BQ2970 Datasheet, PDF (21/31 Pages) Texas Instruments – Cost-Effective Voltage and Current Protection Integrated Circuit

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10.2.2 Detailed Design Procedure
bq2970, bq2971, bq2972, bq2973
SLUSBU9C – MARCH 2014 – REVISED MARCH 2016
NOTE
The external FET selection is important to ensure the battery pack protection is sufficient
and complies to the requirements of the system.
• FET Selection: Because the maximum desired discharge current is 7 A, ensure that the Discharge
Overcurrent circuit does not trigger until the discharge current is above this value.
• The total resistance tolerated across the two external FETs (CHG + DSG) should be 100 mV/7 A = 14.3 mΩ.
• Based on the information of the total ON resistance of the two switches, determine what would be the Charge
Overcurrent Detection threshold, 14.3 mΩ × 4.5 A = 65 mV. Selecting a device with a 70-mV trigger threshold
for Charge Overcurrent trigger is acceptable.
• The total Rds ON should factor in any worst-case parameter based on the FET ON resistance, de-rating due
to temperature effects and minimum required operation, and the associated gate drive (Vgs). Therefore, the
FET choice should meet the following criteria:
Vdss = 25 V
Each FET Rds ON = 7.5 mΩ at Tj = 25°C and Vgs = 3.5 V
• Imax > 50 A to allow for short Circuit Current condition for 350 µs (max delay timer). The only limiting factor
during this condition is Pack Voltage/(Cell Resistance + (2 × FET_RdsON) + Trace Resistance).
• Use the CSD16406Q3 FET for the application.
• An RC filter is required on the BAT for noise, and enables the device to operate during sharp negative
transients. The 330-Ω resistor also limits the current during a reverse connection on the system.
• It is recommended to place a high impedance 5-MΩ across the gate source of each external FET to deplete
any charge on the gate-source capacitance.
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