Datasheet

LTC3553
18
3553fb
NTC Thermistor
The battery temperature is measured by placing a nega-
tive temperature coeffi cient (NTC) thermistor close to the
battery pack. To use this feature connect the NTC therm-
istor, R
NTC
, between the NTC pin and ground and a bias
resistor, R
NOM
, from V
BUS
to NTC, as shown in Figure 1.
R
NOM
should be a 1% resistor with a value equal to the
value of the chosen NTC thermistor at 25°C (R25). The
LTC3553 will pause charging when the resistance of the
NTC thermistor drops to 0.54 times the value of R25
or approximately 54k (for a Vishay curve 1 thermistor,
this corresponds to approximately 40°C). If the battery
charger is in constant-voltage mode, the safety timer also
pauses until the thermistor indicates a return to a valid
temperature. As the temperature drops, the resistance of
the NTC thermistor rises. The LTC3553 is also designed
to pause charging when the value of the NTC thermistor
increases to 3.17 times the value of R25. For a Vishay
curve 1 thermistor this resistance, 317k, corresponds to
approximately 0°C. The hot and cold comparators each
have approximately 3°C of hysteresis to prevent oscillation
about the trip point.
Alternate NTC Thermistors and Biasing
The LTC3553 provides temperature qualifi ed charging if
a grounded thermistor and a bias resistor are connected
to NTC. By using a bias resistor whose value is equal to
the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are preprogrammed
to approximately 40°C and 0°C, respectively (assuming
a Vishay curve 1 thermistor).
The upper and lower temperature thresholds can be
adjusted by either a modifi cation of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modifi ed but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tem-
perature trip points can be independently programmed
with the constraint that the difference between the upper
and lower temperature thresholds cannot decrease.
Examples of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, used
in the following examples, has a nominal value of 100k
and follows the Vishay curve 1 resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
R
NTC|COLD
= Value of thermistor at the cold trip point
R
NTC|HOT
= Value of the thermistor at the hot trip point
r
COLD
= Ratio of R
NTC|COLD
to R25
r
HOT
= Ratio of R
NTC|HOT
to R25
R
NOM
= Primary thermistor bias resistor (see Figure 2)
R1 = Optional temperature range adjustment resistor (see
Figure 2)
OPERATION
Figure 1. Typical NTC Thermistor Circuit
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
V
BUS
NTC_ENABLE
3553 F01
NTC BLOCK
TOO_COLD
TOO_HOT
0.76 • V
BUS
(NTC RISING)
0.35 • V
BUS
(NTC FALLING)
0.017 • V
BUS
(NTC FALLING)
–
+
20
15