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BQ25895: Help determining KILIM for input current monitoring

Peter Lindner
Prodigy 40 points
Part Number: BQ25895

hello there,

i have here a custom board with a BQ25895. on the pin ILIM there is a resistor with 180 ohm to ground.

according to the datasheet (section 8.2.12) i can monitor the input current (the current in VBUS) by measuring VILIM (what i do with an ADC on ILIM pin). according to formula 4 i shold be able to calculate the input current.

but i don't understand what KILIM is.

the electrical characteristics notes that it is a value between 320 and 390.  which leads to a quite big difference in the calclated input current.

i measured the input current on VBUS (360mA) and the voltage on ILIM when charging is enabled (178mV). then i calculated back the KILIM with the above formula. but i get a KILIM of approx. 290.

can someone pleas help me with my understanding of KILIM.?

Is this a constant? how can i determine the value to calculate my input current? shall i take min, max or typical value of KILIM to calculate the current?

Did I understand it correctly that VILIM has a maximum voltage of 0.8V?

thank you very much for your help!

best regards peter

  • 0
    TI__Mastermind 31840 points

    Hi Peter, 

    K_ILIM is a constant used to both set ILIM value and as part of the formula for monitoring I_IN via the ILIM pin. When determining the input current limit you are attempting to set with ILIM pin KILIM = 390 max should be used. In your case that is setting ILIM = 390/180 = 2.167 A. 

    Peter Lindner said:
    which leads to a quite big difference in the calclated input current.

    The ILIM pin provides a reference of the input current, but will not be a precise measurement. Section 8.2.12 of the datasheet provides an example that explains with a 260 ohm resistor and V_ILIM = 0.4 the input current should be in the range of 0.615A to 0.75A (based on the min and max values of K_ILIM). Also the tolerance on your resistor between ILIM and gnd as well as any error in your ADC measuring ILIM pin can lead to further inaccuracy.  

    Peter Lindner said:
    Did I understand it correctly that VILIM has a maximum voltage of 0.8V?

    VILIM can be higher than 0.8V. ILIM voltage exceeding 0.8V is an indication the device is entering input current regulation. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 40 points

    Thank you garrett for your answer,

    which value of KILIM do i use now for calculating the I_IN? min/typ/max?

    I_IN is the current on (VBUS) isn't it?

    thank you very much for your help

    best regards,

    peter

  • +1 in reply to Peter Lindner
    TI__Mastermind 31840 points

    Hi Peter, 

    I would recommend using the typ KILIM value for calculating I_IN. 

    Peter Lindner said:
    I_IN is the current on (VBUS) isn't it?

    Correct I_IN is the input current into the IC at Vbus

    Best Regards,

    Garrett