[HN Gopher] Reverse-engineering the LM185 voltage reference chip... ___________________________________________________________________ Reverse-engineering the LM185 voltage reference chip and its bandgap reference Author : picture Score : 58 points Date : 2022-04-09 18:24 UTC (4 hours ago) (HTM) web link (www.righto.com) (TXT) w3m dump (www.righto.com) | antattack wrote: | Great 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"_ | kens wrote: | Antifuses are used in modern chips, e.g. FPGAs: | https://www.microsemi.com/product-directory/fpga-soc/1641-an... | kens wrote: | Author here for all your voltage reference questions :-) | colejohnson66 wrote: | How do you tell if a chip's substrate is N or P? | monocasa wrote: | 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. | adrian_b wrote: | For this case of lateral PNP transistors, the reason is as | Ken has said. | | Because both the emitter and the collector are on the | surface, making them both circular ensures that the distance | between them, which is the same as the width of the base, is | constant. | | The properties of the bipolar transistors vary very strongly | with the width of the base. If the width is not constant, | then the current becomes crowded in only a part of the base | and many characteristics become worse. | | Unlike in lateral transistors, in vertical transistors the | width of the base is not determined by geometry, but by | doping doses and diffusion times, so the form of the emitter | is less important. | | Nonetheless, in early planar transistors the emitter was also | circular. The reason is that in bipolar transistors with very | narrow bases, the resistance of the narrow base layer becomes | large and in the center of the base under the emitter, the | base-emitter voltage drops to a lower value than at the | terminals of the transistor, which makes the central part of | the emitter and base non-functional (i.e. only a very small | fraction of the current passes through there). | | So in vertical transistors, only the periphery of the emitter | matters. When it is circular, the symmetry guarantees that | the current is uniformly distributed on the periphery, for | maximum current capability. | | Unfortunately, increasing the density of current per | peripheral length of the emitter over a threshold triggers a | positive feedback that will destroy the transistor if the | current is not limited externally. This is usually the main | factor that determines the specification of a maximum current | for a bipolar transistor. If the current is non-uniform over | the periphery, the threshold will be reached at a much lower | current than computed by multiplying the threshold density | with emitter perimeter. | | Because there is a limit for amperes per millimeter of | emitter periphery, to increase the maximum current in a given | area, the form of the emitter must be changed from a circle | to a form with a longer perimeter, without increasing the | occupied area. | | Early power transistors had various fancy forms for the | emitters, e.g. Christmas tree, snow flake and so on. However, | it was quite difficult to ensure that the current is | distributed uniformly on the periphery of such complex forms. | | Later, after the photolithography had improved and smaller | dimensions were no longer problem, instead of having an | emitter with a complex sinuous boundary, 2 simpler solutions | have been adopted to increase the perimeter of the emitter. | Either the transistor had a large number of small emitters | connected in parallel, or it had one large emitter, but with | a large number of small holes in the emitter (mesh emitter). | kens wrote: | 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.) | cushychicken wrote: | Got any good references for how to identify silicon structures | as certain components? | kens wrote: | 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. | dasudasu wrote: | 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? | DiabloD3 wrote: | 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. | kens wrote: | I mostly look at chips from the 1970s, since modern chips | have features that are too small for my microscope. So I | don't have any recommendations for a "modern" book. | londons_explore wrote: | Lots of microcontrollers have a bandgap reference built in... And | typically they have really rather terrible voltage tolerances - | eg. on atmel devices the 1.1v reference doesn't even have | guaranteed minimum and maximum voltages across the whole range of | supply voltage and temperature, but you can only expect it to be | somewhere between 0.9 volts and 1.3 volts.... | | So why are these circuits so bad? Do they use a different design? | adrian_b wrote: | The transistors available in modern digital CMOS processes are | worse and worse for analog functions, the more recent that | process is. | | The very poor device characteristics may be mitigated only | using very complex schematics for the analog circuit, together | with various auto-calibration methods. | | The additional cost may be deemed too much for a voltage | reference in a cheap microcontroller. | | One can always use a good external voltage reference, but that | may cost as much as a microcontroller. ___________________________________________________________________ (page generated 2022-04-09 23:00 UTC)