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output pin source current of gates

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Feb 23, 2011
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What is the typical current that can be sourced from the output of a gate of CD40xx series (especially CD4093).
Can we source higher current from 74xx series gates?
For future reference, what should one look in a Datasheet to find this detail?

You will see two kinds of specs, a VOH@IOH and an output
short circuit current. The former will give you an on-resistance
which should be a rough guide, the output current will start
to roll over at higher output voltage but ought to be close
to what you'd figure as myVOH/Ron. The short circuit limit
is usually one sided and very sandbagged.

CD40xx came in several families with different VDD ratings,
output drive varies hugely with VDD.

Older TTL (H, S) series had higher output drive. Bus transceiver
types tended to be 24mA vs the usual 4mA or 8mA output
buffers and this carried over to CMOS branches as well. You
can parallel buffers if their inputs are co-driven.

A CMOS based power MOSFET driver will give you beaucoup
drive, limited probably by your thermal setup.

Texas Instruments datasheets for CD4xxx ICs show detailed graphs of minimum and typical output current sink and source at 3 supply voltages. The graphs allow you to select a load voltage drop (I use an LED forward voltage). For example with a 10V supply the typical shorted output source current is 20mA or 18mA into a white 3V LED. Each output transistor is allowed to dissipate 100mW so with the 3V LED and the 10V supply the voltage across the output transistor is 7V and its current is 18mA so it dissipates 7V x 18mA= 126mW so a current-limiting resistor is needed to share the heat.
A 74xx IC sinks 16mA but sources a low current. A 74HCxxxx high speed Cmos IC has a max supply voltage of 6V or 7V and a max output source or sink current of 25mA. I have used the 74HC132 IC which is the same as a CD4093 but with different pins and different Schmitt Trigger threshold voltages with a 3V supply that drops to 2V driving a 1.7V red LED brightly.

I think the output currents of CD4xxxB ICs are the same for all gates and counters, etc but buffers are different.


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