Voltage reference IC teardown stuff: pictures, analyses, links - Page 1
Forum section: electronics / general

author: magic
author_rank: Super Contributor
timestamp: August 16, 2021, 07:16:28 am
content: It occurred to me that it is perhaps not an ideal situation when attempts to figure out the internal workings of reference ICs end up polluting random threads, such as "T.C. + Hysteresis measurements on LT1027"It would be better to have one place where all that information can be found and to leave unrelated threads alone. Toward that end, I am going to index here all die images and schematics which have been published on the forum and elsewhere that are known to me. And from now on, I will use this thread for my own posts related to IC internals that don't really have a better place to go. I hope that from time to time I will be able to produce something not seen before. I plan to start with more details on LT1027 soon.Advanced zenersThese have their dedicated threads so I'm leaving them alone.Integrated buried zenersAnalog AD586Crude image of top metal layer (there are two) is included in old revisions of the datasheet. Good enough for identification, I suppose.Likely similar circuit to 587/588.Analog AD587NoopyDiscussion and partial schematic in this threadAnalog AD588Noopy, rev. ANoopy, rev. CDiscussion in this threadFull schematic of rev. A in this threadLinear LT1021-5ZeptobarsApparently same silicon as LT1236LS8.Linear LT1021-10NoopyApparently same silicon as LT1021-5/LT1236 with different metal layer.Linear LT1027Die image in this threadAnother image and discussion further in the threadNoopy99% schematic down belowLinear LT1236LS8NoopyDedicated teardown thread, images and partial schematicMaxim MAX6250Fragment of a broken die in this postAlmost identical to MAX6325 but ESD diodes are different - not the same silicon.Maxim MAX6325NoopyDiscussion in this threadBandgap referencesNot sure if I want to catalog all of them. There is way too many.Multiple images in Noopy's threadNoopy's website indexAnalog AD584ZeptobarsVarious TL431Zeptobars; won't bother linking them. Some use a different reference circuit than TI.Ken Shirriff (useful introduction to reverse engineering bipolar circuits in general)Chinese Wing Shing TL431 die image and schematic in this post, alternative topologyA short summary of the various buried zenersAnaloguses a fairly straightforward reference circuit, laser trimmed for tempco and bootstrapped to the 10V output (what about AD586?) to achieve PSRR. Residual tempco is S-shaped according to datasheets. The reference is scaled and buffered by a 741-style amplifier.Burr-Brownis a mystery so far, it seems. Supposedly laser trimmed. REF102 datasheet shows S-shaped curve.Linearlikes to drive the reference diode with a high-PSRR "super mirror" that permits relatively low dropout voltage in 5V versions. Trimming by fuses and complex thermal compensation schemes are employed, resulting in higher order drift curves according to datasheets.Maximis not fully understood at this time. Seems to contain multiple buried zeners and CMOS circuitry of unknown function.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2021, 12:45:50 pm
content: And here's the first delivery of the promised LT1027 analysis, showing some really cool analog wizardry from Linear TechnologyTo recap, the reference voltage produced in this IC is a weighted average of a buried zener and a Vbe multiplier. This is shown in the datasheet. The topmost resistor is adjustable by fuses, changing the weighting ratio and hence output voltage and drift. Additional final adjustment is possible by zener zaps on the NC pins, which affects the buffer amplifier and its feedback network.The old LT1021-5 is fairly similar. The input stage of its buffer amplifier is deliberately unbalanced, causing offset and drift (which cancels with an opposite drift of the reference cell). The exact amount of offset and drift can be slightly altered by blowing fuses on the NC pins, but their relationship is fixed and hence two grades are available: one adjusted for minimum drift and one adjusted for accuracy at 25°C.LT1027 is an evolution of this idea, where some serious thought and a fair bit of die area too have been devoted to making an amplifier whose drift can be controlled without incurring offset voltage at 25°C. Of courseThe devil hides in the details, which in this case is the bias circuit not shown in the datasheet. So here's the bias circuit.I0, Q0, D0are simply a startup trigger. They prevent an otherwise valid mode of operation where all transistors are cut off and only leakage currents flow through the circuit. Q0 is reverse biased in normal operation.Q1~Q5, R1~R5are a very simple Brokaw cell bandgap reference. Transistor area ratio (4:1) and resistor ratios are determined approximately by eyeballing and simulate too well to be a coincidence. I only cheated by choosing a jellybean transistor model which works best with those component values, but other models were