# LTC3649 step down controller (lab supply) add current limit feedback led?

On the 20th page LTC3649 there is a good typical application picture. How can I add a led if the set current equal the actual current? It is possible? Thanks.

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 – 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.

• Ok, thanks. I would prefer using only LTC3649, and not using LT3086 because if i power something with 1.2V/2A and the input voltage is 24V, a lot of heat is generated, so i would like to use LTC3649 only. In this case how can I add a feedback led for current limit? – Bruk Sep 1 '16 at 18:10
• Same thing, but this time, you are looking at IMON and comparing it to 2V. The end of page 5 discusses this. – Andrew Spott Sep 1 '16 at 18:13
• Super! I read the datasheet and will do it this. What is the good PNP transistor for this project? Could you link one please? – Bruk Sep 1 '16 at 19:27
• Any pnp that has the characteristics I mentioned: low Vbc and Vce(sat). Go on mouser or digikey and plug in those values. – Andrew Spott Sep 1 '16 at 19:31
• Ok, I think it is good: BC369 Agree? – Bruk Sep 1 '16 at 19:55

I believe you set the output current limit via the Iset pin and there is also an Imon pin which indicates the actual current through the inductor. Basically, if A>B then turn the LED on via a comparator. You will need to adjust the resistor connected to Imon to scale it to be in the same range as Iset this might be a tad fiddly.

Alternatively the Pgood output will tell you when the output is failing to be in regulation and this could also indicate that current limit is occuring.