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LP8860-Q1: Current dimming mode

Dan Schnell
Prodigy 120 points
Part Number: LP8860-Q1

I got recommendation from the TFT supplier to reduce forward current of backlight at high temperature to reduce artifacts that our SW application team was observing.

4 channel configuration.  Each string rated at 80mA max.  Would like to reduce from 80mA per string starting at 65C to 40mA per string at 85C.  There is a 10k NTC  (NCP18XH103) available on tft flex circuit, but I could easily add another thermistor to main or pcb circuit.  ISET is open at the moment.

 

Can the knee point and gain be properly calculated without iset?  Datasheet wasn't clear to me whether 'ITEMP" was the slope or output value of TSENSE block after ramp.

  • 0
    TI__Intellectual 2180 points

    Hi Dan,

    LP8860 has two options for high temp protection with external NTC, LED Current Limitation and LED Current Dimming.

    We recommend using LED Current Limitation instead of LED current dimming if possible (please refer to Section 7.3.9.3 in datasheet) . It is simpler to implement since you are just setting the temp limits and the device will automatically set up or down as necessary to protect the device from reaching these limits. 

    For LED Current Dimming, I've attached an excel sheet calculator that helps define the R1/R2 values and setting the knee point and gain registers. Current values on the calculator show approximate limits that your application requires (80mA at 65C and 40mA at 85C).

    To use LED current dimming with external NTC sensor, you need to use external RISET to set maximum ILED current, this will influence R1/R2 values as well as knee point/gain registers so we cannot properly calculate without actual ISET and RISET values. Also note that EN_EXT_LED_CUR_CTRL must be enabled for setting ISET with external  RISET resistor. 

    LP8860_NTC_Calculator.xlsx

    Hope this helps,

    Salome

  • 0 in reply to Salome Ramirez Carrion
    Prodigy 120 points

    Do you have a recommendation for ext temp time?  Only problem I see with fastest time is there may not be a temperature change in that duration.

  • 0 in reply to Dan Schnell
    TI__Intellectual 2180 points

    Hi Dan,

    EXT_TEMP_PERIOD[4:0] register value, will have to be chosen depending on thermal performance of the system and will have to be optimized accordingly. But you are correct, setting the fastest time might not allow enough time for the temperature to decrease even though the LED current has already been brought down, so setting fastest time might result in dimming LED current too much before the temperature has stabilized. Setting periods of 32s or higher is more common to allow temperature to decrease before unnecessarily dimming LED current further down.

    Best Regards,

    Salome

  • 0 in reply to Salome Ramirez Carrion
    Prodigy 120 points

    What is the VBG ref?  Bandgap Voltage?

    Is there internal pullup to VDD on TSENSE input?

  • 0 in reply to Dan Schnell
    TI__Intellectual 2180 points

    Hi Dan,

    Yes, VBG is the band gap voltage 1.2V.

    There is no internal pullup to VDD from TSENSE pin.

    Best,

    Salome