MC34844AEP-How to generate a constant output

JCgor · ‎07-09-2023

Hi

I am trying to use MC34844AEP to generate 53.8V output to a 8 channels' LED backlight display from a 12V input. Imax of each channel is 80mA. Now, I have the LED conntrol board, where there is a MC34844AEP working on, but I haven't had the LED backlight unit yet. Therefore, I just test the MC34844AEP without LED now.

1.The Vout of the IC is 62V by tested, but not around 53.8V as I expected. I am wonder which part or parameter is used to determine the voltage output of this IC.

2.Also, The the rising edge is not monotonous as the attached waveform shown.

3.I used a FPGA to gennerated PWM to control brightness of LED. I can change the duty cycle of PWM to change the ouput current of MC34844. In result, the brightness of LED got change. Is my understanding correct?

Below I am attaching my circuit design for your refernce, and these are my design requirement.

Vin = 12V, Vout = 53.5V, Voutmax = 59.4V, Ioutmax = 80mA per channel

I need help about what will the causes for this problem and how can I resolve this.

Thank you so much

JCgor · ‎07-13-2023

I set the LED controller in I2C slave mode, and I connceted a LED board between output voltage and LED channel. The LED can flash when power on, but the light will goes out after LED flash 2-3 times. I measured VDC1/2/3, and they worked fine, and MC34844 can be found through I2C. I also can write register. I try set the EN bit, SetI2C bit, PWR_ON bit, but the LED cannot turn on again. I measured the VDC1/2/3 again, and found out their voltage become zero. I seperated the LED Controller and tested in individually. The result is that VDC1 and VDC2 are short circuit to ground.

Please help. Thank you

guoweisun · ‎07-09-2023

1:You'd better mount the LEDs for testing, because of the VOUT will change according the value of current flow into LEDs. Same with your quesitons2.

2:.I used a FPGA to gennerated PWM to control brightness of LED. I can change the duty cycle of PWM to change the ouput current of MC34844. In result, the brightness of LED got change. Is my understanding correct？

[gw]Correct.