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LP8861-Q1 Vmin Boost for Vstep

Ryuji
Guru 20090 points

Hello,

In the datasheet P.18, there is the equation of

Vstep=(Vmax boost - Vmin boost) / 256

What is this Vmin boost?

I understood that the Vmax boost is came from the equation (1).  Vmax boost = (Vbg/R2 + 0.0387) * R1 + Vbg.
However, I couldn't fount the explanation about Vmin boost.

Best Regard,
Ryuji Asaka

  • 0
    Guru 20090 points
    Hello team,

    I'm looking forward to the reply.

    Best Regards,
    Ryuji Asaka
  • 0
    TI__Intellectual 1235 points
    Hello Ryuji Asaka,

    Thanks for your inquire. The Vmin boost is mainly defined by your LED string configuration. In other words, the adaptive boost control function adjusts the boost output voltage to the minimum sufficient voltage for proper LED current sink operation. The device detects the output with highest number of LEDs and the boost voltage is adjusted according to the required voltage based on forward voltage x number of LEDs plus some headroom voltage.
    Please refer to section 7.3.6.1 of datasheet for additional details regarding adaptive boost control and functionality of LED fault comparators.

    Kind Regards,
    David D Quintana
  • 0 in reply to David Diaz Quintana1
    Guru 20090 points
    Hello David,

    Thank you for the reply.
    Thus the calculation of Vboost_min is the following.

    Vmin_boost = LED Vf(min) * Series LEDs # + Vsat(typ)

    If the minimum Vf of LED is 3V and 4 series,
    Also Vsat is 0.4V(typ). (I couldn't found the minimum voltage of Vsat)

    the Vmin_boost = 12.4V

    Is my understanding correct?

    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    Guru 20090 points
    Hello David

    I'm looking forward to your reply
  • 0 in reply to Ryuji
    TI__Prodigy 200 points

    Hi Asaka-san,

    Yes, your basic idea is correct.  Normally we refer to the voltage on the LED driver current sink pins (OUT1..OUT4) as the driver headroom voltage.  This is a little different than what the Vsat voltage means, let me explain.  When the voltage on the OUT1...OUT4 pins (driver headroom voltage) reaches the saturation voltage (Vsat) level then it means the LED driver sinks are entering "saturation condition" and the LED current is 90% of the ideal current (set by ISET resistor).  This could happen is the boost drooped for some reason or if you did not set the maximum boost output level (resistor divider) high enough for your LED Vf load.  Under normal operation the adaptive boost control loop will adjust the boost voltage so that all the driver headroom voltages are higher than the Vsat level (so that LED current is at 100% of correct level).  We have multiple voltage comparators on each OUTx pin as shown in Figure 14 in the datasheet.  The adaptive boost control loop will adjust the boost voltage so the headroom of the string with your highest LED Vf will be between the LOW_COMPARATOR and MID_COMPARATOR levels.  The minimum driver headroom voltage is equal to the LOW_COMP level (which is still higher than Vsat spec).  So minimum headroom voltage would be (1.1 * Vsat)+0.2V = 0.64v based on the equations below figure 14.

    So assuming your minimum LED Vf is 3v per LED and you have 4 series LEDs per string, then your minimum boost voltage is (4 LEDs x 3v Vf) + 0.64V min headroom = 12.64V.  Hope that helps!

    Thanks,

    Jeremy