6 Current Measurements – Two Cell and Single Cell

Current measurements were carried out to determine the I_CC and I_Batt of the MSP430 + TPS60313 one-cell thermostat system. They are summarized in Table 1 for the condition V_CC = 3.0 V and V_Batt = 1.5 V.

Figure 6 shows when each of the one second processes is executed with respect to time. The total time required to run through count = 1 to count = 6 is six seconds.

The average current consumption for each second is calculated by finding the area under I(t) vs t plot. Current required by each of the one-second processes is calculated based on the amount of the time spent by the MSP430 device in each of the states for which current consumption was measured (see Table 2).

Finally, the expected battery life of the single-cell thermostat is calculated based on the mA•hr ratings given for Duracell AA and AAA batteries. The method for calculating the battery life is as follows.

First convert the battery rating of mA•hr to µA•sec.

µA•sec = (mA•hr) (1000 µA / 1 mA) (60 min / 1 hr) (60 sec / 1 min)

Then divide the battery rating in µA•sec by the average current consumption in µA for one second to get the estimated number of seconds the battery will last.

sec = µA•sec / µA

Now that the seconds are known, calculate the equivalent years:

years = (sec) (1 min / 60 sec) (1 hr / 60 min) (1 day / 24 hr) (1 year / 365 days)

The results in Table 3 are based on the assumption that V_Batt remains a constant 1.5 V throughout the battery’s lifetime. In reality, the voltage falls over time as the battery discharges, so the TPS60313 has been designed to be most efficient at 1.2 V, as this represents the battery’s average voltage over its lifetime. Optimizing for operation at 1.2 V helps to maximize the overall system lifetime.