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LP8860-Q1: Problem with output power

Alessio Cocchi
Prodigy 110 points
Part Number: LP8860-Q1

Hi all,

i'm using lp8860 led driver in a design that have a nominal input Voltage of 13.5V.

Qhen the voltage is lowered up to 9.1V, the power consumption of the whole board begin to lower, because the led driver don't deliver all the necessary output power (I can see the Led backlight change luminosity).

Not all DUTS have this behaviour, in fact i have only two duts with this problem.

the values in eeprom registers are:

{ LP8860_EEPROM_REG_0, 0xef },
        { LP8860_EEPROM_REG_1, 0xff },
        { LP8860_EEPROM_REG_2, 0xdc },
        { LP8860_EEPROM_REG_3, 0xb1 },//60mA
        { LP8860_EEPROM_REG_4, 0x87 },
        { LP8860_EEPROM_REG_5, 0x05 },
        { LP8860_EEPROM_REG_6, 0xf2 },
        { LP8860_EEPROM_REG_7, 0x77 },
        { LP8860_EEPROM_REG_8, 0x77 },
        { LP8860_EEPROM_REG_9, 0x71 },
        { LP8860_EEPROM_REG_10, 0x3f },
        { LP8860_EEPROM_REG_11, 0xb7 },
        { LP8860_EEPROM_REG_12, 0x17 },
        { LP8860_EEPROM_REG_13, 0xef },
        { LP8860_EEPROM_REG_14, 0xb0 },
        { LP8860_EEPROM_REG_15, 0xE7 },
        { LP8860_EEPROM_REG_16, 0xEE },
        { LP8860_EEPROM_REG_17, 0x03 }, //BOOST FREQ TO 3 = 400KHz
        { LP8860_EEPROM_REG_18, 0xD9 },
        { LP8860_EEPROM_REG_19, 0x27 },
        { LP8860_EEPROM_REG_20, 0x30 },
        { LP8860_EEPROM_REG_21, 0xB7 },
        { LP8860_EEPROM_REG_22, 0xde },
        { LP8860_EEPROM_REG_23, 0x88 },
        { LP8860_EEPROM_REG_24, 0x3E },

any suggestion?

Thanks.

Alessio

  • 0
    TI__Intellectual 1235 points
    Hello Alessio,

    Thank you for reaching out. Could you please do a reading back of the faults to check whether any fault is being triggered when you lower Vin to 9.1V? How may DUTs have you tested?
    Kind Regards,

    David
  • 0 in reply to David Diaz Quintana1
    Prodigy 110 points

    Hi David,

    When i read Fault register (0x0F), the value is 08, from datasheet is overcurrent Fault. I've also monitored Isense Pin, (i've a 25mohm resistor), and i don't have a voltage over 200mV (i've set OC Protection to 8 amps). The voltage on Isense pin is 136mV (with 306mV spikes during FET transition, but are few nanoseconds)

  • 0 in reply to Alessio Cocchi
    Prodigy 110 points

    I add some details: the Output Led strings have a maximum voltage of 38V (typical about 36/37V), and a power consumption of 60mA for each string. here's the schematic:

  • 0 in reply to Alessio Cocchi
    TI__Intellectual 1235 points
    Hi Alessio,

    Thank you for providing the additional information.

    Could you please swap functional / ‘failing’ devices and boards? Could you please label the ‘failing devices, the two PCBs where those two ‘failing’ devices were mounted, two functional devices, and the two PCBs where those two functional devices were mounted? Then, could you please mount the two ‘failing’ devices (bad IC) and mounted them on those two PCBs that didn’t show any overcurrent fault when Vin~9.1V (good PCB)? Similarly, could you please mount two devices that have worked well by not triggering the overcurrent fault at Vin=9.1V (good IC) and mount those two functional devices on the two PCBs where the two ‘failing’ devices were previously mounted (bad PCB)? Could you please repeat the experiment by sweeping VIN from 13V to 9V and check behavior on those four boards (bad IC_1+good PCB_1, bad IC_2+good PCB_2, good IC_1+bad PCB_1, good IC_2+bad PCB_2).

    Kind Regards,

    David