Datasheet

LTC2942-1
10
29421f
applicaTions inFormaTion
Note that the internal digital resolution of the coulomb
counter is higher than indicated by qLSB. The internal
charge resolution is typically 299µAs.
ADC Mode B[7:6]
The LTC2942-1 features an ADC which measures either
voltage on SENSE
–
(battery voltage) or temperature via
an internal temperature sensor. The reference voltage and
clock for the ADC are generated internally.
The ADC has four different modes of operation as shown
in Table 3. These modes are controlled by bits B[7:6] of
the control register. At power-up, bits B[7:6] are set to
[00] and the ADC is in sleep mode.
A single voltage conversion is initiated by setting the bits
B[7:6] to [10]. A single temperature conversion is started
by setting bits B[7:6] to [01]. After a single voltage or
temperature conversion, the ADC resets B[7:6] to [00]
and goes to sleep.
The LTC2942-1 also offers an automatic scan mode where
the ADC converts voltage, then temperature, then sleeps
for approximately two seconds before repeating the voltage
and temperature conversions. The LTC2942-1 is set to this
automatic mode by setting B[7:6] to [11] and stays in this
mode until B[7:6] are reprogrammed by the host.
Programming B[7:6] to [00] puts the ADC to sleep. If
control bits B[7:6] change within a conversion, the ADC
will complete the current conversion before entering the
newly selected mode.
A conversion of either voltage or temperature requires 10ms
conversion time (typical). At the end of each conversion,
the corresponding registers are updated. If the converted
quantity exceeds the values programmed in the threshold
registers, a flag is set in the status register and the AL/CC
pin is pulled low (if alert mode is enabled).
Accumulated Charge Register (C,D)
The coulomb counter of the LTC2942-1 integrates current
through its internal sense resistor over time. The result of
this charge integration is stored in the 16-bit accumulated
charge register (registers C, D). The amount of charge for
a given register contents (C[7:0]D[7:0]) and prescaler
setting M can be calculated by:
Q mAh
M
C D= +
( )
0 085
128
256. • • •
The ACR should be read in a single I
2
C Read transaction
(see Figure 10). If C and D are read in individual single-
byte transactions, each with a STOP condition, the register
may change between the first and the second transaction
due to coulomb count events, causing erroneous charge
readings.
As the LTC2942-1 does not know the actual battery status
at power-up, the accumulated charge register (ACR) is set
to mid-scale (7FFFh). If the host knows the status of the
battery, the accumulated charge (C[7:0]D[7:0]) can be
either programmed to the correct value via I
2
C or it can be
set after charging to FFFFh (full) by pulling the AL/CC pin
high if charge complete mode is enabled via bits B[2:1]. In
this case, FFFFh represents a fully charged battery. If the
actual battery capacity is smaller, the host can subtract the
excess charge whenever doing the charge calculation, and
set the low charge threshold (registers G,H) to the value
representing an empty battery. This procedure essentially
shifts the zero point of the scale upwards. Before writing
the accumulated charge registers, the analog section
should be shut down by setting B[0] to 1.
Voltage and Temperature Registers (I, J),(M, N)
The result of the 14-bit ADC conversion of the voltage at
SENSE
–
is stored in the voltage registers (I, J), whereas
the temperature measurement result is stored in the tem-
perature registers (M, N). The voltage and temperature
registers are read only.
As the ADC resolution is 14-bit in voltage mode and 10-bit
in temperature mode, the lowest two bits of the combined
voltage registers (I, J) and the lowest six bits of the
combined temperature registers (M, N) are always zero.
From the result of the 16-bit voltage registers I[7:0]J[7:0]
the measured voltage can be calculated as:
V V
RESULT
FFFF
V
RESULT
SENSE
h
h
DEC
–
• •= =6 6
65535