close too.Q5, R5is also a constant current sink.Q6~Q8, R6~R7is where the magic starts to happen. I don't fully understand this circuit, but it clearly splits Q5 current into two parts depending on Q6 Vbe. The differential voltage between Q6 base and collector becomes a logarithm of the split ratio due to Q7 and Q8 emitters. And the really cool part is that the circuit is designed to make the difference zero at 25°C. Simulation shows 30~50°C depending on model choice and I can bring it down near 25°C by changing R5 to 19kΩ. I need to think more about this circuit but this doesn't look like a coincidence at all.Q10~Q13split and remix two currentsI1, I2into emitters of the input pair (represented by diodesD1, D2) as commanded by the former circuit block.Nothing exciting happens when the two currents are equal. But when they are unequal, the output currents become unbalanced away from 25°C and we get this beautiful straight line as their ratioThe circuit is almost ready as shown above, because linear current ratio drift is converted into linear offset drift by the logarithmic nature of the input transistor BE junctions. However, the thermal voltage which appears in the logarithms is not constant but proportional to absolute temperature. This is compensated by making the current sink also PTAT, as below. Now the ratio of input stage currents isn't linear (not show), but the input offset voltage should be. PTAT is the usual way of biasing opamp input stages so there is nothing extraordinary about this part.At last, we can look if the circuit would still work if we replace the constant sink Q5 with a PTAT sink. It can be made similar, but it's not linear anymore. I take this as a further bit of evidence that the Brokaw-like circuit really is a proper Brokaw bandgap reference, designed for thermal stability.

author: magic
author_rank: Super Contributor
timestamp: August 18, 2021, 08:02:56 am
content: A small correction is in order. My offset calculations would be valid if the current unbalancing circuit were placed in the collector load of the input stage, as in LT1021. However, the two current sinks draw current from the emitters of the input pair (nodesXandY) and the collectors are loaded with an ordinary 1:1 mirror. Thereforeinput stage collector currents and base-emitter voltages are equalbecause they need to be for the output to be in regulation.What really happens is that there is a resistor betweenXandYwhich bridges the two emitters and their current sinks and permits equalization of currents. (Also controls open loop gain). Therefore offset voltage produced by the unbalanced bias generator is simply the voltage across the balancing resistor.Offset voltage is proportional to the difference between currents from the unbalancer, not to the logarithm of their ratio or anything like that.I gave up on simulating the exact characteristic of this circuit because adding a model of the input stage introduced its own drift and nonlinearity. Turns out, a single stage opamp which doesn't equalize collector voltages of the input pair or cancel mirror base current errors is not exactly perfectBottom line:the unbalancer induces temperature-variable offset voltage in the input stage. At one particular temperature, the offset voltage is zero. This happens when Q7,Q8 currents and emitter voltages are equal and diff pairs Q10,Q11 and Q12,Q13 are driven equally. This is most likely intended to happen at 25°C.The critical part is the Q6,R6,R7 "gadget" which splits Q5 current. Zero offset occurs when the current is split equally. Since R5 voltage is 1.2V and R7 is similar value to R5, this occurs when R7 voltage is near 0.6V and R6 near 30mV. So it depends on the absolute value of Q6 Vbe. If Linear can control Vbe to ±20mV then zero offset point is controlled to ±10°C and so on.And now the factory adjustment networks are starting to make sense. This is what I think hides under those damn bond wires in the north-east corner (drift adjustment):And in the south-east corner (gain adjustment):And a schematic which puts things into context. The drift adjustment Y-network is loaded with two equal current sources and controls the Q14,Q15 splitter in the input stage unbalancer, IN- is the inverting input of the opamp. OUTS is the output sense pad separate from OUTF, GNDS comes from a dedicated ground sense pad for precision circuits.The resistor networks are pretty self-explanatory, I think. The zeners can be turned into shorts by "zapping" them with a voltage of appropriate polarity between appropriate pins - many combinations are possible. DZXXL are the weird devices that Noopy wondered about. I still cannot explain their structure, but due to circuit constraints there is only one thing that they can possibly be: large zeners, acting as isolation between voltages up to a few volts while being able to conduct zapping currents in either direction, without getting zapped. Fairly cleverAnd now we know why there is 5V absolute maximum rating on the TRIM pin.ZAP!

