Datasheet

LTC2942-1
14
29421f
applicaTions inFormaTion
where T
NOM
= 27°C (or 300K) and R
PP
(T
NOM
) is from
the Electrical Characteristics table. This means that the
resistance between SENSE
+
and SENSE
may drop by
26% if die temperature changes from 27°C to –40°C
or increase by 23% for a 27°C to 85°C die temperature
change. Ensure that total voltage drop between SENSE
+
and SENSE
, caused by maximum peak current flowing
in/out of SENSE
:
V
DROP
= I
PEAK
• R
PP
(T
DIE(MAX)
)
does not exceed the application’s requirements.
Limiting Inrush Current
Inrush currents during events like battery insertion or
closure of a mechanical power switch may be substan-
tially higher than peak currents during normal operation.
Extremely large inrush currents may require additional
circuitry to keep currents through the LTC2942-1 sense
resistor below the absolute maximum ratings.
Note that external Schottky clamp diodes between SENSE
+
and SENSE
can leak significantly, especially at high tem-
perature, which can cause significant coulomb counter
errors. Preferred solutions to limit inrush current include
active Hot Swap™ current limiting or connector designs
that include current limiting resistance and staggered pins
to ensure a low impedance connection when the connector
is fully mated.
Power Dissipation
Power dissipation in the R
PP
resistance when operated
at high currents can increase the die temperature sev-
eral degrees over ambient. Soldering the exposed pad
of the DFN package to a large copper region on the PCB
is recommended for applications operating close to the
specified maximum current and ambient temperature. Die
temperature at a given I
SENSE
can be estimated by:
T
DIE
= T
AMB
+ 1.22 • θ
JA
• R
PP(MAX)
• I
SENSE
2
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
=
-
Both T
AMB
and T
DIE
temperature may be measured using
the internal temperature sensor included in the LTC2942-1.
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
.
S R
ALERT RESPONSE ADDRESS DEVICE ADDRESS
29421 F08
A
1
0001100 11001001
0 1
P
A
Figure 8. LTC2942-1 Serial Bus SDA Alert Response Protocol
S10ms W
ADDRESS REGISTER S
29421 F09
A A ADDRESS
0
1100100 08h 1
0 0 1100100
0
P
R
0
A
F1h
DATA
80h
DATA
A
1
A
S W
ADDRESS REGISTER DATA
A A
0
1100100 01h BC
0 0
P
Figure 9. Voltage Conversion Sequence
S W
ADDRESS REGISTER S
29421 F10
A A ADDRESS
0
1100100 02h 1
0 0 1100100
0
P
R
0
A
80h
DATA
01h
DATA
A
1
A
Figure 10. Reading the LTC2942-1 Accumulated Charge Registers (C, D)