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BQ2040SN-C408 Datasheet(PDF) 6 Page - Texas Instruments |
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BQ2040SN-C408 Datasheet(HTML) 6 Page - Texas Instruments |
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6 / 36 page  Voltage Thresholds In conjunction with monitoring VSR for charge/discharge currents, the bq2040 monitors the battery potential through the SB pin. The voltage potential is deter- mined through a resistor-divider network per the fol- lowing equation: R R MBV 2.25 5 4 1 =− where MBV is the maximum battery voltage, R5 is con- nected to the positive battery terminal, and R4 is con- nected to the negative battery terminal. R5/R4 should be rounded to the next higher integer. The voltage at the SB pin (VSB) should never exceed 2.4V. The battery voltage is monitored for the end-of- discharge voltages (EDV1 and EDVF) and for alarm warning conditions. EDV threshold levels are used to de- termine when the battery has reached a programmable “empty” state. The bq2040 generates an alarm warning when the battery voltage exceeds the maximum charg- ing voltage by 5% or if the voltage is below EDVF. The battery voltage gain, the two EDV thresholds, and the charging voltage are programmable in the EEPROM. If VSB is below either of the two EDV thresholds, the associ- ated flag is latched and remains latched, independent of VSB, until the next valid charge. EDV monitoring may be disabled under certain condi- tions. If the discharge current is greater than the value stored in location 0x2c and 0x2d in the EEPROM (EE 0x2c/0x2d), EDV monitoring is disabled and resumes af- ter the current falls below the programmed value. Reset The bq2040 is reset when first connected to the battery pack. On power-up, the bq2040 initializes and reads the EEPROM configuration memory. The bq2040 can also be reset with a command over the SMBus. The software reset sequence is the following: (1) write MaxError (0x0c) to 0x0000; (2) write the reset register (0x64) to 0x8009. A software reset can only be performed if the bq2040 is in an unlocked state as defined by the value in location 0x3d of the EEPROM (EE 0x3d) on power-up. Temperature The bq2040 monitors temperature sensing using an in- ternal sensor. The temperature is used to adapt charge and self-discharge compensations as well as to monitor for maximum temperature and ∆T/∆t during a bq2040 controlled charge. Temperature may also be accessed over the SMBus with command 0x08. Layout Considerations The bq2040 measures the voltage differential between the SR and VSS pins. VOS (the offset voltage at the SR pin) is greatly affected by PC board layout. For optimal results, the PC board layout should follow the strict rule of a single-point ground return. Sharing high-current ground with small signal ground causes undesirable noise on the small signal nodes. Additionally, in refer- ence to Figure 1: n The capacitors (C1 and C2) should be placed as close as possible to the SB and VCC pins, and their paths to VSS should be as short as possible. A high-quality ceramic capacitor of 0.1 µf is recommended for VCC. n The sense resistor capacitor (C3) should be placed as close as possible to the SR pin. n The bq2040 should be in thermal contact with the cells for optimum temperature measurement. Gas Gauge Operation The operational overview diagram in Figure 2 illus- trates the operation of the bq2040. The bq2040 accumu- lates a measure of charge and discharge currents, as well as an estimation of self-discharge. Charge currents are compensated for temperature and state-of-charge of the battery. Self-discharge is temperature-compensated. The main counter, RemainingCapacity (RM), represents the available battery capacity at any given time. Battery charging increments the RM register, whereas battery dis- charging and self-discharge decrement the RM register and increment the internal Discharge Count Register (DCR). The Discharge Count Register is used to update the FullChargeCapacity (FCC) register only if a complete battery discharge from full to empty occurs without any partial battery charges. Therefore, the bq2040 adapts its capacity determination based on the actual condi- tions of discharge. The battery's initial full capacity is set to the value stored in EE 0x60-0x61. Until FCC is updated, RM counts up to, but not beyond, this threshold during subsequent charges. The battery’s empty state is also programmed in the EEPROM. The battery low percentage (EE 0x2e) stores the percentage of FCC that will be written to RM when the battery voltage drops below the EDV1 threshold. 1. FullChargeCapacity or learned-battery capacity: FCC is the last measured discharge capacity of the battery. On initialization (application of VCC or reset), FCC is set to the value stored in the EEPROM. Dur- 6 bq2040 Not Recommended For New Designs |
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