Decapping the LT1236LS8

[Andreas]

There are actually 4 fuses you may break via pins:

pin8 - fuse - pin7 - fuse - pin4_GND
pin1 - fuse - pin3 - fuse - pin4_GND

Ok I found some references in my drawer and did some (non destructive testing).
The LT1021 showed all fuses intact.
On the LT1236ACN8-5 the fuse from Pin4 to Pin7 was obviously blown (1.3 KOhm or 0.83V in diode test instead of 1-3 Ohms)
The pins 7+8 have around 9mV against pin 4 in this case.

So the idea was to repair the blown fuse and record the output voltage.
Vout 5000.74 mV with fuse open
Vout 4993.76 mV with fuse repaired (shorted between pin 4+8)
The difference is 7mV so nearly the value (9mV) of pin 7+8 against ground.

with best regards

Andreas

[magic]

Good find. The result is close to theoretical predictions.

It seems that with right resistance between pin 4 and 7, the output could be adjusted to zero. A potentially interesting question is whether thermal coefficient could be adjusted to zero that way. I think it should be affected by this modification to some extent.

[iMo]

This is how it looks with all 4 fuses blown (the resistances estimated from the picture).

[magic]

Slightly more accurate measurement with an image editor on Zeptobars high-res LT1021 image. Distances taken between centers of metal contacts.

R4 = R1
R2 = 0.58 R1
R5 = 0.79 R1

R3 = R6 = 2.34 R1

R4 and R1 appear to be equal and, as said above, could be inserted simultaneously to reduce sensitivity to either R2 or R5.

edit
By the way, these pins may be somewhat ESD sensitive because each goes to an emitter of an NPN whose base is clamped to ground by the base of the other NPN.

Also, the resistors appear to be made of P silicon and their isolation island is biased to the zener cathode. So the pins are one PN junction away from the zener. Applying more than 7V is not recommended

[magic]

I was curious about thermal behavior of this buffer so I whacked together a simplified simulation.

All resistors are scaled proportionally from 1300Ω. Input stage current is set to produce about 9mV at pin 7. Q2 is made of four NPNs because its emitter diameter appears to be 1.9x greater than the others, close enough. The second stage is three NPNs without biasing, it still yields good performance (160dB loop gain at DC) and I mainly cared about getting the right number of diode drops to set the operating point of Q6/Q8 collectors correctly.

Reference voltage is set to produce 5.0006V at the output, close to the chip measured by Andreas. Shorting the missing fuse produces 4.9935V. The difference is -7.1mV, close to the real thing.

Contrary to my initial suspicion, offset voltage of the buffer isn't zero. Maybe my sim is wrong, maybe it's meant to be that way

As predicted, the buffer has significant thermal coefficient: -110uV/°C. That's -22ppm/°C. Shorting the missing fuse adds further -2.5ppm/°C.

Blowing fuses around R3 and R4 at the same time reduces output voltage by 0.8mV and affects tempco by 0.14ppm/°C. I naively expected no change, oh well.
Sorry for misinformation, this was with the fuse between pin 7 and ground blown. If this fuse is present, then blowing fuses P8-P7 and P1-P3 has no effect in simulation. A real world change may occur due to resistor tolerance or Early effect in Q5 and Q8.

[Noopy]

I could sent you a faulty LT1021dmh-10, pm if you're interested.

Got it and done some analysis:

https://richis-lab.de/REF08.htm








- The layers under the metal layer are the same as in the LT1236 but the metal layer is a lot different.

- The "1210" in the metal layer seems to stand for the 10V-type in contrast to the "1205" in the 5V-type.

- The structures don´t really fit to the zeptobars picture of the LT1021-5V.
But the structures fit very well to the circuit diagram in the LT1021 datasheet.
I don´t think the difference is due to the 5V-type.
Strange...

- There is a resistor divider between the zener and the opamp like in the LT1236, which is not shown in the datasheet.

- There are more fusible links than in the LT1236. But not all of them are placed in the "R1-path", some adjust the feedback before and after the buffer.

- And yes, the transistor above the zener is a buffer in this circuit.


Thanks Soulman!