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BQ25896: REG0C NTC Fault, REG10 TS Voltage relation

Werner Goebel
Prodigy 90 points
Part Number: BQ25896
Other Parts Discussed in Thread: BQ40Z50

Question 1 - REG10 TS voltage: slusc76c manual (bq25896 documentation) describes register content as ADC convertion of TS voltage with values from 21% to 80%. Chapter 9.2.7.5.1 describes the JEITA guideline compliance in charge mode, relating the TS voltage to T1-T5 temperature range. What I cannot find in the document is a formula how to convert the REG10 register reading to a temperature value.

Question 2 - REG0C NTC Fault indication for overtemperature: attached screenshot shows oscillating charge current; due to a design mistake the NTC is not measuring the battery temperature, but is placed near to the BQ25896 (will be modified), charging is done for a LiIon1s battery. Recorded data show that NTC_FAULT indication is toggling between  010 and 000 (meaning TS Warm and Normal), REG10 reading is at 45.6%.
According documentation current should be reduced to 0 for hot condition, recorded data look different; also NTC_FAULT indication is never showing "hot".


(Information to data in screenshot:
- first graph shows current values, first 3 values are cyclically polled from battery (bq40z50) reading the registers 0x14, 0x0a and 0x0b; last value is the analog measurement value from bq25896)
- second graph shows battery voltage, register 0x09 of bq40z50
- third graph shows battery temperature, register 0x08 from bq40z50
- fourth graph shows REG0C values and REG10 TSPCT value from bq25896

  • 0
    TI__Guru 54491 points

    Hi,

    We will check and get back to you.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    TI__Mastermind 27660 points

    Hi Werner, 

    1) The percentage values output from the ADC refer to the voltage observed at TS as a percentage of REGN. They are in accordance with the percentages listed for VT1-VT5 in the electrical characteristics table on page 10 of the datasheet. The relation between the percentage and the actual temperature is flexible and will depend on the thermistor and resistor values used. In general higher percentage means cold temperature and lower percentage means a higher temperature. If your TS pin configuration matches the example from datasheet page 23 then 73% is approximately 0C and 34.3% is approximately 60C and so on. 

    2) The device has both warm and hot temperature ranges. TS warm is between VT3 and VT5 and TS hot will be beyond VT5, which is a voltage percentage below 34.3%. REG10 = 45.6% is right on the edge between normal range and TS warm, as shown on page 10 of datasheet (and shown below). Possible oscillations between normal operation and JEITA warm operation may occur with any slight change in voltage at TS pin. Charge current will not go to zero in TS warm conditions, only TS hot conditions.

     

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 90 points

    Hi Garrett,

    thanks for your reply; I assume you wanted to say "REG10 = 45.6% is right on the edge between normal range and TS warm.." (not hot) in item 2 above, correct? (also assume that a slight change in voltage at TS pin should not cause oscillations, because there is 1% hysteresis, so oscillations should only occur if voltage change is bigger 1%).
    As the oscillations occur around T3 temperature, only reason I can see for this is that the charging voltage is reduced by 200mV with this condition; with battery voltage near 4V at this time, this could cause the reduction in charge current.
    Need to check this again, because I think this 200mV reduction should already be disabled in my configuration by setting bit 4 / JEITA_VSET in REG09 register, at least that's what this bit should do according documentation.
    Can you confirm that setting of JEITA_VSET should disable the 200mV reduction in charge voltage above T3? (table on page 10 is not mentioning this function).

  • +1 in reply to Werner Goebel
    TI__Mastermind 27660 points

    Hi Werner, 

    Apologies for the typo. Yes you are correct it should read "REG10 = 45.6% is right on the edge between normal operation range and TS warm." Also I agree that voltage changes at TS pin will need to be larger than 1% of V_REGN to cause a change from normal operation to TS warm and vise versa. 

    You should set REG09 bit 4 = 1 to disable the 200mV reduction to VREG setting. With REG09 bit 4 = 1 there should be no change in charger IC operation between NTC_Fault = Normal and NTC_Fault = TS Warm. 

    Best Regards,

    Garrett