SN74HCS594: VOL Spec extrapolation

Hey Cameron,

To get V_OL there's three things you need to know:

(1) The output is a MOSFET

(2) MOSFET's act like resistors when the voltage is close to the rail

(3) The datasheet gives you voltage and current, which lets you calculate resistance

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I guess I should include a 4th, which is don't use the low current values b/c they have a lot of guard-banding and won't give you very accurate results -- but that's more of a quality thing rather than the basic requirements.

In this example, at V_CC = 4.5V, I_OL = 6 mA, the output voltage maximum is 0.30 V

Using Ohm's Law:  R_OL = V_OL / I_OL = 0.30V / 0.006A = 50 ohms

Don't forget that this is a max -- the device will never have 50 ohms or more. The next question that always comes up is: well, how _low_ can it go?

Since there's no minimum spec, that's not easy to answer, but I can give you an estimate. The typical value will usually be about half of the max, and the minimum will be half again.

So, we can expect that R_OL for this device at 4.5V will be:

Min: 12.5 ohms*

Typ: 25 ohms*

Max: 50 ohms

* Estimated values

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It may also help to know that MOSFETs have lower resistance as the bias voltage increases, so 4.5V will be the worst case for the 5V operating range.

Also, we have several FAQ's that may help with this topic:CMOS Output FAQs 

And a new video about how CMOS outputs work: Basics of Logic - CMOS Outputs