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LP8556: I can’t tell if the output is pulsed even when controlling the level by I2c

Stephen Nashburn
Prodigy 40 points
Part Number: LP8556
Other Parts Discussed in Thread: TPS61169

Hello TI support,

I have been researching multiple backlight LED drivers for controlling a string of LEDS. For example 6 LEDS per channel. I need the LEDs to be turned on at a steady current for 2 seconds, turned off for about 7 seconds and then repeat forever.  We are backlighting a series of semitransparent objects that are passing by a global shutter camera.  Using the exposure control of the camera to (somewhere between 10 and 40usec/image) capture images of the objects as they move past the camera. Expect drive current range is 20 to 40 mA. The part numbers for the LEDS are: "LB P4SG", "ATS2012UV415". We can't use a PWM or pulsed output because it will reduce resolution in each image captured. Therefore, I want to use the pure analog control for controlling the brightness of the LEDs. Since I will be driving the LEDs in a range from 20mA to 40mA, I would think that pure analog control will work since I am driving greater than 25% of the LED current. Please let me know what your thoughts are and if this product will work or if there are any other parts that might be better for my application. I have four LED drivers that I picked out, this one in this support ticket has all three dimming options, PWM control, analog control, and hybrid PWM dimming. In addition, please let me know if there is anymore information that I need to provide to you. 

  • 0
    TI__Genius 9630 points

    hello Nashbu

          This thread has been assigned to me, and I will feedback to you ASAP.

  • 0
    TI__Genius 9630 points

    hello Nashbu

          May I ask how many channels you need? According to your description, pure analog dimming is preferred with the current range 20mA to 40mA.

         Right now we have one channel backlight product TPS61169 for analog dimming only, it can handle up to 400mA LED current.

  • 0 in reply to Mason Wang
    Prodigy 40 points

    I would be using at minimum 3 channels with 6 LEDs in series per channel. This new part you proposed is for white LEDs. Would it be ok to use different colored LEDs? We have a plan to use different colored LEDs at different wavelengths. For example at blue 470nm LED.   I see this proposed part you suggested is a boost topology from the inductor and zener diode in series. I am concerned about that If I use pure analog dimming, how much ripple will occur when starting up, and turning off. Needs to be quick since the camera is capturing images. I see the ripple of the part you proposed is is 9mA/d and the ripple does not seem bad. In addition, it appears to turn on and shut off fast. That is what I need. Multiple channels, pure analog control, little to no ripple, fast turn on time and turn off time, constant current. 

  • 0 in reply to Stephen Nashburn
    Prodigy 40 points

    Schottky Diode. Not Zener diode. My apologies.

  • 0 in reply to Stephen Nashburn
    Prodigy 40 points

    Could you explain to me how the pure analog dimming works for TPS61169? I looked at the data sheet and all it talks about is PWM dimming.... Which is not the same as pure current control. 

  • +1 in reply to Stephen Nashburn
    TI__Genius 9630 points

    hello Nashburn

            The difference between analog dimming and PWM dimming is that analog dimming controls the amplitude of LED current, while the PWM dimming control the LED on and off (LED current high and zero) to adjust average LED current.

          For TPS61169, it control the amplitude of LED current and the current reference is set by the Ctrl Pin. 

         LED current= Imax*D

        D refers to the input PWM duty cycle of Ctrl Pin.