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LP8860-Q1EVM: Cluster mode analog dimming of 4 separate LED strings

Dave Bell1
Intellectual 390 points

Part Number: LP8860-Q1EVM

I recently purchased the EVM and set it up under WIndows 10.

My goal is to determine the configuration settings I need for full analog dimming control of all driver outputs, but I am having trouble understanding the interaction of the (many!) control functions.

I have the EVM Users' Guide, and have read the relevant sections on software operation, but really could use some over-all guidance on configurations from someone familiar with the software tool.

Only once, and not repeatably, I managed to get 4 strings set to individually controlled brightness or current.

Eventually, I want to save the configrations as an EEPROM load so the final application boards can be pre-set to my normal operation and the microcontroller can control the srtrings' intensity with simple write operations.

  • 0
    Intellectual 390 points

    For reference, these are the main settings I have currently.

    All 4 strings are on, at equal brightness, which is controllable by both Current and Brightness sliders of Display/Cluster 1 only.

  • 0 in reply to Dave Bell1
    TI__Expert 5475 points

    Hi Dave,

    May I ask what is your application? Why would you like to control the current of the four channels individually?

    Regarding your question of why you were not able to control individually, have you checked if the EEPROM value is actually changed? Please make sure the EEPROM is unlocked before configuring to other values. You can check if LED string configuration is set successively by reading the EEPROM back.

  • 0 in reply to Shirley He
    Intellectual 390 points

    Hi, Shirley,

    Back, after holiday shutdown...

    For my application, I need to separately control the currents of 4 LED strings. The LP8860-Q1 provides considerably more features then we absolutely need, but initialization from EEPROM and otherwise minimal MCU interface requirements are desirable.
    Perhaps you could eccomend another TI device that might be suitable.

    As to operating the EVM with the supplied GUI, I have only scratched the surface of the feature set. I was not aware that I needed to actually write to the EEPROM for GUI-driven, interactive settings. I figured that once I had the combination of LED Driver settings, Brightness modes, etc., that I could *then* write that configuration to EEPROM. Then, upon start-up in the future, the EEPROM settings would be the defaults, and writing to the Brightness registers would control the strings.
    Is there no direct, interactive way to set up the Driver config? I do get *some* changes in operation (at least I think I do), but not to where I can control each string individually.

    The mode I believe I am looking for is Cluster:

    8.1.4 Cluster Mode
    In Cluster mode LED strings have independent control but fewer features enabled than in Display Mode.
    Brightness (PWM and current) are independently controlled for all 4 outputs. When there is an unequal number
    of LEDs per channel, the LP8860-Q1 adaptive voltage control is not used in Cluster mode; therefore, boost
    output voltage is fixed (or externally controlled or powered).
    In Cluster mode PWM frequency can be set through EEPROM, and Phase Shift PWM mode is enabled.
    Cluster mode does not support the PWM input pin, hybrid dimming, slope control or dither mode.

    8.3.2 Brightness Control (Display Mode)
    The LP8860-Q1 LED outputs can be configured to display or cluster mode. The following sections describe
    display mode options. Cluster mode is a special mode with individually controlled LED outputs. See Cluster
    Mode section for details.

    8.3.5 Cluster Mode
    Cluster is a simplified mode which allows independent current and PWM control for every string in cluster mode.
    In this mode brightness control is limited to conventional PWM through the SPI/I2 C brightness registers. The
    PWM input pin, Hybrid PWM and Current dimming mode, slope control, or dither are not available. Brightness for
    different LED strings depends on <DISP_CL1_BRT[15:0]>, <CL2_BRT[12:0]>, <CL3_BRT[12:0]> and
    <CL4_BRT[12:0]> registers. If OUT1 is in cluster mode, only 13 MSB are used. If all LED outputs are in the
    cluster mode, LED output PWM resolution is always 13 bits, and frequency depends on
    <PWM_RESOLUTION[1:0]> bits (see Table 13).

    Table 9. Description of the LED Output Modes
    MODE     LED_STRING_CONF[2:0]         DESCRIPTION
       7                         111                                All strings are used in cluster mode with 90° phase shift

    Do I understand correctly that this Mode setting should appear in the EEPROM here?  [0DH  CONFIGURATION  0000 0000]

    When I change the mode selections with the pull-down menu on the LED Drivers tab, the EEPROM bits above do not change, regardless of EEPROM Lock or Unlock setting, nor do any other registers.

    EEPROM appears to be Unlocked: [1AH  EEPROM_UNLOCK  1110 1111]

    Thanks for the guidance,

    Dave

  • 0 in reply to Dave Bell1
    Intellectual 390 points

    Hi, Shirley,

    I've made some progress in understanding the EEPROM configuration.

    I had completely missed the fact that the GUI side bar display is the live registers (00 - 1A), NOT the EEPROM (60 - 78), contributing to my confusion.

    I figured out the sequence of operations to actually write to the EEPROM, and changed the LED_STRING_CONFIG (address 63h) from the default F1h to 8Fh.

    When I wrote to the EEPROM, the LEDs momentarily appeared to go to the expected settings (4 strings, each at different, dim, settings), then blanked out.

    The Faults and Status tab, once updated, then shows all 4 strings in Fault, and Short LED string fault. (In case it wasn't clear, I am using the LED array board supplied with the EVM, in its deefault configuration)

    Restoring the EEPROM address 62h to F1h and re-burning returned the EVM to working, but do you have any insight as to why I am getting LED Faults in Cluster Mode?

  • 0 in reply to Dave Bell1
    TI__Expert 5475 points

    Hi Dave,

    What is the initial voltage setting? And what is voltage of each LED string? You can refer to the table below for the LED short fault detection mechanism to see if the short fault is your case can be explained. Since in cluster mode the adaptive mode control is disabled, and the current of each string may be different, it should be very careful to set a proper boost initial voltage. Or you can disable the short detection in cluster mode to avoid any mistrigger.