[SI-LIST] Re: VOH datasheet question
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <ray.anderson@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 9 Mar 2006 13:35:54 -0800
Chris
Voh is a DC parameter as others have pointed out. All it means is the
output, if loaded with a 18 mA load will have at least 2.4 volts. Typically
most drivers we see will really be much stronger than the output current
specification at the 2.4V level. Most will put out more than 2 or 3 times
that amount of current. Look at the pullup table in your model to confirm
that, it will be listed next to 3.0-2.4V or 0.6V.
The rise or fall times into a capacitive load will be dominated by the RC
time constant of the output impedance of the driver and the load
capacitance. The driver will have a non-linear output impedance in most
cases.
Since it is a capacitive load and if the output is truly a CMOS pullup then
the output will always get to Vcc or 3.0 Volts in your example if enough RC
time constants have passed. The capacitor will have charged and the output
current is then Zero.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ray Anderson
Sent: Thursday, March 09, 2006 12:33 PM
To: si-list@xxxxxxxxxxxxx
Cc: Ray Anderson
Subject: [SI-LIST] VOH datasheet question
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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