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------------------------ >> ------------------------------------------ >> >> 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 forum is accessible at: >> http://tech.groups.yahoo.com/group/si-list >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> > -- Steve Weir IPBLOX, LLC 1580 Grand Point Way MS 34689 Reno, NV 89523-9998 www.ipblox.com (775) 299-4236 Business (866) 675-4630 Toll-free (707) 780-1951 Fax All contents Copyright (c)2013 IPBLOX, LLC. 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If you are not the intended recipient, please destroy all records and notify the sender. ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu