Datasheet

LTC2941
9
2941fa
applicaTions inFormaTion
The value of the external sense resistor is determined by
the maximum input range of V
SENSE
and the maximum
current of the application:
R
mV
I
SENSE
MAX
≤
50
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 (q
LSB
) of the accumulated charge
(registers C, D) is given by:
q
LSB
= 0.085mAh •
50mΩ
R
SENSE
•
M
128
or
q
LSB
= 0.085mAh •
50mΩ
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
SENSE
= 50mV/I
MAX
is not sufficient in
applications where:
A. the battery capacity (Q
BAT
) is very large compared to
the maximum current (I
MAX
):
Q
BAT
> I
MAX
• 5.5 hours
B. the battery capacity (Q
BAT
) is very small compared to
the maximum current (I
MAX
):
Q
BAT
< I
MAX
• 0.1 hours
For case A: In low current applications using a large bat-
tery, choosing R
SENSE
according to R
SENSE
= 50mV/I
MAX
can lead to a q
LSB
smaller than Q
BAT
/2
16
and the 16-bit
accumulated charge may underflow before the battery is
exhausted or overflow during charge. Choose in this case
a maximum R
SENSE
of:
R
SENSE
≤
0.085mAh • 2
16
Q
BAT
• 50mΩ
In an example application where the maximum current is
I
MAX
= 100mA, calculating R
SENSE
= 50mV/I
MAX
would
lead to a sense resistor of 500mΩ. This gives a q
LSB
of
8.5µAh and the accumulated charge register can represent
a maximum battery capacity of Q
BAT
= 8.5µAh • 65535 =
557mAh. If the battery is larger, R
SENSE
must be lowered.
For example, R
SENSE
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, q
LSB
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:
q
LSB
= 0.085mAh = 306mC
The battery capacity then corresponds to only 1176 q
LSB
s
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 q
LSB
to better match the accu-
mulated 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
LSB
= 0.085mAh •
50m
Ω
R
SENSE
•
M
128
To use as much of the range of the accumulated charge
registers C and D as possible the prescaler factor M should