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BQ27500 connections with TPS65950 charger module

Gandhar Dighe
Genius 15100 points
Other Parts Discussed in Thread: TPS65950, BQ27500, BQ27505-J4, BQ27510

Hi,

I support the TI PMIC solutions (TPS659xx) for OMAP3 family. TPS65950 has an integrated battery charger module which can be used for charging single cell Li-Ion battery.

I have a customer who will use the integrated charger and the BQ27500 fuel gauge. I need some help to determine how TPS65950 and BQ27500 can be connected on the system. Please help me with the questions below:

1. Is it necessary that BQ part should be connected to the battery thermistor? Can we connect a 10K to BQ instead of actual thermistor?
Reason to ask the above question: TPS65950 ADCIN1 pin must be connected to the thermistor, it acts as a battery detection for the device. If BQ part doesnt need to be connected to thermistor and still can function normal then that will be ideal. I dont know if temperature sensing is essential for gauging purpose.

2. BQ specs mention that the thermistor can be shared, but in this case TPS65950 may not read the accurate resistance (based on the thermistor current) to report the correct battery temperature.

I am trying to understand the best connectivity for these parts.

If further discussion is required then I can meet someone in Forest Lane or can have a phone call in case the support person is outside Dallas.

Customer will be finalizing schematics and I need to ensure we help them soon.

 

Thank you.

Gandhar.

 

  • 0
    TI__Expert 4050 points

    Gandhar,

     

    1. bq27500 is using the temperature for accurate gauging. If accuracy is not required then 10k resistor connect to TS is ok.

    2. Normally we are not recommend to share the thermistor as it may need more components. Can second thermistor be used?

     

    You can contact me or David Maxwell.

     

    Thanks

    Ming Yu

  • 0 in reply to Ming Yu
    TI__Genius 14985 points

    Yes, temperature information is critical for accurate gauging of battery run-time.  You can configure the bq27500 to use the internal temp sensor but you still need to fool the bq27500 into thinking a battery pack is inserted with the network  on BI/TOUT & TS.  (Newer gauges like bq27505-J4 have firmware commands which can over-ride the battery insertion detection and allow the host to tell the gauge that a battery is present.)

    Usually a charger or PMIC only needs to know temperature-out-of-range for charging purposes and doesn't really care about the precise temperature.  I would argue that you should allow the gauge to connect to the thermistor in the pack and then use the BAT_GD pin to enable/disable the charger when the temperature is out of range.  There are data flash setting inside the gauge that allow you to specify the allowable charging range.  It can be configured so the BAT_GD pin will toggle when it is outside of this range (polarity is actually configurable).  Below is one example of this configuration with a bq27510 and TPS658610.  When temperature charging is allowed, BAT_GD is pulled low and the PMIC thinks the temperature is nominal due to the resistor divider.  When charging is to be disallowed, BAT_GD will open up and the PMIC's TS pin will float high.  It will consider this to be the extreme cold temp and stop charging.  (You could use 2Mohms in place of the 10k to reduce current.)