Datasheet

LTC2941
6
2941fa
operaTion
Overview
The LTC2941 is a battery gas gauge device designed for
use with single Li-Ion cells and other battery types with
a terminal voltage between 2.7V and 5.5V. A precision
coulomb counter integrates current through a sense
resistor between the battery’s positive terminal and the
load or charger.
Coulomb Counter
Charge is the time integral of current. The LTC2941 mea-
sures battery current by monitoring the voltage developed
across a sense resistor and then integrates this information
to infer charge. The differential voltage between SENSE
+
and SENSE
–
is applied to an auto-zeroed differential analog
integrator to convert the measured current to charge.
When the integrator output ramps to REFHI or REFLO
levels, switches S1, S2, S3 and S4 toggle to reverse the
ramp direction. By observing the condition of the switches
and the ramp direction, polarity is determined.
A programmable prescaler is incremented or decremented
every time the integrator changes ramp direction. The
prescaler effectively increases integration time by a factor
M programmable from 1 to 128. At each under or overflow
of the prescaler, the accumulated charge is incremented or
decremented one count. The value of accumulated charge
is read via the I
2
C interface.
Power-Up Sequence
When V
SENSE
+
rises above a threshold of approximately
2.5V, the LTC2941 generates an internal power-on reset
(POR) signal and sets all registers to their default state.
In the default state, the coulomb counter is active. The
accumulated charge is set to mid-scale (7FFFh), the low
threshold registers are set to 0000h and all the high thresh-
old registers are set to FFFFh. The alert mode is enabled
and the coulomb counter prescaling factor M is set to 128.
–
+
–
+
–
+
+
S4
REFHI
REFLO
S3
S2
S1
LOADCHARGER
SENSE
+
R
SENSE
BATTERY
I
BAT
1 5
SENSE
–
6
GND
2
POLARITY
DETECTION
CONTROL
LOGIC
PRE-
SCALER
M
I
2
C/
SMBus
REGISTERS
AL/CC
3
SCL
4
SDA
2941 F03
STATUS
CONTROL
ACR
THRESHOLDS
V
SUPPLY
Figure 3. Coulomb Counter Section of the LTC2941