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LED polarity: discrepancy between manual and effective behaviour

Francesco Valla16
Prodigy 50 points
Other Parts Discussed in Thread: DLPC350

Hello,

I'm using a DLP LightCrafter 4500 with direct USB HID command for control.

I need to set the LED current; the DLPC350 Programmer's Guide (on page 31) states:

If the LED PWM polarity is set to normal polarity, a setting of 0xFF gives the maximum PWM current.

But if I send an USB LED Driver Current Control command (CMD2: 0x0B, CMD3: 0x01) with 0xFF as arguments, the LEDs are turned off, even if the polarity is set to normal through the LED PWM Polarity command (CMD2: 0x1A, CMD3: 0x05 with payload 0x00).

Sniffing the USB packets between the official GUI and the board led me to the same conclusion.

Is this normal (and thus an error exists in the documentation) or something is wrong in my board?

Thanks

Francesco

  • 0
    TI__Genius 13235 points
    Hi Francesco,

    Thanks for posting. What you are seeing is normal behavior for the LightCrafter4500's LED driver circuit. Due to how the circuit was implemented on this platform, a 0% duty cycle LED driver current control value corresponds to max current and 100% duty cycle corresponds to min current.

    If you take a look at the GUI source code, you can see that we take this into account in the function that sends the set LED current command. It takes the user input values and subtracts them from 255 (0xFF). This allows for the LightCrafter4500 GUI input values to align with the DLPC350 programmer's guide.

    Thanks,
    Clinton
  • 0 in reply to Clinton Rodgers
    Prodigy 50 points
    Ok, got it.
    Thank you very much!

    Francesco