LTC2941
11
2941fb
For more information www.linear.com/LTC2941
be chosen for a given battery capacity Q
BAT
and a sense
resistor R
SENSE
as:
M≥128 •
BAT
16
•
SENSE
50mΩ
M can be set to 1, 2, 4, 8, …128 by programming B[5:3]
of the control register as M = 2
(4 • B[5] + 2 • B[4] + B[3])
. The
default value after power up is M = 128 = 2
7
(B[5:3] = 111).
In the above example of a 100mAh battery and a R
SENSE
of 50mΩ, the prescaler should be programmed to M = 4.
The q
LSB
then becomes 2.656µAh and the battery capacity
corresponds to roughly 37650 q
LSB
s.
Note that the internal digital resolution of the coulomb
counter is higher than indicated by q
LSB
. The digitized
charge q
INTERNAL
is M • 8 smaller than q
LSB
. q
INTERNAL
is
typically 299µAs for a 50mΩ sense resistor.
V
BAT
Alert B[7:6]
The V
BAT
alert function allows the LTC2941 to monitor
the voltage at SENSE
–
. If enabled, a drop of the voltage
at the SENSE
–
pin below a preset threshold is detected
and bit A[1] in the status register is set. If the alert mode
is enabled by setting B[2] to one, an alert is generated at
the AL/CC pin. The threshold for the V
BAT
alert function
is selectable according to Table 3.
Accumulated Charge Registers (C, D)
The coulomb counter of the LTC2941 integrates current
through the sense resistor. The 16-bit result of this charge
integration is stored in the accumulated charge registers
C and D. As the LTC2941 does not know the actual battery
status after initial power-up, the accumulated charge is
set to mid-scale (7FFFh). If the host knows the status of
the battery , the accumulated charge registers C[7:0] and
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]). Before writing the accumulated charge
registers, the analog section should be shut down by setting
B[0] to 1. In order to avoid a change in the accumulated
charge registers between reading MSBs C[7:0] and LSBs
D[7:0], it is recommended to read them sequentially, as
shown in Figure 8.
Threshold Registers (E, F), (G, H)
For battery charge, the LTC2941 features a high and a
low threshold register. At power-up the high threshold
is set to FFFFh while the low threshold is set to 0000h.
Both thresholds can be programmed to a desired value via
I
2
C. As soon as the accumulated charge exceeds the high
threshold or falls below the low threshold, the LTC2941
sets the corresponding flag in the status register and pulls
the AL/CC pin low if alert mode is enabled.
I
2
C Protocol
The LTC2941 uses an I
2
C/SMBus compatible 2-wire open-
drain interface supporting multiple devices and masters on
a single bus. The connected devices can only pull the bus
wires LOW and they never drive the bus HIGH. The bus
wires should be externally connected to a positive sup
-
ply voltage via a current source or pull-up resistor. When
the bus is idle, both SDA and SCL are HIGH. Data on the
I
2
C-bus can be transferred at rates of up to 100kbit/s in
standard mode and up to 400kbit/s in fast mode.
Each device on the I
2
C/SMBus is recognized by a unique
address stored in that device and can operate as either a
transmitter or receiver, depending on the function of the
device. In addition to transmitters and receivers, devices
can also be classified as masters or slaves when perform
-
ing data transfers. A master is the device which initiates a
data
transfer
on the bus and generates the clock signals to
permit that transfer. At the same time any device addressed
is considered a slave. The LTC2941 always acts as a slave.
Figure 4 shows an overview of the data transmission on
the I
2
C bus.
applicaTions inFormaTion
