Datasheet

LTC2941-1
14
29411f
applicaTions inFormaTion
where the factor 1.22 approximates the effect of sense
resistor self-heating, R
PP(MAX)
is the maximum pad-to-
pad resistance at nominal temperature (27°C) and θ
JA
is
the thermal resistance from junction to ambient. The θ
JA
data given for the DFN package is valid for typical PCB
layouts; more precise θ
JA
data for a particular PCB layout
may be obtained by measuring the voltage V
P-P
between
SENSE
+
and SENSE
–
, the ambient temperature T
AMB
, and
the die temperature T
DIE
, and calculating:
θ
JA
DIE AMB
P P SENSE
T T
V I
=
–
•
-
For evaluation of printed circuit board layouts during devel-
opment, both T
AMB
and T
DIE
temperature may be measured
using a pin compatible LTC2942-1 internal temperature
sensor. I
SENSE
should be set to zero to measure T
AMB
,
and high enough during T
DIE
measurement to achieve a
significant temperature increase over T
AMB
.
Measuring Current
In some applications, it may be desirable to measure the
current I
SENSE
flowing through the internal sense resistor.
Since charge measured by the coulomb counter is the
time integral over I
SENSE
, differentiation of the contents
of the accumulated charge register (ACR) over time may
be used to measure average current.
Accuracy of such an indirect current measurement is limited
by the basic accuracy of the coulomb counter, the accuracy
of the timebase within the host system, quantization caused
by the prescaler setting, and time delays caused by I
2
C
transactions. Still, especially at higher currents, useful
results may be obtained by reading the accumulated charge
register twice, with a defined time interval in between, and
dividing the charge difference by the time interval. The
time interval may be increased at low currents to limit time
quantization errors to the desired accuracy. For quicker
current measurement at low currents, prescale factor M
may be temporarily decreased, sacrificing some coulomb
count accuracy for higher current resolution.
Extending Coulomb Counter Range
To increase the range of the coulomb counter for battery
capacities higher than 5.5Ah, the host controller can either
regularly poll the accumulated charge register (ACR) or use
the threshold registers to determine when the accumulated
charge register approaches the minimum or maximum
limits. At this point it can add or subtract a fixed charge
quantity and rewrite the result into the ACR. The added
or subtracted charge quantities can then be tracked in
software, increasing the effective ACR range.
PC Board Layout Recommendations
Keep all traces as short as possible to minimize noise and
inaccuracy. Use wider traces from the resistor to the bat-
tery, load and/or charger (see Figure 10). Put the bypass
capacitor close to SENSE
+
and GND. Provide adequate
copper area on exposed pad for heat sinking.
LTC2941-1
ELECTRICALLY ISOLATED HEAT SINK
CONNECTED TO EXPOSED PAD ONLY
2941 F10
TO BATTERY
TO
CHARGER/LOAD
4
5
6
3
2
1
C
Figure 10. Recommended Layout