I'm using an AL8843 LED driver and have assembled the circuit according to the datasheet. I used a 390mΩ Rset resistor to set the output current to 256mA. As per the datasheet, I left the CTRL pin floating to operate the driver at maximum power.

However, after powering up the circuit (Input Voltage: 29.5V), I measured 0.330V on the CTRL pin with a multimeter, and the LED is not running at full power. Since the pin is floating, shouldn't it be pulled high internally? What could be the reason for this voltage drop?

Any insights would be appreciated.

 

I am not familiar with that particular IC device, but it seems that has some linear types of functions.
VERY FEW POINTS IN ACTIVE LINEAR DEVICES ARE AT "ZERO VOLTS". Linear systems tend to always be at some potential because of the nature of linear semiconductor devices.
What does the extended data sheet say about the function and driving of the "control" pin?? It will either sink or source current as some function is controlled, is my guess.. Thus it will have some voltage present during the IC's operation.

 

According to the datasheet, the control pin should be left floating for normal operation. It states that when left open, the device should operate at maximum power by referencing its internal voltage. Even though I have followed these procedures strictly according to the datasheet, I am still not getting the expected output. Why might the IC fail to reach full power despite the pin being left floating?

 

I measured 0.330V on the CTRL pin with a multimeter,

0.3V gives low current. Have you tried pulling the input higher? I don't know why it is not at 2.5V.

It is common for the wrong inductor to be used. The inductor might be saturating from too much current. Please tell us all you can about the inductor.

 

0.3V gives low current. Have you tried pulling the input higher? I don't know why it is not at 2.5V.

It is common for the wrong inductor to be used. The inductor might be saturating from too much current. Please tell us all you can about the inductor.

I used 33uH 1.7A Inductor called SRN8040-330M. I want the use CTRL pin at full power and stable output for my leds.

 

I don't know why the CTRL pin is only 0.3V. Try adding a 10k pull up resistor to the supply. Or somehow gently pull up.

That is a good inductor.

 

and the LED is not running at full power.

How do you figure that?
Meaning have you measured the current through the LED?
Is 256ma enough to power the LED at max illumination?
Link to the LED.

 

How do you figure that?
Meaning have you measured the current through the LED?
Is 256ma enough to power the LED at max illumination?
Link to the LED.

I haven't measured the exact current through the LED with a multimeter yet, but I can visually tell that the brightness is quite low when they are connected.

Also, I am running into a new issue: when I drive the CTRL pin with 3V, the LEDs don't stay on solidly. Instead, they continuously turn on and off (cycling/blinking).

As for the link to the LEDs, unfortunately I don't have one because I salvaged (desoldered) them from another board

 

Also, I am running into a new issue: when I drive the CTRL pin with 3V,

What is this drive circuit exactly?

 

what is your construction method? are you sure that thermal pad is soldered and has enough copper for cooling? without proper cooling many products will limit output - this product included (OTP = over temperature protection). is the board clean (no leakage current)? how did you size inductor? datasheet states it should be in 33-100uH and you value at the bottom end of that range. what diode you use? it need to be fast Schottky. next is load... you need to know the load details to properly size the inductor.