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LT3080 documentation states under absolute maximum ratings:

SET Pin Voltage (Relative to OUT) ±0.3V

(http://cds.linear.com/docs/en/datasheet/3080fc.pdf page 2)

In the documentation there's also figure named "Lab supply" (bottom of page 18) with 2 LT3080s. I simulated this circuit in LTspice and it seems that in constant voltage mode the current limiting LT has SET about 1V higher then OUT. Similar thing happens with voltage setting LT in constant current mode.

green trace is OUT, blue is SET (both on U1), circuit set to 10V 1A, load draws 10mA

Is this how this should work?

Note 7 about SET pin current states:

SET pin is clamped to the output with diodes. These diodes only carry current under transient overloads

How is this situation supposed not to damage these diodes (in steady state)?

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  • \$\begingroup\$ That first LT3080 is a current limit (of 1 A) see figure on page 17 of spec sheet. \$\endgroup\$ – George Herold Oct 18 '17 at 17:18
  • \$\begingroup\$ Yes, I undarstand that part. My question is why is this configuration exceeding the absolute maximum ratings? \$\endgroup\$ – ja2142 Oct 18 '17 at 21:12
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    \$\begingroup\$ Right, looking at the cartoon of the circuit, when the loop is closed and working it will be less. But when not limiting the current the pass transistor is on hard. It doesn't seem to hurt the part so maybe a bit of a mis-print. You could ask LT, they are pretty good at customer service. (If you really are worried about it.) \$\endgroup\$ – George Herold Oct 19 '17 at 13:39
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I asked LT and they answered me (surprisingly fast) this:

Your point is valid in that the nature of the Lab Supply application circuit violates the SET-OUT abs. max. However, this is the way the circuit is supposed to work. Your simulation results that also sound correct.

In the case of the Lab Supply application circuit, you can ignore the SET-OUT abs. max. This is because SET is not being driven externally to 1V away from OUT. Instead, SET is driven internally by a relatively week 10uA current source which can’t drive enough current through the parasitic OUT-SET diodes to cause the LT3080 to misbehave or damage itself.

So I guess that 0,3V absolute maximum means that I can't use external voltage 0,3V higher or lower then OUT.

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