LTC3789 LINER [Linear Technology], LTC3789 Datasheet - Page 22

• Manufacturer Part Number: LTC3789
• Description: High Voltage High Current Controller for Battery Charging and Power Management
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTC4000
Battery Temperature Qualified Charging
To use the battery temperature qualified charging feature,
connect an NTC thermistor, R
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.
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:
or
22
R3 =
R3 = 1.857 • R
R
NTC
Figure 9. NTC Thermistor Connection
THERMALLY COUPLED
WITH BATTERY PACK
at cold_ threshold
LTC4000
NTC RESISTOR
NTC
3
at hot _ threshold
BIAS
NTC
BAT
NTC
R3
, between the NTC pin
R
4000 F09
NTC
C
BIAS
Notice that with only one degree of freedom (i.e. adjusting
the value of R3), the user can only use one of the formu-
las 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
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
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
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.
Figure 10. NTC Thermistor Connection with
Desensitizing Resistor R
NTC
THERMALLY COUPLED
WITH BATTERY PACK
at cold_threshold.
LTC4000
NTC RESISTOR
NTC
BIAS
NTC
BAT
at cold_threshold. Similarly, if the
D
R3
R
D
R
4000 F10
NTC
D
C
in effect reduces
BIAS
4000f