LTC2944
9
2944f
For more information www.linear.com/LTC2944
operaTion
Overview
The LTC2944 is a battery gas gauge designed for use
with multicell batteries with terminal voltages from 3.6V
to 60V. It measures battery charge and discharge, battery
voltage, current and its own temperature.
A precision analog coulomb counter integrates current
through a sense resistor between the battery’s positive
terminal and the load or charger. Battery voltage, battery
current and silicon temperature are measured with an
internal ADC.
Coulomb Counter
Charge is the time integral of current. The LTC2944 mea-
sures charge by monitoring
the voltage developed across
a sense resistor. The differential voltage between SENSE
+
and SENSE
–
is applied to an auto-zeroed differential analog
integrator to infer charge.
When the integrator output ramps to REFHI or REFLO
levels, switches S1, S2, S3 and S4 toggle to reverse the
ramp direction (Figure 2). By observing the condition of
the switches and the ramp direction, polarity is determined.
This approach also significantly lowers the impact on offset
of the analog integrator as described in the Differential
Offset Voltage section.
A programmable prescaler effectively increases integration
time by a factor M programmable from 1 to 4096. At each
underflow or overflow of the prescaler, the accumulated
charge register (ACR) value is incremented or decremented
one
count. The value of accumulated charge is read via
the I
2
C interface.
Voltage, Current and Temperature ADC
The LTC2944 includes a 14-bit No Latency ΔΣ analog-to-
digital converter, with internal clock and voltage reference
circuits.
The ADC can be used to monitor the battery voltage at
SENSE
–
or the battery current flowing through the sense
resistor or to convert the output of the on-chip tempera-
ture sensor.
Conversion of voltage, current and temperature are trig-
gered by programming the control register via the I
2
C
interface. The LTC2944 includes a scan mode where
voltage, current and temperature conversion measure-
ments are executed every 10 seconds. At the end of each
conversion the corresponding registers are updated and
the converter goes to sleep to minimize quiescent current.
The temperature sensor generates a voltage proportional
to temperature with a slope of 2mV/K resulting in a volt-
age of 600mV at 27°C.
Power-Up Sequence
When SENSE
+
rises above a threshold of approximately
3.3V, the LTC2944 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 while
the voltage, current and temperature ADC is switched
off. The accumulated
charge register is set to mid-scale
(7FFFh), all low threshold registers are set to 0000h and all
high threshold registers are set to FFFFh. The alert mode
is enabled and the coulomb counter prescaling factor M
is set to 4096.
Figure 2. Coulomb Counter Section of the LTC2944
2944 F02
REFHI
S1
V
CC
S2
S3
S4
REFLO
GND
SENSE
+
R
SENSE
I
BAT
BATTERY
+
LOADCHARGER
SENSE
–
1
8
2
M
PRESCALER
ACR
–
+
–
+
–
+
CONTROL
LOGIC
POLARITY
DETECTION
