The BQ2057 continuously monitors temperature by measuring the voltage between the TS and VSS pins. A negative- or a positive-temperature coefficient thermistor (NTC, PTC) and an external voltage divider typically develop this voltage. (See Figure 10-9.) The BQ2057 compares this voltage against its internal V(TS1) and V(TS2) thresholds to determine if charging is allowed. (See Figure 10-10.) The temperature sensing circuit is immune to any fluctuation in V_CC, since both the external voltage divider and the internal thresholds (V_(TS1) and V_(TS2)) are referenced to VCC.

The resistor values of R_(T1) and R_(T2) are calculated by the following equations:

For NTC Thermistors

Equation 3.

Equation 4.

For PTC Thermistors

Equation 5.

Equation 6.

Where R_(TC) is the cold temperature resistance and R_(TH) is the hot temperature resistance of thermistor, as specified by the thermistor manufacturer.

R_T1 or R_T2 can be omitted If only one temperature (hot or cold) setting is required. Applying a voltage between the V_(TS1) and V_(TS2) thresholds to pin TS disables the temperature-sensing feature.

Figure 10-9 Temperature Sensing Circuits

Figure 10-10 BQ2057 TS Input Thresholds