Datasheet

DS2715: NiMH Battery Pack Charge Controller
TEMPERATURE SENSE
Accurate temperature sensing is needed to determine end of charge by dT/dt and to detect over temperature fault
conditions. Connecting an external 10k NTC thermistor between THM and V
SS
, and a 10k bias resistor between
Div and THM allows the DS2715 to sense temperature. To sense the temperature of the cell pack, locate the
thermistor close to the body of a cell, preferably in the middle of the cell pack. Several recommended 10k
thermistors are shown in Table 3.
Min, Max Temperature Compare
The voltage thresholds of the THM input (V
THM-MIN
, V
THM-MAX
) are set to allow charging to start if the thermistor
temperature is between 0C and 45C when using the recommended 10k bias resistor and 10k thermistor
circuit. If pre-charging is in progress, and the voltage on THM reaches V
THM-STOP
, pre-charging stops and a fault
condition is generated. If the voltage on THM reaches V
THM-STOP
during FAST-CHARGE or TOPOFF, charging
stops and the DS2715 enters the DONE state. FAST_CHARGE will complete normally and TOPOFF will begin if
the voltage change on THM exceeds the equivalent T
TERM
ºC per minute (dT/dt Detect specification).
Table 3. THM THRESHOLDS
TEMPERATURE (C)
THM
THRESHOLD
RATIO
OF V
CBIAS
THERMISTOR
RESISTANCE
()
Semitec
103AT-2
Fenwal
197-103LAG-A01
173-103LAF-301
MIN 0.73 27.04k
0C 4C
MAX 0.33 4.925k
45C 42C
STOP 0.29 4.085k
50C 47C
Used with a 10k resistor, the Semitec 103AT-2 provides about 0.9% full scale per degree sensitivity. This linearity
is shown in the curve in Figure 4. The left axis is the ratio of the sensed voltage to the divider’s input voltage
(V
Cbias
).
Figure 4. RATIO OF THM PIN TO CBIAS PIN OVER TEMPERATURE
Divider Output
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0 1020304050
Degrees Centigrade
Voltage Ratio