[SI-LIST] Re: Drive strength to Load conversion
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxxxxxx>
- To: "'Chen, Sherman'" <sherman.chen@xxxxxxx>, <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 18 Aug 2014 17:47:25 -0700
Sherman
Yes, that is the implication. The equivalent output impedance could be much
lower.
But it is best to view this as what it is in most cases, a CMOS device that
is large enough to sink the spec's current and provide an "IR Drop" of less
than 0.4V in this example. There are many drivers that have a built in
series resistance that will dominate the output current over the operating
current range and then you mostly have a true IR drop in the output. I've
seen these series resistor in series with the output and sometimes in series
with just the pulldown or pullup device. But most drivers have no internal
series resistors so the IV characteristics seen by the outside world is the
DC characteristics of the FET. At low Vds the output looks like a resistor
and at higher Vds the FET looks like a current source/sink.
A typical 8 mA driver may sink 10 to 30mA or higher at 0.4V.
Regards,
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
tom@xxxxxxxxxxxxxxxxx
www.teraspeedlabs.com
971-279-5325 office
503-430-1065 cell
-----Original Message-----
From: Chen, Sherman [mailto:sherman.chen@xxxxxxx]
Sent: Monday, August 18, 2014 5:11 PM
To: tom@xxxxxxxxxxxxxxxxx; weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Drive strength to Load conversion
Tom,
If a device' spec claims its output remains below 0.4V when sinking 8mA, the
actually Vo could be way below 0.4V which mean a much smaller Rseries. And
it's the Rseries that determines the max sinking current - correct me if not
the case.
And you are right, normally high speed buffer spec don't give the output
current rating/capacitance, they just give output voltage/slew rate at test
load.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
-----Original Message-----
From: Tom Dagostino [mailto:tom@xxxxxxxxxxxxxxxxx]
Sent: Tuesday, August 19, 2014 5:23 AM
To: Chen, Sherman; weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Drive strength to Load conversion
Be very careful looking at current specs of drivers. An 8mA driver is
really saying that at 0.4V the part will sink at least 8 mA. In all likely
hood it will sink much more than 8 mA at 0.4V and during the transition from
high to low (I'm only talking about the pulldown device for a typical CMOS
driver here, there is a similar set of specs for the pullup device) the
device can sink much more current, likely 100mA or more. Look at the IBIS
model for the driver to see its current capabilities.
I have never seen an output current spec with a capacitive load. Capacitive
load specs are only given for delays or if present rise and fall times. The
current specs of a driver are DC specs, not AC specs.
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
tom@xxxxxxxxxxxxxxxxx
www.teraspeedlabs.com
971-279-5325 office
503-430-1065 cell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chen, Sherman
Sent: Monday, August 18, 2014 1:52 PM
To: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Drive strength to Load conversion
While Steve explained the essentials of the matter from both low slew rate
and high slew rate scenarios, I think Ajay's question is about why in most
spec the output current is given with specified capacitive loads. For
example, a HCSL clock buffer gives the current capability of its output with
the load condition specified as a 100Ohm tline with two 2pf shunt caps to
GND at the end on each +/- line. And many lower freq. chips give their tr,tf
spec. with the test load as 10-50pF //1k resistor.
So Ajay,
For the low freq. case, the spec. of tr,tf are actually telling how much is
the output resistance which is the R value in the formula tr(10%-90%) =
2.2RC.
For the high freq. case (the HCSL one above), I guess this is the one you
are asking about, first with the cap load at the end of tline there will be
some ringing following each transition edge of the signal - if the signal
rate is not that fast so the charge/discharge of the cap can be finished
soon after each edge before next edge. The same Ohmic law is still the rule
that determines the voltage/current relationship here - even in high freq.
case. So using the RLC charging/discharging formula you can calculate the
peak current of the ringing which's rms value I think should be the output
current capacity in the spec. A simplified way of calculation is to use RC
instead of RLC with R replaced with Z which is the characteristic impd. of
the tline. You need to create a lattice diagram if doing the calculation
manually.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Sunday, August 17, 2014 5:41 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Drive strength to Load conversion
If we take the rash assumption that your rise and fall times are so slow
that you can treat your traces as simple ohmic interconnects, then you can
analyze your timing like it was 1985. The rational disconnect here is that
if your rise and fall times are that slow, then why would you be concerned
about skew from capacitive loading? Do you have an enormous fan out?
If your rise and fall times are fast enough that the interconnects are
transmission lines, then if you have so much capacitance at the far end that
|Ztxline|*C >> [Trise, Tfall] then you have a reflection issues and
|Ztxline|failing
to control your interconnect impedance is going to make that even worse. If
your timing budget cannot handle capacitive charging at your loads, then it
is extremely unlikely that it will tolerate a number of round trip delays to
settle out. If any of the signals are clocks or timing strobes then you
could be walking into multiple transition nightmares.
I think that what you need to do is understand your signaling requirements
first. That will tell you how sloppy your timing can be and whether you
need to engineer as though it is the current or past century.
Steve.
On 8/16/2014 11:20 PM, Ajay Dhingra wrote:
> Hi Lee
> Thanks for your reply.
> I further have two things in my mind.
>
> First: I don't have a controlled impedance environment.
> Second: for any Transmission line, there is a cap load at the end of
> the transmission line. The cap load requires certain amount of charge
> so that it's voltage reaches logical threshold with a desired ramp. at
> the end of the day cap will change the ramp and hence timing. And
> eventual ramping of that voltage is determined by drive strength of
> the driver. though it happens in stages, first charging the
> transmission line and then transmission lines forwards the same ramp
> to cap load. So my question is how to approximate the drivers
> capability for a particular load or what max load a drive can drive
> based
upon its drive strength.
>
> Thanks
> Ajay
>
>
> On Sat, Aug 16, 2014 at 11:50 PM, Lee <leeritchey@xxxxxxxxxxxxx> wrote:
>
>> We don't drive capacitive loads any more. We drive impedances on the
>> order of 50 ohms. transmission lines.
>>
>>
>> -----Original Message----- From: Ajay Dhingra
>> Sent: Saturday, August 16, 2014 11:09 AM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: Drive strength to Load conversion
>>
>>
>>
>> Sent from my Huawei Mobile
>>
>> -------- Original Message --------
>> Subject: Re: [SI-LIST] Drive strength to Load conversion
>> From: Ajay Dhingra <ajay.dhingra@xxxxxxxxx>
>> To: Lee <leeritchey@xxxxxxxxxxxxx>
>> CC: si-list@xxxxxxxxxxxx
>>
>> Just for first order estimation if that particular drive strength is
>> sufficient for an equivalent cap load. Or in other words what max cap
>> load can be driven by x mA drive strength.
>>
>> in a situation when only drive strength in mA information is
>> available how to assess the driver capability.
>>
>> Thanks
>> Ajay
>>
>> Sent from my Huawei Mobile------------------------
>> ------------------------------------------
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>
--
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