Reverse-engineering the LM185 voltage reference chip and its bandgap reference
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Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#2Author here for all your voltage reference questions :-)
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#3Author here for all your voltage reference questions :-)
Is there any chance you can expand on the reasoning behind the circular/coaxial designed transistors? The normal sorta logic style transistors are straight forward, and the serpentine nature of the high current transistors makes sense for the surface area, but I never understood the reasoning for the round transistors.
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#4Author here for all your voltage reference questions :-)
Got any good references for how to identify silicon structures as certain components?
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#5Author here for all your voltage reference questions :-)
Is there any chance you can expand on the reasoning behind the circular/coaxial designed transistors? The normal sorta logic style transistors are straight forward, and the serpentine nature of the high current transistors makes sense for the surface area, but I never understood the reasoning for the round transistors.
Theoretically you could make a PNP transistor by reversing the doping of an NPN transistor. The main problem is that boron diffuses rapidly, making it hard to fabricate a buried P-layer. Boron also has less solubility than phosphorus, making it hard to dope the emitter. Also, holes have only 1/3 the mobility of electrons, so PNP and NPN aren't symmetrical. To deal with these issues, PNP transistors are usually built with lateral construction (i.e. horizontally). The ring structure ensures that almost all of the carriers injected by the emitter are intercepted by the collector.
(This is based on The Art of Analog Layout, p280. I don't know all this doping stuff myself.)
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#6Author here for all your voltage reference questions :-)
Got any good references for how to identify silicon structures as certain components?
I got an old, cheap copy of "The Art of Analog Layout" (Hastings), which describes these structures in detail. For the most part, the structures are fairly easy to recognize after you've seen them once or twice. But then there are the bizarre mystery circuits that require some puzzle-solving. For instance, where they combine a couple of transistors to save a bit of space.
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#7Earlier quoted context omitted.
Got any good references for how to identify silicon structures as certain components?
I got an old, cheap copy of "The Art of Analog Layout" (Hastings), which describes these structures in detail. For the most part, the structures are fairly easy to recognize after you've seen them once or twice. But then there are the bizarre mystery circuits that require some puzzle-solving. For instance, where they combine a couple of transistors to save a bit of space.
Seeing that the latest edition of The Art of Analog Layout came out of 2005, is there any other book you'd recommend on analog layouts specifically?
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#8Earlier quoted context omitted.
I got an old, cheap copy of "The Art of Analog Layout" (Hastings), which describes these structures in detail. For the most part, the structures are fairly easy to recognize after you've seen them once or twice. But then there are the bizarre mystery circuits that require some puzzle-solving. For instance, where they combine a couple of transistors to save a bit of space.
Seeing that the latest edition of The Art of Analog Layout came out of 2005, is there any other book you'd recommend on analog layouts specifically?
I'd hazard a guess that the 20 to 50 year old tech described in that book hasn't changed much; anything that is novel is probably still under patent and will someday make it into a newer edition.
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#9Great write-up and drawings. I've heard of fuses in processors, MCUs, etc but not of anti-fuses. Are anti-fuses a feature of older designs, based on large process size?
"The second type of fuse is an "antifuse", which has the opposite behavior: it does not conduct until a high current is applied"
Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference
#10Great write-up and drawings. I've heard of fuses in processors, MCUs, etc but not of anti-fuses. Are anti-fuses a feature of older designs, based on large process size? "The second type of fuse is an "antifuse", which has the opposite behavior: it does not conduct until a high current is applied"
Antifuses are used in modern chips, e.g. FPGAs: https://www.microsemi.com/product-directory/fpga-soc/1641-an...