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LP8863-Q1: High dimming ratio implement

Terry Wu2
Expert 2110 points

Part Number: LP8863-Q1

Hello Sirs,

May I know how to use LP8863 EVM to get 10,000 :1 dimming ratio ?

I can't enter very low duty into GUI . LP8863 EVM.pptx

Pls refer attach file .  TKS !

  • 0
    TI__Intellectual 2180 points
    Hello,

    Unfortunately, 0.1% is minimum resolution allowed by uController on EVM. In order to achieve lower PWM input %s, you'll need to use an external function generator and connect directly to PWM input for this case.
  • 0 in reply to Salome Ramirez Carrion
    Expert 2110 points

    Hi Salome,

    Should I rework something in EVM before external PWM signal feed into EVM J32 pin4 (SDOPWM) ?

    So I could get high dimming ratio just to use external 152Hz PWM input , right ?

    TKS !

  • 0 in reply to Terry Wu2
    TI__Intellectual 2180 points
    Hi,

    Yes please remove R81 or R38 resistors to disconnect PWM pin from uController and then connect PWM signal of function generator at J32 pin (SDO_PWM) and GND.

    And yes, you are correct higher dimming ratio will be achieved with lowest PWM input frequency set.
  • 0 in reply to Salome Ramirez Carrion
    Expert 2110 points
    Hi Salome,

    Thanks your help ! Now we tie LED0~2 together to get 375mA/string max LED current .

    - May I know how to adjust LED current ? I need to measure max and min brightness by optical scope .
    Iset is to determine max LED current . Is LEDx_current[11:0] this register the keypoint ? If yes, where I need to adjust from EVM GUI ?
    - Could you help to explain Cluster_Brightness and Grouping as EVM GUI ?

    TKS a lot !
  • 0 in reply to Terry Wu2
    TI__Intellectual 2180 points
    Hi Terry,

    For 375mA total from 3 channels you would have to set each LED output to 125mA by setting RISET = ~25kOhm. This is what sets your max output current at 100% brightness (100% PWM duty). To adjust the brightness you change the PWM input duty which will be reflected also on the output PWM of the LEDs. So setting 50% PWM input duty in your case would result in an also setting 50% duty at the output with a max current of 125mA, resulting in an average current of 125*0.5 = 62.5mA per channel.
    On top of setting max current with RISET resistor for all channels, you can also set individual max current per channel using LEDx_current registers. This is mainly used if there is a high LED Vf variation between channels and customer user needs to match variation better so can set max current slightly less/more for each one. Also this is used in the case of using single LP8863-Q1 device for driving two different displays. For example you want to drive display1 with 3 channels (0-2) and display2 with other 3 channels(3-5) at different currents. In this case you would set first 3 channels to GROUP 0 and set max current with Region 0 Current Register and assign other 3 channels to GROUP 1 and set that max current with Region 1 Current Register.
    If you are using all strings for single display then most likely you only need to set max current with RISET resistor and use PWM duty to dim the LEDs.

    Hope this helps.