Ok, it took me a while to understand this circuit. It seems to have no real value compared to using a similar circuit without Rt to directly control the current. Why would anyone want to use this sort of added complexity compared to a series regulator which can work fairly well?
Well, if you have a few volts extra headroom, you could run it from the ground pin of a 78L05 with its output floating. Still SO8, but only one chip. ;) They even come in BGA!
The other thing is a BF862 with a BAT54 in series with the source (gate connected to cathode). That comes in pretty reliably at about 3-5 mA. Using BAT54Cs, that's only 1.5 parts per channel.
317s are good for lots of stuff, because they're pretty hard to blow up. If you protect them from input overvoltage, and don't discharge any caps backwards through their outputs or adjustment pins, about the only thing you can do to kill them is hit them with a hammer or set them on fire.
Of course there are all sorts of creative ways to get input overvoltage--iirc George had a good one a year or so back. ;)
I don't remember an over voltage problem, perhaps you're thinking of my method for defeating the thermal/current shut down of the LM395.. which involved putting the "right" size coil/inductor on the output. Maybe you can blow up a '317 in the same way?
For a 5mA source, ignore the 'thermal connections'; we don't need 1% matching, it's driving unmatched LEDs. Matched pairs aren't expensive (this one is under 0.1 USD in quantity) but it's a different story for quads or higher. So, each of two matched pairs gets an emitter degeneration resistor. 60 ohms or more should do it. The current regulator, probably cheapest to go with LM317 and a (1.25V/.005A) = 250 ohm program resistor. BOM is now: one IC, two dual transistors, three resistors. You could use a quad 120 ohm resistor pack instead (two in series for the LM317).
That data sheet carefully avoids mentioning thermal coupling. It's not even clear if the transistors are electrically isolated from one another. There is no transistor-transistor max voltage spec. There is no single-transistor theta or max power. Thermal runaway could wreck the matching specs.
Here's a thermal image of a similar NEC part:
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One transistor is hot, one is cool. It wouldn't make a very good pure current mirror, little better than two separate transistors.
So, use big emitter resistors.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
The LM317 and LM395 are the same chip with different metallization.
IIRC you hit the thermal limit, the chip tried to turn off, and the inductive kick killed it by overvoltage. Like I said, creative. ;)
I might well have been bitten the same way--one time I had a power stage oscillating so hard that the decoupling bead melted its way right through a Pomona clip.
I can only take half the credit. The lm395 drives a 50 ohm cell heater circuit with banana plugs for the heater. Like the heater, the big coil also has banana jacks, mix in physics students, and it was only a matter of time. :^)
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