Datasheet

LTC2942
10
2942fa
applicaTions inFormaTion
Alert/Charge Complete Configuration B[2:1]
The AL/CC pin is a dual function pin configured by the
control register. By setting bits B[2:1] to [10] (default)
the AL/CC pin is configured as an alert pin following the
SMBus protocol. In this configuration the AL/CC pin is a
digital output and is pulled low if one of the three mea-
sured quantities (charge, voltage, temperature) exceeds
its high or low threshold or if the value of the accumulated
charge register overflows or underflows. An alert response
procedure started by the master resets the alert at the
AL/CC pin. For further information see the Alert Response
Protocol section.
Setting the control bits B[2:1] to [01] configures the AL/CC
pin as a digital input. In this mode, a high input on the
AL/CC pin communicates to the LTC2942 that the battery
is full and the accumulated charge register is set to its
maximum value FFFFh. The AL/CC pin would typically
be connected to the “charge complete” output from the
battery charger circuitry.
If neither the alert nor the charge complete functionality
is desired, bits B[2:1] should be set to [00]. The AL/CC
pin is then disabled and should be tied to GND.
Avoid setting B[2:1] to [11] as it enables the alert and the
charge complete modes simultaneously.
Choosing R
SENSE
To achieve the specified precision of the coulomb counter,
the differential voltage between SENSE
+
and SENSE
–
must
stay within ±50mV. For differential input signals up to
±300mV the LTC2942 will remain functional but the preci-
sion of the coulomb counter is not guaranteed.
The required value of the external sense resistor, R
SENSE
,
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 equal to:
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 = 3.6C (coulomb).
Choosing R
SENSE
= 50mV/I
MAX
is not sufficient in ap-
plications where the battery capacity (Q
BAT
) is very large
compared to the maximum current (I
MAX
):
Q
BAT
> I
MAX
• 5.5 Hours
For such low current applications with a large battery,
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 accu-
mulated charge register 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 capacity is larger, R
SENSE
must be
lowered. For example, R
SENSE
must be reduced to 150mΩ
if a battery with a capacity of 1800mAh is used.