LED Driver Design with PWM Dimming - Page 1
Forum section: electronics / projects

author: ipek.grgc
author_rank: Contributor
timestamp: April 11, 2023, 05:35:38 pm
content: Hello everyone,I have designed a Texas Instruments LM Series LED Driver Board. Regardless of the dimming PWM signal I give (from the DIM pin) the driver fixes the constant current output at 539 mAmps. I am afraid I made a design mistake because of something I did not understand fully.There is an Riadj resistor in the datasheet, which connects the Iadj pin of the driver to GND. Iadj is described as "Current Adjustment Pin. Connect resistor Riadj from this pin to ground to Iadj. Adjust the average output current." So I thought I could pick the average current value my LEDs will be working with, and picked this resistor value 6.25kOhms which makes Iout 500mAmps according to the table in the datasheet. I did not think that this would fix the output to this current value regardless of what the pwm input signal given from the DIM pin.Does anyone know how can I have full control over the current output? Did I choose a wrong type of LED Driver or Topology? Doesn't it have an adjustable range? My LEDs can work between 180mAmps and 1Amps. How do I dim them?Thank you,My bests.

author: TZ400T3B
author_rank: Contributor
timestamp: April 11, 2023, 06:30:18 pm
content: Have you read the datasheet? DIM is internally pulled HIGH. Unless you pull it down with whatever signal you are using for dimming, it will not translate into output current waveform. There is not really any mistake to be made. Try pulling DIM to GND if LED turn off, just to check function. Then design your dimmer circuit, so when PWM is low, it pulls DIM pin low. Even two transistors would work.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 11, 2023, 06:43:44 pm
content: The DIM pin does not adjust the current, it just turns the driver on/off

author: ipek.grgc
author_rank: Contributor
timestamp: April 12, 2023, 03:35:28 pm
content: Thank you all for the answers. Problems Solved. Turns out there were two.1st Problem and SolutionRegarding TZ400T3B's suggestion, I have tried to pull the DIM pin GND and nothing happened. Then I realized instead of using Arduino's GND (I'm providing the PWM signal for the DIM pin from Arduino also) if I use LED drivers own GND it turns off. Which made me think "Aha! The grounds of these two boards are separate so this is why the PWM signal I give is not working as it should be."This solution turned out to be true. I have powered up both Arduino and my LED driver from the same supply where they could also share the ground. Now, I am able to control the brightness of the LEDs via the DIM pin by just using the basic " analogWrite(PinName, 150); " command. With different values of the PWM, I get different brightness levels.2nd Problem and SolutionThe resistor RIADJ is setting the "Maximum Output Current." I had a parallel SMD LED spot on my PCB in case I wanted to change the value of this resistor. I soldered a 10K parallel resistor to the 6.2K resistor. Which led to a higher maximum output current rating ( according to the datasheet).Now I'm able to manipulate the brightness levels of the output LEDs in full scale from 180mAmps to 950mAmps. Everything works as it should be.P.S. I also got a little help from ChatGPT.