So I am using this AND gate which has an open drain output. I am trying to determine how many loads that I can place on this output. I see in the datasheet, that the Iol is over 1mA, but I am unsure as to whether it can drive the loads when in high Z.
So if the input loads have a input current of .5uA, what will supply this current to these loads. Is it the open drain output?
Thanks!
It is the resistor. In a static state you can drive more than 100 loads of this kind, but their input capacitance slows down the transition from low to high. So the real question is: What is the possible switching frequency at a given number of loads or vice versa. Another point is, that slow transitions on inputs without schmitt trigger internally draw a significant amount of additional supply current (see minimum input rise time in datasheet).
The smaller the resistor, the more loads you can pull high (logic 1) - but watch total capacitive load, as previous poster. However, if you make the pull-up resistor too small, too much current will be drawn when the open-drain AND gate output is trying to drive to logic 0. Check your AND gate data sheet for how much is allowed (typically 1-4mA, but can be more, depending on the logic series used).
