Can you use an ACMOS output to drive 10 TTL loads ?

Bean

We have a customer that requires our circuit drive 10 TTL loads.

One of our engineers had an output circuit of a 680 ohm resistor to +5V and a PNP transistor to ground. The base of the transistor is driven by a standard cmos output (10MHz) through a 10K in parallel with a 100pF cap.

The customer is complaining about the slow rise time. We cannot lower the 680 ohm because of the increased current draw.

We would like to remove the transistor and pullup and just put a gate on the output of the cmos clock. I've been told that ACMOS logic will drive 10 TTL gates just fine. But I was wondering if LS or HCMOS or some other logic would be better.

Any thoughts ?

Bean.


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Phil Pilgrim (PhiPi)

According to Horowitz and Hill, you have to sink 1mA to drive a standard TTL load. Chips like the 74AC00 can source and sink 24mA. So the answer to your question, in theory, is "yes" — at least statically. If you need more dynamic oomph to reduce rise and fall times due to capacitive loading, you may have to use a line driver instead of a gate or buffer.

-Phil

Bean

Phil,
Thanks for the comments.
It has been recommended to use a NC7SZ126 buffer.
Since you also recommended a buffer, that is what I will try.

Bean.

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“The United States is a nation of laws -· poorly written and randomly enforced.” - Frank Zappa

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
www.hittconsulting.com
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