SN74AHC125: Vol maximums at 3.3V Vcc

Hi Richard,

We do not have test data to show that particular data point, however it is fairly easy to specify a maximum using the datasheet specs. There are a few different ways to calculate intermediate V_OL's, but I prefer to stick with the easiest and safest method.

We can get the closest value on the datasheet and use that to calculate an r_on, then use that calculated r_on to determine the output drop.

In this case, the math is really easy -- the datasheet shows V_OL at 3V (~3.3V supply -10%) with an output current of 4 mA. 6 mA is a 50% increase in current, and the operating voltage remains in the same range, so I would also expect a worst-case 50% increase in V_OL, giving an output voltage of 0.44*1.5 = 0.66 V.

To double check this, let's get the r_on as I mentioned above.

r_on = V_OL / I_OL = 3 / 0.004 = 110 Ω

Using that value to get our new V_OL:

V_OL = I_OL * r_on = 0.006 * 110 Ω = 0.66 V

You can quickly calculate the power consumed in the FET with P = I²R = 3.96 mW, just to get an idea of the power dissipation (4 mW isn't a concern)

So, that's the simplest method of getting the V_OL, and the estimation is extremely safe -- ie there's a lot of headroom since the operating voltage is increased over the datasheet spec, which means r_on will be decreasing, also the output current isn't a huge increase, so there should be no thermal effects on the output FETs.

I could test the in the lab, but I have no way of testing every variation of PVT. Using the datasheet specs to estimate is the safest solution.