ICS1702 datasheet(14/28 Pages) ETC

14 / 28 page

ICS1702

To set the voltage at the THERM pin for 1.67V at 25

°C, the

equivalent divider looks like Figure 11.

Table 6: Thermistor Voltage Thresholds

Figure 11: Equivalent voltage divider

The parallel resistance R

|| is calculated:

|| = 5V - 1.67V

=20k

Ω.

1.67V/10k

Ω

The internal pull-up resistance R

pu and the parallel resistance R|| are

known so the external fixed resistor can be calculated from:

Rpu R||

Rfix = __________ .

Rpu - R||

Substituting in known values: R

fix

= 27.27k

Ω. A 27kΩ standard

value is used for R

fix.

Since the thermistor resistance R

th is specified by manufacturers at

a particular temperature, the voltage across the thermistor V

th at

that temperature can be calculated from:

Rth (5V)

Vth =

__________ (5V),

Rpu + R||

with the drop across the resistor divider equal to 5V. For this

example, the calculated voltage with R

=18k

Ω at 10°C is 2.37V

and with R

=4.7k

Ω at 45°C the voltage is 0.95V. Table 6 lists the

internal thresholds for hot and cold battery detection. If the voltage

across the thermistor (at the THERM pin) drops below 0.93V, the

ICS1702 will shut down due to a hot battery fault condition and

will not restart unless reset. If the voltage dropped across the

thermistor is above 2.4V before fast charge is initiated, the

ICS1702 will begin a reduced current charge. See the Cold Battery

Charging section for more information.

• Using an NTC thermistor for temperature slope

termination:

As a battery approaches full charge, its accelerated rate of heating

can be used to terminate fast charge by detecting the large change

in the temperature slope. The large change in temperature slope is

proportional to the thermistor voltage change per unit of time. If

the DTSEL pin is programmed for temperature slope termination,

the controller will calculate the thermistor voltage slope and

terminate based on internally set thresholds as listed in Table 1.

The threshold is 40mV per minute for selected charge rates greater

than C/2, 28mV per minute for charge rates selected at or between

C/2 and C/3, and 18mV per minute for selected charge rates less

than C/3. The voltage across the thermistor must change at these

rates or greater to terminate the selected charge rate.

These thresholds correspond to a set change in thermistor

resistance when an external pull-up to 5V is used as shown in

Figure 11. Using the values calculated from the hot and cold

battery detection example, the percent change in the thermistor

resistance per minute for selected charge rates are provided. For

selected charge rates greater than C/2, the thermistor resistance

must decrease 4%/min. to terminate charge. For selected charge

rates at or between C/2 and C/3, the thermistor resistance must

decrease 3%/min. to terminate charge. For selected charge rates

less than C/3, the thermistor must decrease 2%/min. to terminate

charge.

Parameter

Voltage

Battery

Temperature

Cold Battery Thermistor

Voltage

>2.4

<10

°C

Hot Battery Thermistor

Voltage

<0.93

>45

°C

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