LTC2943
APPLICATIONS INFORMATION
If neither the alert nor the charge complete functionality
is desired, bits B[2:1] should be set to [00]. The ALCC
pin is then disabled and should be tied to the supply of
For such low current applications with a large battery,
choosing R
according to R
= ±0mV/I
can
SENSE
SENSE
MAX
16
lead to a q
smaller than Q /2 and the 16-bit accu-
LSB
BAT
2
the I C bus with a 10k resistor.
mulated charge register may underflow before the battery
is exhausted or overflow during charge. Choose, in this
Avoid setting B[2:1] to [11] as it enables the alert and the
charge complete modes simultaneously.
case, a maximum R
of:
SENSE
0.340mAh•216
QBAT
RSENSE
≤
•±0mΩ
Choosing R
SENSE
To achieve the specified precision of the coulomb counter,
+
–
In an example application where the maximum current is
thedifferentialvoltagebetweenSENSE andSENSE must
stay within ±±0mV. With input signals up to 300mV the
LTC2943 will remain functional but the precision of the
coulomb counter is not guaranteed.
I
= 100mA, calculating R
= ±0mV/I
would
LSB
MAX
SENSE
MAX
lead to a sense resistor of ±00mΩ. This gives a q
of
34μAh and the accumulated charge register can represent
a maximum battery capacity of Q
= 34μAh•6±±3± =
BAT
The required value of the external sense resistor, R
,
SENSE
2228mAh. If the battery capacity is larger, R
must
SENSE
should be reduced to
is determined by the maximum input range of V
the maximum current of the application:
and
SENSE
be lowered. For example, R
SENSE
1±0mΩ if a battery with a capacity of 7200mAh is used.
±0mV
IMAX
Choosing Coulomb Prescaler M B[5:3]
RSENSE
≤
If the battery capacity (Q ) is small compared to the
BAT
maximum current (I
) the prescaler value M should
be changed from its default value (4096).
MAX
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
In these applications with a small battery but a high maxi-
–
SENSE for the same current resulting in more precise
mum current, q
can get quite large with respect to the
LSB
coulomb counting. The amount of charge represented by
battery capacity. For example, if the battery capacity is
100mAh and the maximum current is 1A, the standard
equation leads to choosing a sense resistor value of
±0mΩ, resulting in:
the least significant bit (q ) of the accumulated charge
LSB
(registers C, D) is equal to:
±0mΩ
RSENSE 4096
M
qLSB =0.340mAh•
•
qLSB =0.340mAh=1224mC
or
The battery capacity then corresponds to only 294 q
LSB
and less than 0.±% of the accumulated charge register
is utilized.
±0mΩ
RSENSE
qLSB =0.340mAh•
To preserve digital resolution in this case, the LTC2943
includesaprogrammableprescaler.Loweringtheprescaler
when the prescaler is set to its default value of M = 4096.
Note that 1mAh = 3.6C (coulomb).
factor M reduces q
to better match the accumulated
LSB
chargeregistertothecapacityofthebattery.Theprescaling
factor M can be chosen between 1 and its default value
of 4096. The charge LSB then becomes:
Choosing R
= ±0mV/I
is not sufficient in
MAX
SENSE
applicationswherethebatterycapacity(Q )isverylarge
BAT
compared to the maximum current (I
):
MAX
±0mΩ
RSENSE 4096
M
qLSB =0.34mAh•
•
QBAT >IMAX •22Hours
2943fa
11
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Linear [ Linear ]
