LTC2941-1
13
29411f
applicaTions inFormaTion
Internal Sense Resistor
The internal sense resistor uses proprietary* tempera-
ture compensation techniques to reduce the effective
temperature coefficient to less than ±50ppm/K typically.
The effective sense resistance as seen by the coulomb
counter is factory trimmed to 50mΩ. Both measures,
and the lack of thermocouple effects in the sense resis-
tor connections, contribute to the LTC2941-1’s superior
charge measurement accuracy compared to competing
solutions employing a common 1% tolerance, 50ppm/K
tempco discrete current sense resistor.
Like all sense resistors, the integrated sense resistor in
the LTC2941-1 will exhibit minor long-term resistance
shift. The resistance typically drops less than –0.1% per
1000h at 1A current and 85°C ambient temperature; this
outperforms most types of discrete sense resistors except
those of the very high and ultrahigh stability variety. See
the Typical Performance Characteristics for expected
resistor drift performance under worst-case conditions.
Drift will be much slower at lower temperatures. Contact
LTC applications for more information.
For most coulomb counter applications this aging behavior
of the integrated sense resistor is insignificant compared
to the change of battery capacity due to battery aging.
The LTC2941-1 is factory trimmed to optimum accuracy
when new; for applications which require the best possible
coulomb count accuracy over the full product lifetime, the
coulomb counter gain can be adjusted in software. For
instance, if the error contribution of sense resistor drift
must be limited to ±1%, coulomb counts may be biased
high by 1% (use factor 1.01), and maximum operational
temperature and current then must be derated such that
sense resistor drift over product lifetime or calibration
intervals is less than –2%.
Applications employing the standard external resistor
LTC2941 with an external 50mΩ sense resistor may be
upgraded to the pin compatible LTC2941-1 by removing
the external sense resistor.
Voltage Drop Between SENSE
+
and SENSE
–
The LTC2941-1 is trimmed for an effective internal resis-
tance of 50mΩ , but the total pin-to-pin resistance (R
PP
),
consisting of the sense resistor in series with pin and bond
wire resistances, is somewhat higher. Assuming a sense
resistor temperature coefficient of about 3900ppm/K, the
total resistance between SENSE
+
and SENSE
–
at a tem-
perature, T , is typically:
R
PP
(T) = R
PP
(T
NOM
) [1 + 0.0039(T – T
NOM
)]
where T
NOM
= 27°C (or 300K) and R
PP
(T
NOM
) is from the
Electrical Characteristics table. This means that the resis-
tance 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 LTC2941-1 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
*Patent pending.
