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TPS92518: Dimming ration conditions

Terry Wu2
Expert 2110 points
Part Number: TPS92518
Other Parts Discussed in Thread: LEDSPIMCUEVM-879

Hello Sirs,

We want TPS92518 support 9~16Vin(from battery) to drive 2.5A ILED per channel and 10,000 : 1  dimming ratio using 200Hz PWM input .

Would you pls help questions as attach file ? TKS!

TPS92518 Vboot-Vsw-uvlo.pptx

Many thanks !

  • 0
    TI__Mastermind 25500 points

    Hello Terry,

    There is no issue with changing the Cboot value, but it will just change the profile a bit. The issue is at certain lower frequencies and duty cycles you may try to turn on right at the boot uvlo point and it will not enable. This is pretty easy to fix simply by lowering the output capacitor value, or not using one at all. Basically you want the output (and the switch node since it will follow the output during the off time) to discharge far enough below VCCx to make sure Cboot charges enough for the next cycle.

    You should be able to obtain 10,000:1 at 200Hz or lower. Shunt FET dimming will give you even better range if required.

    This device is a controller so >2.5A is possible with the right external components.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 2110 points

    Hello Clint,

    Many thanks your help !

    Q1: Is there more description about "Shunt FET dimming" ?  If we use EVM and feed a PWM from external source , can we meet 10,000 : 1 ?

    Q2: TPS92518 need to use SPI to change register

          LEDx _ PKTH _ DAC[7 : 0]  and TOFF _ DAC[7 : 0]  follow equation 4 or 5, right ?

    Add one more question,

    We use TPS92518 EVM environment could see LED backlight start flick at 200Hz PWM < 0.2% duty .

    Due to we don't have "LEDSPIMCUEVM-879" EVM to send SPI command ( default register value )

    May I know how to avoid this flick ? Need to send correct LEDx _ PKTH _ DAC[7 : 0]  and TOFF _ DAC[7 : 0] ?

    Attach the video capture file .

  • 0 in reply to Terry Wu2
    TI__Mastermind 25500 points

    Hello Terry,

    Sorry for the slow response on this. The answer to Q1 is yes, depending on the PWM frequency. I am checking on the other two with an expert on this device and will get back to you.

    Regards,

    Clint

  • 0 in reply to Terry Wu2
    TI__Intellectual 1115 points

    Is the output LED string 1 or 2 LEDs? If you're using the default values the switching frequency could be as low as ~35kHz! This would not show the best performance. You should request the LEDSPIMCUEVM-879 to test the TPS92518 correctly.

    For very high PWM dimming ratio, you also need to minimize or remove the output capacitance. The EVM default capacitor value will be too high, you could try your testing with all the output capacitance removed. (remove C14,C15,C17,C18)

  • 0 in reply to Tim Sullivan
    Expert 2110 points
    Hello Tim,
    Thanks your suggestion ! I will try to get LEDSPIMCUEVM-879 to test . TKS !