LTC2941I View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC2941I Datasheet PDF : 16 Pages
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LTC2941
APPLICATIONS INFORMATION
The value of the external sense resistor is determined by
the maximum input range of VSENSE and the maximum
current of the application:
RSENSE
≤
50mV
IMAX
The choice of the external sense resistor value influences
the gain of the coulomb counter. A larger sense resistor
gives a larger differential voltage between SENSE+ and
SENSE– for the same current which results in more precise
coulomb counting. Thus the amount of charge represented
by the least significant bit (qLSB) of the accumulated charge
(registers C, D) is given by:
qLSB
= 0.085mAh • 50mΩ
RSENSE
•
M
128
or
qLSB
=
0.085mAh
•
50mΩ
RSENSE
when the prescaler is set to its default value of M=128.
Note that 1mAh = 3.6A•s = 3.6C (coulombs).
Choosing RSENSE = 50mV/IMAX is not sufficient in
applications where:
A. the battery capacity (QBAT) is very large compared to
the maximum current (IMAX):
QBAT > IMAX • 5.5 hours
B. the battery capacity (QBAT) is very small compared to
the maximum current (IMAX):
QBAT < IMAX • 0.1 hours
For case A: In low current applications using a large bat-
tery, choosing RSENSE according to RSENSE = 50mV/IMAX
can lead to a qLSB smaller than QBAT/216 and the 16-bit
accumulated charge may underflow before the battery is
exhausted or overflow during charge. Choose in this case
a maximum RSENSE of:
RSENSE
≤
0.085mAh • 216
QBAT
• 50mΩ
In an example application where the maximum current is
IMAX = 100mA, calculating RSENSE = 50mV/IMAX would
lead to a sense resistor of 500mΩ. This gives a qLSB of
8.5μAh and the accumulated charge register can represent
a maximum battery capacity of QBAT = 8.5μAh • 65535 =
557mAh. If the battery is larger, RSENSE must be lowered.
For example, RSENSE must be reduced to 150mΩ if a bat-
tery with a capacity of 1800mAh is used.
For case B: In applications using a small battery but having
a high maximum current, qLSB can get quite large with
respect to the battery capacity. For example, if the battery
capacity is 100mAh and the maximum current is 1A, the
standard equation leads to choose a sense resistor value
of 50mΩ, resulting in:
qLSB = 0.085mAh = 306mC
The battery capacity then corresponds to only 1176 qLSBs
and less than 2% of the accumulated charge register is
utilized.
To preserve digital resolution in this case, the LTC2941
includes a programmable prescaler. Lowering the prescaler
factor M allows reducing qLSB to better match the accumu-
lated charge registers C and D to the capacity of the battery.
The prescaling factor M can be chosen between 1 and its
default value 128. The charge LSB then becomes:
qLSB
= 0.085mAh • 50mΩ
RSENSE
•M
128
To use as much of the range of the accumulated charge
registers C and D as possible the prescaler factor M should
2941f
10
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