Datasheet

LTC2942-1
7
29421f
Coulomb Counter
Charge is the time integral of current. The LTC2942-1 mea-
sures battery current by monitoring the voltage developed
across its internal sense resistor and then integrates this
information to infer charge. The internal sense resistor is
tied between the SENSE
+
and SENSE
–
pins and is con-
nected to an auto-zeroed differential analog integrator
which converts the measured current to charge.
When the integrator output reaches the REFHI or REFLO
thresholds, 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 effectively increases integration
time by a factor M programmable from 1 to 128. 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 and Temperature ADC
The LTC2942-1 includes a 14-bit No Latency ∆Σ analog-
to-digital converter, with internal clock and voltage refer-
ence circuits.
+
LOADCHARGER
R
SENSE
BATTERY
I
BAT
–
+
–
+
–
+
S4
R
BOND
R
BOND
REFHI
REFLO
S3
S2
S1
V
CC
SENSE
+
1
SENSE
–
6
GND
2
POLARITY
DETECTION
CONTROL
LOGIC
M
PRESCALER
ACR
29421 F02
Figure 2. Coulomb Counter Section of the LTC2942-1
The ADC can either be used to monitor the battery voltage
at SENSE
–
or to convert the output of the on-chip tempera-
ture sensor. The sensor generates a voltage proportional to
temperature with a slope of 2.5mV/K resulting in a voltage
of 750mV at 27°C.
Conversion of either temperature or voltage is triggered
by setting the control register via the I
2
C interface. The
LTC2942-1 features an automatic mode where a voltage
and a temperature conversion are executed every two
seconds. At the end of each conversion the corresponding
registers are updated and the converter goes to sleep to
minimize quiescent current.
Power-Up Sequence
When V
SENSE
+ rises above a threshold of approximately
2.5V, the LTC2942-1 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 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 pre-scaling factor M
is set to 128.
operaTion