LTC4000_15 Datasheet, PDF (21/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

LTC4000
Applications Information
Battery Temperature Qualified Charging
To use the battery temperature qualified charging feature,
connect an NTC thermistor, R_NTC, between the NTC pin
and the GND pin, and a bias resistor, R3, from the BIAS
pin to the NTC pin (Figure 9). Thermistor manufacturer
datasheets usually include either a temperature lookup
table or a formula relating temperature to the resistor
value at that corresponding temperature.
BIAS
LTC4000
NTC
BAT
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY PACK
R3
C_BIAS
R_NTC
4000 F09
Figure 9. NTC Thermistor Connection
In a simple application, R3 is a 1% resistor with a value
equal to the value of the chosen NTC thermistor at 25°C
(R25). In this simple setup, the LTC4000 will pause charg-
ing when the resistance of the NTC thermistor drops to
0.54 times the value of R25. For a Vishay Curve 2 therm-
istor, this corresponds to approximately 41.5°C. As the
temperature drops, the resistance of the NTC thermistor
rises. The LTC4000 is also designed to pause charging
when the value of the NTC thermistor increases to three
times the value of R25. For a Vishay Curve 2 thermistor,
this corresponds to approximately –1.5°C. With Vishay
Curve 2 thermistor, the hot and cold comparators each
have approximately 5°C of hysteresis to prevent oscillation
about the trip point.
The hot and cold threshold can be adjusted by changing
the value of R3. Instead of simply setting R3 to be equal to
R25, R3 is set according to one of the following formulas:
R3 = ^{RNTC at cold_ threshold}
3
or
R3 = 1.857 • R_NTC at hot _ threshold
Notice that with only one degree of freedom (i.e. adjusting
the value of R3), the user can only use one of the formulas
above to set either the cold or hot threshold but not both.
If the value of R3 is set to adjust the cold threshold, the
value of the NTC resistor at the hot threshold is then
equal to 0.179 • R_NTC at cold_threshold. Similarly, if the
value of R3 is set to adjust the hot threshold, the value
of the NTC resistor at the cold threshold is then equal to
5.571 • R_NTC at cold_threshold.
Note that changing the value of R3 to be larger than R25
will move both the hot and cold threshold lower and vice
versa. For example, using a Vishay Curve 2 thermistor
whose nominal value at 25°C is 100k, the user can set
the cold temperature to be at 5°C by setting the value of
R3 = 75k, which automatically then sets the hot threshold
at approximately 50°C.
It is possible to adjust the hot and cold threshold indepen-
dently by introducing another resistor as a second degree
of freedom (Figure 10). The resistor R_D in effect reduces
the sensitivity of the resistance between the NTC pin and
ground. Therefore, intuitively this resistor will move the hot
threshold to a hotter temperature and the cold threshold
to a colder temperature.
BIAS
LTC4000
NTC
BAT
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY PACK
R3
C_BIAS
R_D
R_NTC
4000 F10
Figure 10. NTC Thermistor Connection with
Desensitizing Resistor R_D
For more information www.linear.com/LTC4000
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