Each BQ76972 provides nine pins which can support external thermistors. Most of these pins also can support other features which can be required in the system, such as the Alarm interrupt to the host processor, or a hardware pin-control for FET turnoff. If an application requires support for more thermistors than the BQ76972 can natively support, additional multiplexer circuitry can be included to enable this, as Figure 2-6 shows.

Figure 2-6 Thermistor Multiplexer Block Diagram

Typically, only high-voltage ESS requires more than 9 thermistors measurement for one BQ76972 and do not need these pins configured for other functions so this design only shows the implementation of the thermistor multiplexer when all 9 pins are configured as thermistor inputs. A total of 17 thermistors are measured by one BQ76972 in this design. There are also many variants if the designer wants to reserve some pins for other purposes. These circuits can be modified based on the basic principles demonstrated in the rest of this section.

Figure 2-7 illustrates how the BQ76972 ADC measurements are performed using a continuous repeating loop in normal mode, such that after the device completes each set of measurements, the device immediately initiates a new set of measurements. Each measurement loop (ADCSCAN) contains up to 21 measurement slots. The t_meas slot time default is 3ms, but you can reduce that time to 1.5ms by setting the [FASTADC] bit, with a reduction in conversion resolution. One ADCSCAN takes 31.5ms (FASTADC = 1) or 63ms (FASTADC = 0).

Figure 2-7 BQ76972 NORMAL Mode Measurement Loop

Because this design configures 9 pins as thermistor inputs, the design takes 3 ADCSCAN to measure 9 pins, called as FULLSCAN. One FULLSCAN cycle duration is 94.5ms (FASTADC = 1) or 189ms (FASTADC = 0). This design uses a 4:1 multiplexer to measure 17 thermistors so one full temperature sensing cycle (FULLTEMP) takes 378ms (FASTADC = 1) or 756ms (FASTADC = 1). See also the Improving Voltage Measurement Accuracy in Battery Monitoring Systems technical article.

The timing of when the MUX is changed requires some coordination with the regular measurement loop of the BQ76972, to avoid a corrupted measurement if the MUX was changed in the middle of a measurement. This design uses an approach to automatically control the timing of the MUX changes, shown in Figure 2-6. The TS1 pin is used with a dummy 1MΩ resistor to generate the clock signal for an external binary counter that counts 0 to 3. The count controls a multiplexer that switches 3 thermistors and 1 ground on each MUX into one of 6 pins, thus supporting a maximum of 18 total thermistors. The ground channel is used for multiplexer circuit diagnostic, meaning the multiplexer works correctly if you can see a ground detection every four measurements on one pin. One of the 18 channels is connected to a high-accuracy fix resistor for temperature measurement calibration.

The 9 pins are measured in the sequence of CFETOFF, DFETOFF, ALERT, TS1, TS2, TS3, HDQ, DCHG, and DDSG, but BQ76972 only measures the pins that are configured as thermistor inputs. As TS1 is used as clock input, TS2 is not used as a real thermistor to avoid any MUX settling transients that can affect the measurement because TS2 is measured immediately after the TS1 pin.

Implement the thermistor-related temperature protections through the host microcontroller because the pin temperature of the BQ76972 moves between 3 thermistors and 1 ground.

A silicon linear thermistor has a linear positive temperature coefficient (PTC), TMP61, is used in this design for better temperature measurement accuracy. Unlike an NTC, which is a purely resistive device, the TMP61 resistance is affected by the current across the device and the resistance changes when the temperature changes. The TMP61 has good linear behavior across the whole temperature range. This range allows a simpler resistance-to-temperature conversion method that reduces look-up table memory requirements. The linearization circuitry or midpoint calibration associated with traditional NTCs is not necessary with the device. The linear resistance across the entire temperature range allows the device to maintain sensitivity at higher operating temperatures. How to Achieve ±1°C Accuracy or Better Across Temperature With Low-Cost TMP6x Linear Thermistors shows how to achieve the best accuracy with TMP61.