Rainbow Electronics DS2712 - Manual (Page 10)

DS2711/DS2712: Loose Cell NiMH Charger
10 of 13
Charge Timer
The charge timer monitors the duration of charge in fast and top-off charge phases, and is reset at the beginning of
each phase. The time-out period is set with an external resistor connected from the TMR pin to V
SS
. Resistors can
be selected to support fast-charge time-out periods of 0.5 to 10 hours and top-off charge time-out periods of 0.25 to
5 hours. If the timer expires in fast-charge, the timer count is reset and charging proceeds to the top-off charge
phase. The top-off time-out period is half of the fast charge time-out period. If the timer expires in top-off, charging
proceeds to the maintenance phase. The programmed charge time approximately follows the equation:
t = 1.5 x R / 1000 (time in minutes)
Suspend
Suspension of charge activity is possible by floating the TMR pin. The CC1 and CC2 outputs become high-Z and
the charge timer stops. The state machine and all timers are reset to their presence test conditions.
Temperature Sense
Connecting an external 10k W NTC thermistor between THM1 or THM2 (THMx) and V
SS
, and a 10k W bias resistor
between V
DD
and THMx allows the DS2711 to sense temperature. To sense the temperature of the battery cells,
locate the thermistor close to the body of the battery cell so THM1 monitors the temperature of cell-1 and THM2
monitors the temperature of cell-2. Alternatively, the thermistor can sense ambient temperature by locating it away
from the cells. THM1 and THM2 can be connected together to sense temperature using a single thermistor and
bias resistor. The temperature qualification function can be defeated by connecting THM1 and THM2 to a single
resistor-divider supplying a voltage between the Thermistor-Min and Thermistor-Max threshold voltages. Several
recommended 10k
W thermistors are shown in Table 2.
Min, Max Temperature Compare
The voltage thresholds of the THMx inputs (VTHM-MIN, VTHM-MAX) are set to allow fast charging to start if 0 °C <
T
A
< 45 °C when using the recommended 10kW bias and 10kW thermistor. If fast charging is in progress, and the
voltage on THMx reaches VTHM-STOP, fast charging stops and the maintenance phase begins.
Table 2. THM1, THM2 THRESHOLDS
TEMPERATURE ( °C)
THM
THRESHOLD
RATIO
OF V
DD
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
Figure 7. CELL VOLTAGE SENSE POINTS
Vcell1
Vcell2
VN1
VN0
VP1
VP2
Charge Source
Vcell1 Vcell2
CC1
CC1
CC2
VP1
VN1
VP2
VN0
Series Configuration Parallel Configuration
Charge Source