Electronics > Projects, Designs, and Technical Stuff

Voltage regulators - die pictures

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Noopy:
Are you interested in more die pictures?
I have taken some pictures of a LM317 and soon will upload a high-current voltage regulator.
If you want me to post such pictures here to cheer you and discuss them give me a thumps up.  ;)

And now the LM317:



It´s a 1991 National Semiconductor part in a TO39 package.








Here you can see the bandgap reference (transistor ratio 1:10). A modification of the metal layer can change the ratio to 2:10.




They use two metal fuses and one zener fuse for adjusting the reference voltage.
Metal fuses are nc, zener fuses are no. I assume that they tried to get a midposition after production so they don´t have to open too much metal fuses. Opening metal fuses leads to contamination of the test needles.




I assume the circuit on the left side modifies the bias of the Regulator curcuits depending on the voltage over the LM317. Can somebody confirm that?  :-/O


Some more pictures on my website:

https://www.richis-lab.de/LM317_01.htm

 :popcorn:

magic:
On the schematic, Q2~Q5 looks like the "constant gm bias" circuit. I suppose Q3 has more emitter area than Q5 while Q2 and Q4 and their emitter resistors are identical. It's a trick to generate currents through Q3 and Q5 such that transconductance of those two and other similar transistors maintains some defined value regardless of die temperature.

Q1 and R6 provide a small current which starts the bias generator. Otherwise neither the NPNs nor the PNPs would turn on by themselves because their bases are driven by each other. D1 seems to limit this current when Vin-Vout is high.

edit
Somewhere, there has to be thermal shutdown too.

Noopy:

--- Quote from: magic on June 08, 2020, 07:27:54 am ---On the schematic, Q2~Q5 looks like the "constant gm bias" circuit. I suppose Q3 has more emitter area than Q5 while Q2 and Q4 and their emitter resistors are identical. It's a trick to generate currents through Q3 and Q5 such that transconductance of those two and other similar transistors maintains some defined value regardless of die temperature.

Q1 and R6 provide a small current which starts the bias generator. Otherwise neither the NPNs nor the PNPs would turn on by themselves because their bases are driven by each other. D1 seems to limit this current when Vin-Vout is high.

edit
Somewhere, there has to be thermal shutdown too.

--- End quote ---

Thank you very much, magic.  :-+

I have nothing more to add.  :)

Noopy:
But wait, one more sentence!

With your explanation an some closer examination of the red part it seems to me the red part has to do the thermal shutdown. It can shut down the linear regulator and Q6, Q7, Q9 are somewhat crazy wired. That part has to be the thermal shutdown. Although I have to think about the operation principle.

magic:
Well, Q6 is just an emitter follower. If Q7 Vbe decreases 2mV/°C due to temperature, R10 voltage increases equally and R8 voltage increases 6mV/°C. Q9 Vbe also decreases 2mV/°C so we have 8mV/°C equivalent increase of Q9 base drive. This goes to Q11 and turns it on.

All of that assumes that Q7 base is held at constant voltage which is probably not exactly true, but I suppose similar principles apply. If this circuit isn't doing thermal limiting then I have no idea what else it could do.

I'm also ignoring that the increased Q9 current reduces Q6 current, this is hopefully insignificant.

Corollary: there is no real shutdown with hysteresis, it just throttles the output so as to maintain roughly constant die temperature.

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