25 Aug
2014
25 Aug
'14
2:43 p.m.
I wrote:
By which Tony means "can sink a **MUCH** larger
current than it can source".
If you look at the spec for the TI SN7404, the difference is a factor of 40:
max -0.4 mA source (high), vs. max 16 mA sink (low). Generally speaking,
On Mon, Aug 25, 2014 at 1:07 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Although to be fari, if yo get it to source more
current (sensibly more
current, like the 10mA to light an LED), the IC will not be damaged but
the output voltage may well drop below the specification for a logic 1
output.
Plenty of (commercial) units drove LEDs form a TTL output to ground, even
though it is not good practice. (and here 'TTL' excludes the CMOS
families)
It's beyond "not good practice" and into the realm of "can't be
expected to
work consistently". While it's true that sourcing more than the rated
current from a 74xx output will pull its voltage below the Voh
spec, there's no telling how much lower it will pull it. You could
characterize the voltage of a typical 7404 output sourcing 10 mA
under various conditions (e.g., supply voltage, temperature), and find
that driving the LED works will all the parts you buy today, but parts you
buy tomorrow or from a different vendor might fully meet specifications yet
not be able to drive the LED.
I would consider any product that high-side drives an LED from a TTL
output beyond the rated source current to be defective in design.
Some moron at TI has revised the SN5404/SN54LS04/SN7404/SN74LS04
datasheet to claim that all unused inputs must be held at Vcc or GND, which is
good practice, but to further cite their app note "Implications of
Slow or Floating
CMOS Inputs", which is completely irrelevant to a bipolar TTL device. Sigh.