BQ2040_15 Datasheet, PDF (6/36 Pages) Texas Instruments – Gas Gauge IC With SMBus Interface

Not Recommended For New Designs
bq2040
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 _{5 =} MBV _{− 1}
R₄
2.25
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 0.1µf 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-
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