Analog Devices LTC2941 Datasheet page 10

LTC2941
APPLICATIONS INFORMATION
The value of the external sense resistor is determined by
the maximum input range of V
current of the application:
50mV
R
≤
SENSE
I
MAX
The choice of the external sense resistor value influences
the gain of the coulomb counter. A larger sense resis-
tor 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 (q
mulated charge (registers C, D) is given by:
50mΩ
q = 0.085mAh •
LSB
R
SENSE
or
50mΩ
q = 0.085mAh •
LSB
R
SENSE
when the prescaler is set to its default value of M=128.
Note that 1mAh = 3.6A • s = 3.6C (coulombs).
Choosing R = 50mV/I
SENSE
applications where:
A. the battery capacity (Q
the maximum current (I
Q > I • 5.5 hours
BAT MAX
B. the battery capacity (Q
the maximum current (I
Q < I • 0.1 hours
BAT MAX
For case A: In low current applications using a large bat-
tery, choosing R according to R
SENSE
can lead to a q smaller than Q
LSB
accumulated charge may underflow before the battery is
10
and the maximum
SENSE
) of the accu-
LSB
M
•
128
is not sufficient in
MAX
) is very large compared to
BAT
):
MAX
) is very small compared to
BAT
):
MAX
= 50mV/I
SENSE
16
/2 and the 16-bit
BAT
For more information
exhausted or overflow during charge. Choose in this case
a maximum R
SENSE
0.085mAh • 2
R
≤
SENSE
In an example application where the maximum current is
I = 100mA, calculating R
MAX
lead to a sense resistor of 500mΩ. This gives a q
+
of 8.5µAh and the accumulated charge register can rep-
resent a maximum battery capacity of Q
65535 = 557mAh. If the battery is larger, R
lowered. For example, R
if a battery with a capacity of 1800mAh is used.
For case B: In applications using a small battery but hav-
ing a high maximum current, q
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:
q = 0.085mAh = 306mC
LSB
The battery capacity then corresponds to only 1176 q
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 pres-
caler factor M allows reducing q
accumulated 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:
q = 0.085mAh •
LSB
MAX
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of:
16
• 50mΩ
Q
BAT
= 50mV/I
SENSE
BAT
SENSE
must be reduced to 150mΩ
SENSE
can get quite large with
LSB
to better match the
LSB
50mΩ M
•
R 128
SENSE
would
MAX
LSB
= 8.5µAh •
must be
s
LSB
Rev. C