I simulated this circuit from a reference design but I'm not entirely sure how it's working or how you would go about designing such a thing. In simulation it looks like it's designed to hold the current through D1 constant at around 5mA despite having an input voltage range of up to 25V.
I see the gate voltage for M1 is held at about 1.6V, and the base voltage for the BJT rises as the input voltage rises. So as the voltage rises the current through the BJT increases so it's acting like an adjustable impedance there I guess to hold the gate voltage constant. Is that right?
Is this the kind of thing you just do in spice or is some kind of current mirror circuit that's well defined somewhere and I just don't recognize it?