BQ2040 Datasheet, PDF (10/33 Pages) Texas Instruments – Gas Gauge IC With SMBus Interface

bq2040
Table 3. bq2040 Current-Sensing Errors
Symbol
Parameter
VOS
Offset referred to VSR
INL
Integrated non-linearity
error
INR
Integrated non-
repeatability error
Typical
± 75
±1
± 0.5
Maximum
± 150
±4
±1
Units
µV
%
%
Notes
DISP = VCC.
Add 0.1% per °C above or below 25°C
and 1% per volt above or below 4.25V.
Measurement repeatability given
similar operating conditions.
Error Summary
Capacity Inaccurate
The FCC is susceptible to error on initialization or if no
updates occur. On initialization, the FCC value includes
the error between the design capacity and the actual ca-
pacity. This error is present until a qualified discharge
occurs and FCC is updated (see the DCR description).
The other cause of FCC error is battery wear-out. As the
battery ages, the measured capacity must be adjusted to
account for changes in actual battery capacity. Periodic
qualified discharges from full to empty will minimize er-
rors in FCC.
Current-Sensing Error
Table 3 illustrates the current-sensing error as a func-
tion of VSR. A digital filter eliminates charge and dis-
charge counts to the RM register when -VSRD < VSRO <
+ VSRD.
Display
The bq2040 can directly display capacity information us-
ing low-power LEDs. The bq2040 displays the battery
charge state in either absolute or relative mode. In rela-
tive mode, the battery charge is represented as a per-
centage of the FCC. Each LED segment represents 25%
of the FCC.
In absolute mode, each segment represents a fixed
amount of charge, 25% of the DesignCapacity. As the
battery wears out over time, it is possible for the FCC to
be below the design capacity. In this case, all of the
LEDs may not turn on in absolute mode, representing
the reduction in the actual battery capacity.
When DISP is tied to VCC, the LED1-4 outputs are inac-
tive. When DISP is left floating, the display becomes ac-
tive whenever the bq2040 detects a charge rate of
100mA or more. When pulled low, the segment outputs
become active immediately for a period of approximately
4 seconds. The DISP pin must be returned to float or
VCC to reactivate the display.
LED1 blinks at a 4Hz rate indicating a low battery con-
dition whenever the display is active, EDVF is not set,
and Remaining_Capacity_Alarm is set. VSB below EDVF
(EDVF = 1) disables the display output.
Microregulator
The bq2040 can operate directly from three nickel chem-
istry cells. To facilitate the power supply requirements
of the bq2040, an REF output is provided to regulate an
external low-threshold n-FET. A micropower source for
the bq2040 can be built inexpensively using a 2N7002 or
BSS138 FET and an external resistor. (See Figure 1.)
The value of R11 depends on the battery pack’s nominal
voltage.
Communicating With the bq2040
The bq2040 includes a simple two-pin (SMBC and
SMBD) bi-directional serial data interface. A host proc-
essor uses the interface to access various bq2040 regis-
ters; see Table 4. This method allows battery character-
istics to be monitored easily. The open-drain SMBD and
SMBC pins on the bq2040 are pulled up by the host sys-
tem, or may be connected to VSS, if the serial interface is
not used.
The interface uses a command-based protocol, where the
host processor sends the battery address and an eight-
bit command byte to the bq2040. The command directs
the bq2040 to either store the next data received to a
register specified by the command byte or output the
data specified by the command byte.
bq2040 Data Protocols
The host system, acting in the role of a Bus master, uses
the read word and write word protocols to communicate
integer data with the bq2040. (See Figure 3).
Host-to-bq2040 Message Protocol
The Bus Host communicates with the bq2040 using one
of three protocols:
n Read word
n Write word
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