Is there a way to remove the Vbe dependence on bipolar current limiters, or at least, lessen the effects of process variation & mismatch in the topology below?
Vin is nominal 22V, but could have transient spikes of 50V for < 100us. The circuit below limits current through Rload to ~4mA, but that is dependent on the Vbe(ON) of the darlington pair. A small change in Vbe will affect the limiting threshold for a set Rsense.
Is there a method to improve this current limiting circuit so that current through Rload is more closely matched to Rsense rather than both Rsense & Vbe(ON) ?
Best Answer
Here is better but still simple current source. It has 3.5MΩ output impedance and 0.002%/K temperature drift. Well, at least in theory. All BJTs must be placed in the same conditions or better in one package in order to get minimal temperature drift:
BTW: Replacing Q3 with low power P-MOSFET can raise the output impedance hundred times and make it in the GΩ range.
Another interesting schematic of current source that can be used as a current limiter is 2-pole device that limits the current flowing through it. It has lower output impedance than the above schematic, but still very good temperature characteristics:
NOTE: Used in the simulations voltage reference IC can be replaced with TL431.