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BQ24773: Sense resistor more than 20mE

EASHWAR ARIVAZHAGAN
Intellectual 315 points
Part Number: BQ24773

Dear Sir/Madam,

My REQUIREMENTS as follows,

I have to design a battery charger for charging Ni-Cd batteries. 

6 AA cell each 1.2V (NOM. voltage)  having 1000mAh capacity. All the cells are arranged in series to get total pack voltage of 7.2V@1000mAh.

I have chosen BQ24773 as charger, since I have I2C comm. to change the battery charge voltage and current to safely charge the battery.

AC-DC Adapter for input of BQ24773 which shall produce 12V@2A output.

My Adapter current requirement will be less than 1A, system current 0.3A and battery charge current 1A (battery has 1C rating for fast charging).

After going through all the design process in the datasheet my sense resistor for adapter current  is 41.25mE and battery current sense resistor is 110mE which is greater than 20mE (datasheet recommends less than 20mE). Will this parameter affect the working of BQ24773?

Since the charger is buck converter, what is min. input voltage required to generate 7.2V output of BQ24773?

Does this charger have any facility to determine charge termination for Ni-Cd batteries? 

Kindly provide a solution ASAP.

  • 0
    TI__Genius 11850 points

    Hi Eashwar,

    1) Yes, using such high sense resistor values could be problematic. Please do not exceed 20 mΩ for your sense resistors.

    2) The sleep comparator has a max rising threshold of 370 mV, so VCC must be at least 7.2 + 0.37 = 7.57 V. However, you also need to factor in the voltage drop between the adapter and VCC (in particular, the drop across diode D1 shown below). It's safer to include a little extra margin, so I would recommend an input voltage of 9 V or higher in order to charge your battery up to 7.2 V. A 12 V adapter is fine.

    3) The BQ24773 does not have a built-in termination current setting. The host is responsible for controlling the termination behavior as desired.

    Best regards,

    Angelo