Reverse-engineering the LM185 voltage reference chip and its bandgap reference

https://www.righto.com/2022/04/reverse-engineering-lm185-voltage.html

by picture • 4 years ago

103 31 4 years ago

kens8• 4 years ago

Author here for all your voltage reference questions :-)

monocasa• 4 years ago

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.

kens8• 4 years ago

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.)

adrian_b2• 4 years ago

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).

cushychicken• 4 years ago

Got any good references for how to identify silicon structures as certain components?

kens8• 4 years ago

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.

colejohnson66• 4 years ago

How do you tell if a chip’s substrate is N or P?

kens8• 4 years ago

There are electrical ways to determine it, but I don't have an easy way to tell.

grishka4• 4 years ago

Why am I getting "access denied" when trying to open your website? I had to use a VPN to get past this.

kens8• 4 years ago

Are you trying to access from Russia? Russia is currently blocked.

antattack• 4 years ago

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"

kens8• 4 years ago

Antifuses are used in modern chips, e.g. FPGAs: https://www.microsemi.com/product-directory/fpga-soc/1641-an...

londons_explore• 4 years ago

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_b2• 4 years ago

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.

beeforpork• 4 years ago

The transistors available in modern digital CMOS processes are worse and worse for analog functions, the more recent that process is.

Does that mean that we have a future ahead of us when a 6581 can be produced again with an authentical 'bad' sound? :-)

exmadscientist3• 4 years ago

In addition to what Adrian said, Atmel built-in references are just notoriously bad. Worst-in-class, you might say!

Other vendors' equivalents are usually (usually!) better.

wanderer• 4 years ago

If the following is not well known to electrical engineers, I would appreciate a little explanation of how it is done:

"Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant"

If it well known, a reference would be helpful.

(K. Shirriff, your posts offer a lot, even to those of us who do not have a technical background. Thank you.)

exmadscientist3• 4 years ago

So the absolute resistance depends on the doping, which is tricky enough to control (furnaces, impurities, voltages, sputtering, etc.) that even with good control, the final resistance is within 20-30% of the target.

The relative resistance comes from the shapes themselves: it's the same material for both resistors, with the same properties from the same doping. And photolithography is great at matching shapes and patterns, so you get fantastic relative tolerancing.

There are, of course, still plenty of other sources of error.

wiml• 4 years ago

It very much is a basic technique you learn in analog circuit design (or at least it was 30 years ago when I was in school). Similarly, there are circuits (like that current mirror) which rely on the nonlinear behaviors of diodes or transistors varying in sync — you can build them on the same chip, or you can carefully select matched sets from one manufacturing run.

An example with resistors is a voltage divider. The output voltage depends on the ratio of resistances not on the absolute values (within reason). The resistors in a long-tailed-pair also need to be matched but their precise value is less important. Or you might have a bias current that's temperature-dependent but in a way that matches the temperature-dependent needs of some other bit of circuitry on the output.

There's a famous early integrated opamp which has a four-way-symmetrical die layout to compensate not only for temperature variations across the die but also process variations.

timerol• 4 years ago

It is well known. "Matched components" are common in analog design. (No two things are ever identical, but things fabricated next to each other are more likely to be much closer, and you can get better tolerances if you're going for a specific ratio.)

As a simple example, a voltage divider has the output voltage depend only on the input voltage and ratios of the resistance values. Vout = R1/(R1+R2)*Vin https://en.m.wikipedia.org/wiki/Voltage_divider

ttul• 4 years ago

In my final year of engineering school, we had to build an op-amp integrated circuit and then characterize its performance in the lab once it came back from manufacturing. My project partner was an A-student, so I had no trouble handing him the first half of the project (design and layout) while I went to Hawaii for a week. Unfortunately, when I returned from my week in the sun, I discovered that he had submitted his design without my name on it. The professor was not sympathetic when he said, “I’m not sure how you’re going to finish this class now, considering that 60% of the mark is based on the project report and you have no chip to characterize…”

Fast forward six weeks or so. On the fateful day when the chips were being handed back to us, my former partner was missing. I faithfully picked up the item and headed to the lab. Plugging the device into a breadboard and hooking up a scope, I determined that it did not, in fact, function as an op-amp. After many painful hours, I determined that the A student had flipped a transistor around. With luck, I was able to characterize what _that_ circuit should do and was then able to show this to the professor.

On the day the project report was due, I brought in two copies: one with my name on it, and one with my name and that of the A student. As he turned pale upon realizing that he had missed picking up the part two weeks earlier, I offered him a chance at redemption: hand in the paper with both of our names on it. He did so, and I was able to graduate.

You’re welcome.

Reverse-engineering the LM185 voltage reference chip and its bandgap reference

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