author: magic
author_rank: Super Contributor
timestamp: August 18, 2021, 01:39:52 pm
content: Aaand... it's doneDashed boxes with ?? are stuff I can't figure out (obscured by bonding wires or just too weird).Dotted boxes are common centroid matched pairs.Adjustable resistors are drawn as pots; not gonna bother with all those fuses.Operating points are approximate, assuming Vbe=0.6V, Vzener=6.3V and using resistance ratios.I0a, I0b, Q0a, Q0b, etc- startup circuits, reverse biased in operation.DZ1, Q1, R1~R5- the core reference cell.Q2, Q3- thermal compensation apparently.Q4- I don't get it, seems to barely influence anything. Unloads Q10 to reduce its base current?Q5~Q9- suspected bandgap reference, biases everything including the core reference.Q10~Q14- familiar LTC super mirror with 3x current gain: Q10 sinks mirror base current, Q11 stabilizes its Vce, Q13 is the feedback output.Q15~Q17- drives an internal supply for the opamp, follows zener voltage.Q18~Q20- some more thermal compensation, it seems.Q21~Q23- that's the gadget, running on current from Q9.Q24~Q27- mirrors the same Q9 current at 2×50% ratio downwards.R25~R27- the drift adjustment network.Q28~Q34- input stage bias generator and unbalancer.Q35~Q40- the whole opamp gain stageQ41~Q50- the output stage, high side current limiting unclear.R39~R44- amp feedback, of course.I thought that perhaps it would be possible to figure out the details of thermal compensation and curvature correction, but I give up.

author: magic
author_rank: Super Contributor
timestamp: August 22, 2021, 02:32:12 pm
content: Another correctionI misidentified Q2 collector originally, it is fixed now. The Q2,Q3 and Q18~Q20 circuits must be thermal compensation - they simply sink some current from two taps in R1 and the current sunk just can't be stable or even linear over temperature given how those circuits look. I tried simulating it but that isn't going anywhere without knowledge of absolute resistor values and Vbe.I also added the NC pins to the schematic as taps in appropriate resistors.And now to the old AD587. I added transistor area ratios and resistor values to make a working simulation. I started from R8=4kΩ as given in the datasheet and calculated sheet resistance of 930Ω/sq based on its proportions. Other resistor values are then derived from that (but R2 is trimmed for good output in simulator). For sanity check I tested output impedance of my AD588 and found it to be 4.7kΩ, which I consider close enough because output impedance of the reference node before R8 is predicted almost 1kΩ.I picked some generic 6.2V zener model from LTspice component library which exhibits suitable thermal drift of +2.4mV/K. For whatever reasons though, this simulation isn't working very well too. The curve is an inverted parabola similar to the results shownhere, which is not entirely surprising because the circuits are very similar. But it doesn't agree with the datasheetI considered adding an external "tweak" resistor, shown below as disconnected. Loading the NR pin like that "steals" some Q3 current away from Q1, increasing its drift. (Q3 is not an emitter follower, more like a common base stage - its current is fixed by Q7). The value shown simulates -5ppm/K change in T.C. (real effect unknown) at the cost of -2.5% output droop - not very exciting. A heavier 100kΩ load shuts down my AD588.