LTC3649 step down controller (lab supply) add current limit feedback led?
That circuit uses a linear regulator on the output (LT3086) to smooth out the power to the circuit.  This means there are two different ways to do the current limiting:  through the LT3086, and through the LTC3649.

The LT3086 has a current limit that calls itself a "precision current limit", so it is probably best to use that.

The datasheet for the LT3086 says that the current limit is enabled when the voltage at the ILIM pin is >= 0.8V.  Thus, if you can read the voltage at the ILIM pin, you can tell if device is current limited.

The brute force way to do this is with a comparator, using the comparator to compare the voltage at the ILIM pin to a 0.8V reference voltage.  If the voltage is high, then turn on an LED, if the voltage is low, then turn it off.







simulate this circuit &ndash; Schematic created using CircuitLab

Where INTVcc is a ~3.5V voltage from the LT3649 that will power this circuit, Vref is some 0.8V reference voltage (check mouser), R1 sets the current through the diode, and Q1 is a pnp transistor with a suitably low Vce(sat) so that 

V_D1 + I_diode*R1 + Vce(sat)_Q1 ~ V_intvcc


Note: Q1 should have a |Vbc(on)| that is smaller than 3.5V.