LM3409: Can NOT get load

Hello Kevin,

I'll need more information.

What is Vin? VCC1?

What is SET2 tied to?  voltage?

What is I017 set to?  voltage?

What is tied to the load?  Vf and number of LEDs or overall LED stack voltage?

What is the expected output current?

Best Regards,

What is Vin? VCC1?

1.The Vin is VCC1 and is about 24V;

What is SET2 tied to?  voltage?

2.SET2 is tied to the control voltage and is about 1.4V;

What is I017 set to?  voltage?

3.IO17 is connected to the ENABLE control signal ,is 5V;

What is tied to the load?  Vf and number of LEDs or overall LED stack voltage?

4.the load is 4 LEDs; 

What is the expected output current?

5 the output current is 1.2A

Hello Kevin,

I am assuming the LED Vf is about 3V for these calculations or Vled stack is 12V.  The current ripple is very high to the point the LM3409 is running discontinuous.  The peak current threshold needs to increase to get to 1.2A LED regulation.  Either SET2 needs to increase to 2.2V or Rcs need to drop from 0.1 ohm to 0.0635 ohm.  It should still regulate current as it is setup however the average will be 0.39A.

There is a lot of capacitance on the output.  Have you looked at the output voltage with an oscilloscope?  Also look at Voff (voltage on Coff).  If the voltage on the output cannot rise high enough the LM3409 will run in maximum off-time.  If the output cannot rise during startup it will remain in this mode.  A larger inductor can help with this or the increase in peak current trip threshold which needs to happen anyway.

Have you checked the LED string with a lab supply to make sure it works and the voltage of LED string is correct?

It looks like there isn't enough energy for the LM3409 to start with on pulse so if anything is loading the output at low voltage it may not start.  This is calculated by energy in the inductor at peak current verses voltage rise on the output capacitor from that energy.

Calculations:

Best Regards,