current limiting

I have a project that involves current sensing at 15 amps at room temperature, with a temperature response that will reduce the current limit setpoint as temperature increases. I need a sharp on-off response, not a proportional output signal. I thought to use the circuit below, with R2/R1 creating an offset according to the Schockley diode equation. The equation's thermal voltage term will give the circuit the same temperature coefficient as a diode. v out will go high when Vs = VT ln (R2/R3), where VT= 26 mV at room temp and varies as the reciprocal of absolute temp.

My concern is keeping the collector current through the transistors on the left low enough to avoid self-heating. Current through the left side of the circuit will be about a couple of orders of magnitude higher than on the right. How low do I have to keep it so that the left-hand transistors won't experience self-heating? I could probably live with a degree or three Kelvin. That induces less than

1% error if my thinking is correct. I'm sure a lot of you people have used the BCV mirrors extensively.

supply---+-----Rs-----+---load | | e\\ BCV62 /e |----+---| c/ | \\ | | | out---+ '-----+ | | R1 R2 | | | ,-----+ | | | c\\ | /c |----+---| e/ BCV61 \\ | | '------+-----' | gnd

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Michael Robinson
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