DS2711/DS2712: Loose-Cell NiMH Chargers

10 of 15

Charge Timer

The charge timer monitors the duration of charge in fast and top-off charge phases, and is reset at the beginning of

be selected to support fast-charge timeout periods of 0.5 to 10 hours and top-off charge timeout 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 timeout period is half of the fast charge timeout 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 disconnecting 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

Ω NTC thermistor between THM1 or THM2 (THMx) and V

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

Ω 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 <

voltage on THMx reaches VTHM-STOP, fast charging stops and the maintenance phase begins.

Table 1. THM1, THM2 Thresholds

THM

THRESHOLD

THERMISTOR

RESISTANCE

(

Ω)

TEMPERATURE (

°C)

Semitec 103AT-2

Fenwal

197-103LAG-A01

173-103LAF-301

MIN

0.73

27.04k

0

4

MAX

0.33

4.925k

45

42

STOP

0.29

4.085k

50

47

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