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BQ25883: /BQ25882 understanding TS_ADC readings

Jan Penterman
Prodigy 180 points
Part Number: BQ25883

Hello,

I'm trying to understand the meaning of TS_ADC. I have a 10K NTC installed (Panasonic ERTJ1VG103FA) with the resistor network identical to the one in the datasheet. After having corrected a fault in the Linux driver, it gives me values that make sense being percentages. I'm also measuring TDIE at the same time. These two seem to change in opposite directions: when TDIE goes up, TS_ADC goes down. How should I interpret the values TS_ADC in the context of thermal regulation/shutdown?

Also, the datasheet mentions a range of 20%-80% TS_ADC value. When I'm not charging  and I actively cool the chip with a fan, I get a TS_ADC reading of:

i2cget -y -f 0 0x6b 0x21 w -> 0x6802

flip endiannes -> 0x0268

0x0268 = 616 decimal

616 * 25 / 256 = 60%

The register description doesn't mention I should still add a 20% baseline. Do I need to or is this already included in the ADC reading?

TDIE is 23 degrees Celsius (room) at this reading.

When charging and not actively cooling, TS_ADC goes down to 36% (or should that be 56%) with a  TDIE of 72 degrees.

  • 0
    TI__Genius 10385 points

    Jan,

    The 20% does not need to be added. The range you see in the datasheet is the range TS_ADC percentages that be specify to be accurate at the given conditions. Taking a look at this app note for estimating temperature from ADC values I have run through some calculations in the attached spreadsheet.

    60% =>23.47 C

    36% =>57.08 C

    Thanks,

    Ricardo

    TS_ADC Calculator.xlsx

  • 0 in reply to Ricardo
    Prodigy 180 points

    Thanks, that surely clears things up. Now I still need to figure out why my device stops charging once I have the NTC in place. When I replaced it again with the common 10K resistor that was there before (TS_ADC=58%), the charger started working again. Our application consumes quite a lot of power when switched on, so with the normal 10K installed and the device on the charger, I get IBUS=1600mA and ICHG=400mA. Once I installed the NTC, IBUS went down to 1200mA and ICHG between 0mA and 10mA (while charge mode was reading "Fast charging") . This with TS_ADC at ±50%, which should be around 37 degrees. I wasn't expecting that, I hoped it to stop charging around 45 degrees. NTC status was Warm to Hot with the NTC installed, Normal with the fixed 10K resistor. I am probably overlooking where the thresholds are defined. I found them for OTG but couldn't find them for charge mode. I haven't got any JEITA temperature stuff configured in device tree.

    With the load powered off I don't have the ability to check the registers (huge design mistake) but then the lab view also went down to 1200mA instead of 1600mA. I also saw it oscillate slowly between 1700mA and 150mA when the device was really hot, but couldn't reproduce that later.

    I am not near the device now to try if I can reproduce this for validation but will as soon as I have the oppurtunity.

    I basically need to prove that we've built a device that runs too hot for LiPo charging (which I suspect it does) but the numbers don't quite work along yet... 

  • 0 in reply to Jan Penterman
    TI__Genius 10385 points

    Jan,

    Could you send me the values for all your registers when you have the NTC installed?

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 180 points

    I could, but I checked again and the chip is now responding to temperature very well, not sure what happened on the previous test.

    How would you go about finding the 'right' setting of the JEITA_* registers? To give you a little bit of context, our prototype device runs really hot. It got sent back to us with a swollen LiPo cell, so I'm now ironing out some bugs to get a better idea of what went wrong. Charging at high temperature seems a likely cause, since we didn't have the NTC installed. The battery datasheet is not very detailed, but specifies a charging temp range between 20-60 degrees. Our prototype easiliy runs over 45 degrees up until around 60. 

  • 0 in reply to Jan Penterman
    TI__Genius 10385 points

    Jan,

    Looking at Figure 24, if you would like to allow 100% of ICHG from 45C to 60C you need to set ISETH to 1. I am not sure what your concerns with regards to the charge  voltage at high temperature, soI am not sure how to advise you with a VSET value.

    Ultimately, I would suggest you reference your battery's datasheet or contact the manufacturer/distributor to find the ideal charge profile at high temperatures. I have the concern that setting ISETH to 1 will result in another swollen battery on account of your battery as the charge profile at high temperature would be similar to having a fixed 10kohm instead of a thermistor.

    Thanks,

    Ricardo