ICS1702 datasheet(15/28 Pages) ETC

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ICS1702

Figure 12: Thermal switch to connection to

ground at the THERM pin

Table 7: Thermal Switch Voltage Thresholds

The 4%/min., 3%/min. and 2%/min. decrease in thermistor

resistance for the last minute of charge for the selected charge rate

are applicable for NTC thermistors other than 10k

Ω @ 25°C

provided that the following requirements are met:

•

An external pull-up resistor to 5V is used to provide a

•

thermistor voltage of 1.67V @ 25

°C.

•

The thermistor resistance at 25

°C does not exceed 20kΩ so

that accuracy and adequate noise immunity are maintained.

•

The thermistor resistance increases by a factor of about 1.8

from 25

°C to 10°C and the thermistor resistance decreases by

a factor of about 2.1 from 25

°C to 45°C.

• Using a thermal switch for hot battery detection:

A thermal switch that opens at about 45

°C is recommended. The

thermal switch must be connected between the THERM pin and

ground. When the thermal switch is closed, the voltage at the

THERM pin must be below 0.5V for normal operation. When the

thermal switch opens (see Figure 12), the internal pull-up at the

THERM pin will raise the voltage above 4.2V and the ICS1702

will shut down and will not restart unless reset. Table 7 contains

the internal voltage thresholds used with a thermal switch.

For example, a battery was monitored as it charged at a 1C rate in

°C ambient. In the final minute of charge, the battery

temperature rose from 29.8

°C to 31°C where full charge was

detected. With this data, the typical 10k

Ω @ 25°C thermistor used

in the example above is checked to determine if its characteristics

satisfy the 4% decrease in resistance required for the last minute of

charge. The thermistor measures 8.37k

Ω @ 29.8°C and 8.01kΩ at

°C. For a 1C charge rate, the resistance must decrease at least

4%/min. or more between 29.8

°C and 31°C. The percent decrease

in resistance for the thermistor is calculated as:

8.37k

Ω - 8.01kΩ (100) =4.30%

8.37k

Ω

This thermistor meets the 4%/min. requirement and will result in

termination at full charge at 31

°C. The thermistor must be checked

for a 4%/min. decrease in resistance for the last minute of charge

near the hot and cold battery thresholds.

The battery in the example above was charged in a 25

°C ambient

with its temperature rising 31

°C - 25°C or 6°C. The temperature

rise was 31

°C - 29.8°C or 1.2°C in the last minute before full

charge occurred. This information is used to check the thermistor

characteristics at the ambient extremes. If the selected 1C charge

rate is initiated at 12

°C, the thermistor resistance change must

decrease 4%/min. between 16.8

°C and 18°C. The thermistor

resistance at 16.8

°C is 13.68kΩ and at 18°C the thermistor

resistance is 13.06k

Ω.

13.68k

Ω - 13.06kΩ (100) =4.53%

13.68k

Ω

The thermistor meets the 4%/min. requirement and will result in

termination of fast charge at 18

°C. If the selected 1C charge rate is

initiated at 37

°C, the thermistor resistance change must decrease

4%/min. between 41.8

°C and 43°C. The thermistor resistance at

41.8

°C is 5.48kΩ and at 43°C the thermistor resistance is 5.25kΩ.

5.48k

Ω -5.25kΩ (100) =4.19%

5.48k

Ω

The thermistor meets the 4%/min. requirement and will result in

termination of fast charge at 43

°C.

THERM pin

R = 75k

normally closed thermal switch

opens at 45ºC

Parameter

Voltage

Battery

Temperature

Open Thermal Switch

Voltage

>4.2

>45

°C

Closed Thermal Switch

Voltage

<0.5

<45

°C

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