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TPS92520-Q1: Why analog dimming is limited 16:1

Part Number: TPS92520-Q1

Hello,

TPS92520-Q1 is to support analog dimming up to 16:1. However, according to the equation (5) below, it looks as if capable of 1024:1 (10 bit) analog dimming. What does limit the maximum ratio to 16:1?  

Thank you.

Regards,

Oguri (TIJ automotive FAE)

  • Hello,

    From the datasheet:  "Linear analog dimming response
    with over 16:1 range is obtained by programming the
    10-bit IADJ value via SPI."  This means it can be greater than a 16:1 range

    The reason for 16:1 is due to the accuracy.  If you look at the datasheet Electrical Characteristics table for ILED_REF_DACx = 63 the accuracy is 6.5 mV to 18.5 mV for current sense.  This is approximately 16:1 range which actually gives an accuracy range of 27:1 to 9:1 for a DAC setting of 63.  Since this is a synchronous buck converter you don't get the advantage of  the inductor current going discontinuous for deep analog dimming.  For lower DAC settings the lower range can go negative which means the LEDs will be off on some parts.

    Best Regards,

  • Hi Irvin,

    I am afraid, I could not understand your explanation. Or I cannot understand what this datasheet parameter is meant for.

    What threshold is it? What is ILED_REF_DACx? Does this have to do with LED current regulation described in pp.17-18 of the datasheet?

    Thanks.

    Oguri

  • Hello,

    CHxIAJD is ILED_REF_DACx (which is what is in the electrical characteristics table).  When it is set to 63 the error is as stated:

    Best Regards,

  • Hi Irwin,

    ILED_REF_DACx is only found in the table. It is not listed in the register map in p.36 of the datasheet. No one can understand what it is and what effect the setting of 63 has.

    Regards,

    Oguri

  • Hello,

    CHxIADJ IS ILED_REF_DACx, ILED_REF_DACx IS CHxIADJ

    If you set CHxIADJ to 63 the table applies.

    The accuracy when set 16:1 analog dimming is per the table

    6.5 mV, 12.5 mV, 16.5 mV.  Typical is 12.5 mV, range can be as high as 6.5 mV to 16.5 mV for current sense threshold.

    Best Regards,

  • These parameters are critical to determine the LED current accuracy. I would suggest that you correct the typo as soon as possible. Oguri

  • Sorry if this confusing.  It is under the the "ANALOG ADJUST SETTING AND CURRENT SENSE AMPLIFIER (CSPx, CSNx)" and shows the error in the reference voltage for the error amplifier for different DAC settings.

    Best Regards,