Where does CMOS output short circuit limiting come from?

shorting cmos output

In a recent experiment I performed after fielding another's question in another thread (CMOS NAND), I wired a cd4001BE NOR gate as follows.

Vdd = 6V/5A source
Vss = 0v
A=B=X=Vss

To my surprise I found that the current drawn from the supply was only around 8mA.

6v/8mA=0.75x10^3 = 750Ω

Where did that 750Ω limitng resistor come from? In my research, the highest resistor I've found is 15-30Ω + Rds_on in series with a CMOS output (via cd4007 datasheet.)

What gives? Is an on p-channel MOSFET really 720Ω? (According to a cd4001 datasheet, the 1/4 cd4001 is actually a NAND with inverters on its inputs and output. So, it's really just a single on p-channel device when its inputs are low, not a series.)

I'm perplexed...

Added after 4 hours 7 minutes:

I'm still perplexed, been digging and digging.

Does anyone know?

If you look at the CD4001 datasheet, you will see the pullup characteristics (see below). This is consistent with your measurement.

Thank you. I did see that graph, in miniature mind you, but I saw it. Likewise, I saw the internal schematic.

From these things and, as well, my experiment, I could only conclude that an on p-channel MOSFET is indeed over 700Ω from drain to source.

Isn't that rather high for a full on device??

Are we sure there isn't more to the internal circuit, something that's adding R?

Euler's Identity said:

Thank you. I did see that graph, in miniature mind you, but I saw it. Likewise, I saw the internal schematic.

From these things and, as well, my experiment, I could only conclude that an on p-channel MOSFET is indeed over 700Ω from drain to source.

Isn't that rather high for a full on device??

Are we sure there isn't more to the internal circuit, something that's adding R?

Click to expand...

Logic gates aren't designed to drive relays and motors - they're designed to drive other logic gates. When you consider that the input capacitance of a 4000 series gate is typically 5pF, a fanout of ten (50pF) would yield a risetime of around 50ns - plenty fast for the specs of the CD4000 series.

Vo ---- R ----- Vin ------ C ------ Vss

Vin = Vo(1 - e^-(t/τ))

Vin
----
Vo

=

1 - e^-(t/τ)

so

e^-(t/τ)

=

1 - Vin/Vo

inverting

-t/τ

=

ln(1 - Vin/Vo)

Therefore, w/R=750Ω and C=Cin=50pF,

-t = 750Ω*50pF*ln(1 - 2.5/5) = 37.5ns*-0.693 = -26ns

Ok, I'll buy it.

I never realized Ron was that large. Thanks!