Thevenin Termination, Power + Heat - Page 1
Forum section: electronics / projects

author: Graz
author_rank: Contributor
timestamp: December 31, 2015, 08:25:20 pm
content: In a project I'm working on, I'm replicating part of an existing design which uses Thevenin termination on a backplane running at 5V. I'm also going with the same scheme. I'm concerned somewhat about power consumption, but more about power dissipation in the resistors. I built up the circuit using 330R to 5V and 470R to ground. When the midpoint is not driven, it floats at ~2.9V and each line sinks 6.25mA from the power source. When the terminated line is actively pulled low, the sink will draw 15mA through the 330R resistor, dissipating 75mW.For space and routing reasons, I used pairs of 4-way SMT 1206 resistor arrays. Each array could end up dissipating 300mW in each 1206 package if all four lines were actively pulled low for any length of time, which seems excessive. There are 13 such pairs in this design. When powered, the termination resistors get noticeably warm to the touch - enough that it's uncomfortable to leave the back of a finger on it for any length of time. This bothers me. The data sheet for the resistor (CTS742C083331 for the 330R) says the 70 degree power per resistor is 63mW, the power-per-package is just 4 x this, and the maximum operating temperature is 125 degrees, so it seems that this is right at the edge of the spec. It doesn't state any kind of maximum. I'm hesitant to leave this as is.The actual value of the effective termination resistance probably isn't that important within reason. Traces are long, speeds are low (~15MHz) and the number of loads per line is effectively unknown as this is a backplane. I could probably bump the resistance values up by a fair bit.What should I do? Leave it alone? Bump the resistors up? Going with anything other than the 1206 arrays would mean a board respin and might not even fit.

author: Andreas
author_rank: Super Contributor
timestamp: December 31, 2015, 09:13:49 pm
content: Quote from: Graz on December 31, 2015, 08:25:20 pmThe speed is not important. The only relevant thing is the rise time of the driver IC.And with heavy capacitive loading the line impedance can easily drop down to 50 Ohms.So even the 330/470 terminator with 200 Ohms is only a bad compromise.(which works only for selected TTL bus families and maximum a VME backplane).Why dont you use specialized terminator networks?http://www.digikey.de/product-detail/de/4610X-104-331%2F471L/4610X-4-331%2F471L-ND/1089271with best regardsAndreas

author: Graz
author_rank: Contributor
timestamp: December 31, 2015, 09:52:21 pm
content: Quote from: Andreas on December 31, 2015, 09:13:49 pmYeah, I realize that. However, the drivers are TTL and this is effectively a "VME-like" backplane, so it should be okay. I hope.Quote from: Andreas on December 31, 2015, 09:13:49 pmI was trying to stick with all SMT except for connectors. The board is dense enough that I need both sides for routing. There are some SMT dual terminators, but they're horrendously expensive and once the buses get wider, routing becomes very hard. Honestly, I'd ballparked the math on power dissipation and hadn't realized how close to the resistors' spec I'd get.Perhaps on V1.1 of the board, I'll rethink this. For now, I'd like to get the prototypes working well enough that I can find and fix any other respin triggers.If I remove either the high or low side resistor, this effectively would become series termination to ground or +5V, respectively. Would it help to do that and fix-up the resistor values?Another thing that might work is that I have a 3.3V supply available. I could tie the high side of the terminator to +3.3V and/or do series termination to +3.3V (adjusting resistors appropriately, of course). Would that work?Sorry, my domain is digital - RTL and micro-controllers, that kind of thing. I actively suck at analog and high frequency stuff. I actually don't understand why you'd choose Thevenin termination over other termination strategies. I just followed the original design (which did use big old through-hole networks).

author: Andreas
author_rank: Super Contributor
timestamp: December 31, 2015, 11:39:10 pm
content: Quote from: Graz on December 31, 2015, 09:52:21 pm220R to 3.3V and 1K8 to GND would be the equivalent.You will need 2.9V idle voltage and around 200 Ohms termination resistor.You could use also a active termination (L2722) with only 1 resistor per signal.(dont forget the bypass capacitors in this case).With best regardsAndreas