Rainbow Electronics DS2715 - Manual (Page 11)

DS2715: NiMH Cell Pack Charge Controller
11 of 12
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
W NTC thermistor between THM and V
SS
, and a 10k W 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
W
thermistors are shown in Table 2.
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 0 °C < T
CELL
< 45 °C
when using the recommended 10k W bias and 10kW thermistor. 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 top-off, charging stops and the DS2715 enters the DONE state. Fast charging will
complete normally and top-off will begin if the voltage change on THM exceeds T
TERM
ºC per minute (dT/dt charge
termination).
Table 2. THM1, THM2 THRESHOLDS
TEMPERATURE ( °C)
THM
THRESHOLD
RATIO
OF V
CBIAS
THERMISTOR
RESISTANCE
( W)
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 10 20 30 40 50
Degrees Centigrade
Vo
lt
ag
e R
at
io