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BQ21040: Using a 100k NTC instead of the recommended 10k NTC

Darin Shaw
Prodigy 10 points
Part Number: BQ21040

On a new design, I am looking to use the BQ21040 battery charger. This is a part my company has used on a lot of designs. The attraction to it is cost and the ability to run stand-alone. The catch on this new design is that the Single-Cell LiPo battery we will be using has a built-in 100k NTC (Murata NCP15WF104F03RC), not 10k like the BQ21040 wants.

Is there a way to make a 100k NTC work with the BQ21040 so it can still operate in stand-alone mode and turn on/off at the right temperatures? Some type of resistor network that can be used around it?

I appreciate any help offered.

  • 0
    TI__Genius 12885 points

    Hi Darin,

    Based on the specifications of the provided Thermistor (Beta of 4250K to 4299K, R - 100K @ 25C). Unfortunately, I don't believe there is a simple way to adjust to this new resistor. The thermal protection works by outputting a 50uA bias current and reading voltage at the TS pin. The Voltage threshold for 0C is about 1.25 V and for 45C is about 0.275 V which approximately aligns with a 10K resistor with a beta of 3370.

    Any external solutions to this issue would likely require either another power source for biasing or another temperature dependent resistance which I believe in your case defeats the purpose of simply using the provided NTC. But I'm having a tough time identifying a fixed resistor network that could adjust resistance at both 0C and 45C to achieve expected resistance or voltage values.

    Best Regards,

    Juan Ospina

  • 0 in reply to Juan Ospina
    Prodigy 10 points

    Thank you Juan. I also struggled to find a solution which is why I reached out. I was hoping somebody was more clever than me Slight smile