LTC4071
11
Rev. D
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APPLICATIONS INFORMATION
The GND pin of the top device is simply connected to
the VCC pin of the bottom device. Care must be taken in
observing the HBO status output pin of the top device as
this signal is no longer ground referenced. Likewise for
the control inputs of the top device; tie ADJ and LBSEL
of the top device to the local GND or VCC pins. Also, the
wall adapter must have a high enough voltage rating to
charge both cells.
NTC Protection
The LTC4071 measures battery temperature with a nega-
tive temperature coefficient thermistor thermally coupled
to the battery. NTC thermistors have temperature char-
acteristics which are specified in resistance-temperature
conversion tables. Internal NTC circuitry protects the bat-
tery from excessive heat by reducing the float voltage for
each 10°C rise in temperature above 40°C (assuming a
Vishay thermistor with a B25/85 value of 3490).
The LTC4071 uses a ratio of resistor values to measure
battery temperature. The LTC4071 contains an internal
fixed resistor voltage divider from NTCBIAS to GND with
four tap points; NTCTH1–NTCTH4. The voltages at these
tap points are periodically compared against the voltage at
the NTC pin to measure battery temperature. To conserve
power, the battery temperature is measured periodically
by biasing the NTCBIAS pin to VCC about once every 1.5
seconds.
The voltage at the NTC pin depends on the ratio of NTC
thermistor value, R
NTC
, and a bias resistor, R
NOM
. Choose
RNOM equal to the value of the thermistor at 25°C. RNOM
is 10k for a Vishay NTHS0402N02N1002F thermistor with
a B
25/85
value of 3490. R
NOM
must be connected from
NTCBIAS to NTC. The ratio of the NTC pin voltage to the
NTCBIAS voltage when it is pulsed to VCC is:
RNTC
R
+R
When the thermistor temperature rises, the resistance
drops; and the resistor divider between RNOM and the
thermistor lowers the voltage at the NTC pin.
An NTC thermistor with a different B25/85 value may also
be used with the LTC4071. However the temperature trip
points are shifted due to the higher negative temperature
coefficient of the thermistor. To correct for this difference
add a resistor, RFIX, in series with the thermistor to shift
the ratio:
FIX +
NTC
RFIX +RNTC +RNOM
Up to the internal resistive divider tap points: NTCTH1
through NTCTH4. For a 100k thermistor with a B25/85
value of 3950, e.g. NTHS0402N01N1003F, at 70°C (with
RNOM = 100k) choose RFIX = 3.92k. The temperature trip
points are found by looking up the curve 1 thermistor R/T
values plus RFIX that correspond to the ratios for NTCTH1
= 36.5%, NTCTH2 = 29%, NTCTH3 = 22.8%, and NTCTH4
= 17.8%. Selecting RFIX = 3.92k results in trip points of
39.9°C, 49.4°C, 59.2°C and 69.6°C.
Another technique may be used without adding an addi-
tional component. Instead decrease RNOM to adjust the
NTCTH thresholds for a given R/T thermistor profile. For
example, if RNOM = 88.7k (with the same 100k thermis-
tor) then the temperature trip points are 41.0°C, 49.8°C,
58.5°C and 67.3°C.
When using the NTC features of the LTC4071 it is important
to keep in mind that the maximum shunt current increases
as the float voltage, VFLOAT_EFF drops with NTC condition-
ing. Reviewing the single-cell battery charger application
with a 12V wall adapter in Figure2; the input resistor should
be increased to 165Ω such that the maximum shunt cur-
rent does not exceed 50mA at the lowest possible float
voltage due to NTC conditioning, VFLOAT_MIN = 3.8V.
Thermal Considerations
At maximum shunt current, the LTC4071 may dissipate
up to 205mW. The thermal dissipation of the package
should be taken into account when operating at maximum
shunt current so as not to exceed the absolute maximum
junction temperature of the device. With θ
JA
of 40°C/W, in
the MSOP package, at maximum shunt current of 50mA
the junction temperature rise is about 8°C above ambi-
ent. With θJA of 76°C/W in the DFN package, at maximum
shunt current of 50mA the junction temperature rise is
about 16°C above ambient. The junction temperature,
TJ, is calculated depending on ambient temperature,