Dears:
Although a designer should know which corner case will degrade the performance of your designed circuit. But it is maybe the worst case which is out of 3-sgma ramge. If trying to overcome the worst case, the area and power of the circuit will raise. Will you do that? How many corner cases to sure the circuit being mass produce?
corner case:
rratio, cratio, i_ratio(internal current), ixp_ratio(external currnet), mos(tt,ff,fs,....)......
BR. purefen
Re: how many corner cases to sure the circuit being mass pro
Hi,
From my experience, i usually use the folloing condition ;
(PS: Take 0.35um process as examples for consumer product)
1. VCC : 2.9V, 3.3V, 3.7V
2. TEMP : -20C, 40C, 100C
3. Process corner: Typ, FNFP, SNSP, FNSP, SNFP
4. Resistor value: 70% , 100%, 130%
5. Cap value : 80%, 100%, 120%
So the total condition= 3x3x5x3x3 = 405 condition.
Re: how many corner cases to sure the circuit being mass pro
Survivor is in the way, but corner analysis give quite pessimistic results. That means that you must oversize your design (are and power consumption) to satisfy that your circuit works well in every corner (as survivor said: a lot).
Normally, even if this is more time consuming, Monte Carlo analysis, if you have some correlations defined between the corners of your devices, give more realistic results. AMS has written some presentations showing another method called statistical corner, which takes into consideration that corners parameter of your devices are not independent of each other. This results in a correlation between parameters, reducing so the number and modifying the "form" of the space of corners.
Re: how many corner cases to sure the circuit being mass pro
Dears:
Thanks for your replies. Read all the replies of your, I thinsk that Humungus's suggestion is more realitic and logical. The more corner is, the more tired the designer is. As the result, I will try to find out the flow that Humungus mention and verify it. Thank you a lot.
BR. purefen
405 corners? no, designer's job is to produce circuits, not sim 7-transistor op amp for 6 weeks.
my opinion is that fs/sf are rare, and i use them to find out which flavor of transistor is misbehaving when a circuit falls off a cliff in ff/ss.
the reason fs/sf are rare is because main factor in fastness is tox, which is about equal for all mosfets in a given area.
actually, i try not to depend on corners for figuring out what i should already know - for example let's think about an op amp. main specs are gain and phase. gain is worst at hot/slow, so satisfy gain using hot/slow. phase margin is worst at cold/fast, so use cold/fast to satisfy phase margin.
only if you don't understand a circuit should you be running all the corners (meaning fs sf). and then you should be running this corner over temp to look for the delta.
ie - cold makes fast, hot makes slow, so -40, +140 at ff lets you see how each device acts as it slows down in it's fastest zone, you get a slope within a slope. understand that, and you'll crank out 10 chips in the time that others run 405 corners on a single block..
ps - typ typ is a given. you should probably draw in typ typ initally, but i wouldn't spend a whole lotta time running voltage and temp corners on typ typ if your circuit passes those corners at fast & slow!.
just my opinion, and this time i won't even charge you the two cents i usually do.
Re: how many corner cases to sure the circuit being mass pro
You can employ Monte Carlo analysis for it.
For example, .TRAN 1n 10n SWEEP MONTE=val
You can use 30 for val to achieve a 99% possibility from normal 80% possibility.
Re: how many corner cases to sure the circuit being mass pro
It all depends if your management understand what you are doing - are technical or not.
If not you'll end up simulating all corners and MonteCarlo.
Have fun
You should also consider that best cases (+3sigma) do not happen. But -3sigma does happen relatively often (in a large scale of things) so you should run WS. MonteCarlo is more realistic for sure.
It also depends if you can live with idea of scrapping a lot if it is WS one.