[SI-LIST] Re: VOH datasheet question
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 09 Mar 2006 21:05:44 -0500
Chris,
To take Tom's comments a step further, when the data sheet states that
VOH is 2.4V with VDD equal to 3.0V and an IOH of -18mA, this infers a
maximum p-channel ON resistance of 600mV/18mA or 33-Ohms at that dc
operating point. As Tom points out, the output channel resistance is a
non-linear function of the voltage across the channel and the current
through it. However, with a 10pF load, if you were to assume the RC
time constant to be around 330ps, you will at least have a rough idea of
what to expect for signal risetime (=2.2*RC) from either simulation or
lab measurements.
-Bill
/************************************
/ billw@xxxxxxxxxxx /
/ /
/ Advanced Electronic Concepts, LLC /
/ www.aec-lab.com /
************************************
Tom Dagostino wrote:
> 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
>
> **********************************************************************
> Please note: CRLO Displays has been renamed to Forth Dimension Displays =
> Limited.
> This communication contains information which is confidential and may =
> also=20
> be privileged. It is for the exclusive use of the intended recipient(s). =
> Please note that any unauthorised distribution, copying or use of this =
> communication, or the information in it, is strictly prohibited. If you =
> have received this communication in error, please notify us by email, by =
> telephone: +44 1383 828800, or fax: +44 1383 828801, then delete the =
> email and any copies of it.
>
> This communication is from Forth Dimension Displays Limited 1 St.David's =
> Drive, St. David's Business Park, Dalgety Bay, Dunfermline, Fife. =
> KY119PF. Scotland. UK.
>
> This footnote also confirms that this email message has been checked for =
> the presence of computer viruses.
>
> =A0
>
>
> ------------------------------------------------------------------
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
