21 Mar
2005
21 Mar
'05
10:28 p.m.
I wrote about the 7438/7406/7407 buffers:
The 74LSxx, 74HCxx, 74HCTxx, etc.
"equivalents" are NOT acceptable, as they have significantly lower
rated drive current.
Wai-Sun wrote:
How low? 2x?
About 24 mA (vs. the needed 48 mA) for 74LS, less for 74HC and HCT.
74S is pretty good, but I don't remember the numbers.
What if one were to parallel the drivers (perhaps in
piggyback manner)?
That would probably work. Can't really be guaranteed, since if Vol is
lower on one driver than the other, the load won't be balanced between
the two.
If using a 7438/7406/7407 is really a problem for you, there are various
more modern alternatives. For instance, a 74ABT125 or 74ABT126 can be
used as a quad 64 mA open collector driver, by tying the inputs to
ground and using the enables as the data input. However, 74ABT outputs
will have much faster edges than plain 74xx, which in this application
is not a particularly good thing.
The 7438, 7406, and 7407 are actually quite good for this application.
They are just about as close to bullet-proof OC drivers as you're
going find.
Eric