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CD74ACT245: Maximum VCC or GND current for short transient events (<50ns)

Nicholas Reilly
Prodigy 60 points

Part Number: CD74ACT245

Hello,

The CD74ACT245 datasheet specifies +/-100mA Vcc or GND current maximum, which increases to 200mA when all 8 outputs are sourcing or sinking current.  

 

It appears that this is to be interpreted as a DC current - is there a higher limit for Vcc or GND current for short (<50ns) transient conditions?  

 

If no higher transient rating can be provided, what is the risk of exceeding this current rating for very short durations? 

 

Thank you,

Nick

  • 0
    TI__Guru* 97676 points

    Hey Nick,

    Generally speaking, TI will only guarantee operation within the data sheet specs, so I'd treat that absolute maximum value as a hard limit for most devices, but there are a couple of exceptions.

    (1) The data sheet _also_ specifies a load capacitance that the device will safely operate with -- ie a value of loading that we have extensively tested the part with.

    With a direct capacitive load, the current can greatly exceed the continuous current limit, but as long as you remain within the capacitor size limit provided, you can be assured that the device will operate reliably.

    For the CD74ACT245, the load capacitance is rated to 50pF, so you're definitely safe up to 50pF. With a 50pF load, I would expect to see the current spike up to around 220mA for 2 to 3ns. I usually recommend to stay within about 150% of the value listed, so up to 75pF is pretty safe.

    (2) For the AC and ACT families specifically, there's a special test condition at 75mA drive for up to 1s, which is much more than your typical logic gates will provide. If you need to limit the current, then 75mA is a very reasonable value to use as long as that 1s time is met.

    Nicholas Reilly said:
    If no higher transient rating can be provided, what is the risk of exceeding this current rating for very short durations?

    The risk is always with reliability. We rate our parts to 100,000 hours of operation (about 10 years), but that's only within the values we provide. We don't test under other random conditions, so we have no basis to guarantee reliability for extended operation.