull up i should use less then 30kOHM so ill probably use 22k , do you think ill have problems with this amount of pull up? there was no lower limit in the datasheet , that i saw.

    Best regards,

    Robert

  • 0 in reply to Robert Alfred Kecs
    Guru 316760 points

    The only lower limit is the absolute maximum rating for the output current.