FreeLists / si-list / [SI-LIST] VOH datasheet question

["Ray Anderson" <ray.anderson@xxxxxxxxxx>]

I'm forwarding the following message to the list for Chris since he is =
experiencing e-mail problems at this time that are preventing the list =
from accepting his postings.

-Ray


---------------------------------------------------
From: "Chris Chalmers" <cchalmers@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>

Si List,


This is a bit of a basic question, I just need a push in the right =
direction.

In datasheets for a CMOS buffer running from 3V, the VOH min=A0 is =
stated as=20
2.4V at -18mA. If you are using this buffer to drive another CMOS input =
with a gate capaci tance of 10pF, for example, how does the current =
-18mA relate to the 2.4V VOH min?

I mean if you are putting through 10Hz through the buffer and change =
from 0 to 1, the input gate capacitance would be fully charged and won't =
be drawing any current.=A0 It surely won't be sitting at 2.4V.

So if we put together the situation of a CMOS buffer driving a 10pF =
capacitor at 70MHz over a very short line.=A0 The driver will be looking =
straight in to the capacitor.=A0 If the capacitor is big (100pF) then =
the VOH value will be affected since for a particular value of IOH it =
will take a certain time to charge up the cap.=A0 If the frequency of =
operation is high then it will be discharging before it has fully =
charged.=A0 I can see this.=20
However from the datasheet this relationship is not obvious if my =
thinking is correct.


If I know my load capacitance and have an IBIS model, can I ignore this =
VOH in the datasheet and verify that that the VOH of the chip works by =
using the slow setting of the IBIS model and simulating?


Thanks in advance


Chris

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