Datasheet

Functional Description
General Operation
The bq2040 determines battery capacity by monitoring
the amount of charge put into or removed from a re
-
chargeable battery. The bq2040 measures discharge
and charge currents, estimates self-discharge, and
monitors the battery for low-battery voltage thresholds.
The charge is measured by monitoring the voltage
across a small-value series sense resistor between the
battery's negative terminal and ground. The available
battery charge is determined by monitoring this voltage
over time and correcting the measurement for the envi
-
ronmental and operating conditions.
Figure 1 shows a typical battery pack application of the
bq2040 using the LED capacity display, the serial port,
and an external EEPROM for battery pack program
-
ming information. The bq2040 must be configured and
calibrated for the battery-specific information to ensure
proper operation. Table 1 outlines the configuration in
-
formation that must be programmed in the EEPROM.
An internal temperature sensor eliminates the need
for an external thermistor—reducing cost and compo
-
nents. An internal, temperature-compensated time-
base eliminates the need for an external resonator,
further reducing cost and components. The entire cir
-
cuit in Figure 1 can occupy less than
3
4
square inch of
board space.
3
bq2040
V
CC
V
OUT
REF
SMBC
SMBD
P STAT
SB
SR
DISP
ESCL
ESDA
LED1
LED2
LED3
LED4
bq2040
V
SS
2040LED.eps
301K BSS138
100K
100K
100K
100K
100K
100K
100K
86.5K
604K
806K
604K
499K
806K
499K
604K
909K
BSS138
BSS138
2N7002
BSS138
2N7002
2N7002
2N7002
2
R5 R4 Q1R11
No. of Cells
Chart 1
For bq2040 With No D8
Li-IonNiMH
3
4
6
8
9
10
12
499K
499K
698K
698K
806K
909K
909K
(Optional)
(Optional)
Figure 1. Battery Pack Application Diagram—LED Display