2 BQ769x2 Temperature Measurement System

The BQ769x2 device can support up to nine external thermistors on multifunction pins (TS1, TS2, TS3, CFETOFF, DFETOFF, ALERT, HDQ, DCHG, and DDSG). All nine pins are connected to the one internal pullup resistor through an internal multiplexer, meaning that only one thermistor draws current from the bias supply at a time, which allows for the lowest current consumption when multiple thermistors are used.

The internal pull-up resistor has two options which can set the pull-up resistor to either 18kΩ or 180kΩ (or none at all). The resistor values are measured during factory production and stored within the device for use during temperature calculation. Figure 2-1 shows the block diagram of BQ769x2 temperature measurement system.

Figure 2-1 BQ769x2 Temperature Measurement System

The temperature of the pin configured as thermistor input can be calculated using the BQ769x2 internal polynomial by writing the specific coefficients into the temperature model registers or using the host microcontroller by reading the measured voltage raw count.

The voltage raw counts are measured in a continuous repeating measurements loop that consists of multiple measurement slots, as shown in Figure 2-2.

Figure 2-2 BQ76972 NORMAL Mode Measurement Loop

Each measurement loop (ADCSCAN) contains up to 21 measurement slots. The last three slots are used for multifunction pins measurement only when the pins are configured as thermistor inputs or ADCIN. The nine pins ( if configured to thermistors inputs or ADCIN) are measured in the sequence of CFETOFF, DFETOFF, ALERT, TS1, TS2, TS3, HDQ, DCHG, and DDSG, which take three measurement loops to collect the full data as one FULLSCAN cycle. The tmeas slot time default is 3ms, but that time can be reduced 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), so one FULLSCAN take 94.5ms (FASTADC = 1) or 189ms (FASTADC = 0). The internal temperature polynomial is calculated every 250ms.