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BQ77PL900DL Iout range

Camillo Ta
Prodigy 60 points

Hi guys,

I need to define the Rsense value between SRBGND and SRPGND in BQ77PL900DL, but i can't find the extremes of IOUT range (pin 29). Can you help me? My idea was:

(VIOUTmax - 1.2V )/ (Ipackmax * Igain) = RSENSE

Thank you

  • 0
    TI__Guru 54715 points

    That is a good starting point based on the equation on page 33 of the datasheet. You will also want to include a term for the input offset spec on page 13 of the datasheet, Voffset x Igain.  So the equation may look more like:

    (VIOUTmax - 1.2V - (Voffset * Igain))/(Ipackmax * Igain) = Rsense

    The datasheet does not indicate that the VIOUT range is otherwise limited. You may want to consider if there are limitations based on the converter used or the interface filter/amp circuitry. You will likely want to characterize several systems as well as calibrate each unit's current measurement.

    Other considerations in picking a sense resistor might include:

    • Keeping the sense input voltages within the abs max ratings of the device during maximum short circuit current of the cells
    • Keeping the desired current protection limits in the range of the part
    • Standard or available values
    • Power rating
    • Temperature coefficient
  • 0 in reply to WM5295
    Prodigy 60 points

    Thank you for the answer!

    I decided to use last points you underlined in the prevoius reply relatively to the maximum current flowing through RSENS but i don't find abs max ratings for VIOUT 

  • 0 in reply to Camillo Ta
    Prodigy 70 points

    I have some problems to compute the value of the charge current  using the IOUT pin output

    These are the paramenters :

    IGAIN = 50

    Vcell_off = 1136 mV

    Rsense = 0,5 mOhm

    Vcurr = 1109 mV

    Ipack =~ 600 mA

    Using the formula of the data sheet pag 33 (VCURR = Vcell_off + (IPACK × RSENSE) × (IGAIN)) to compute the Ipack

    I don't get a reasonable result .What could be my mistake ?

    Thanks

    Giuliano

  • 0 in reply to Giuliano Barbieri
    TI__Guru 54715 points

    It is not immediately clear if the current calibration feature is giving the pedistal offset voltage or that voltage with the differential amplifier offset voltage added.  Since gain is not a condition for the cal mode, the input offset voltage is likely not included.

    Since Vio can be +/- 3 mV, at room temp, with the gain of 50 nominal, this could be +/- 150  mV with no current.  Your input, -.6A x 0.5 m ohm gives -0.3 mV; x 50 predicts a -15 mV output change.  From your values, you see a -27 mV change from the calibration output to the current output.

    I'm certain you were very careful with the layout of the board, it is easy to pick up some unexpected resistance on the board when using small sense resistors.

    I expect you will get best results with a system calibration using no current to obtain an offset voltage and a reference current applied.to calculate a gain.