In order to achieve 10 year lifetime out of a single CR2032 battery, the enable pin (EN) of the LDC0851 can be duty cycled to achieve a low average supply current. Refer to Figure 7-7 to see the three different states of LDC0851 supply current during duty cycle operation. The sum of the Standby, Ramp, and On currents can be used to calculate the average supply current of the LDC0851, which needs to be below 2.5 µA to achieve a 10 year lifetime from a 220 mAh CR2032 battery.

The average supply current can be calculated in the following steps:

1. Select desired system sample rate (ƒ_SAMPLE) based on the given application. In this example, ƒ_SAMPLE is 1 sample per second.
2. Select the sensor characteristics (ƒ_SENSOR, L_SENSOR, C_SENSOR) based on conversion time and current consumption.
   1. ƒ_SENSOR should be increased as much as possible to minimize the conversion time. 10 MHz is chosen as a starting point.
   2. L_SENSOR should be increased as much as possible to decrease the sensor current (I_SENSOR). Based on a reasonable PCB area, 10 µH is a good starting point.
   3. C_SENSOR is calculated to be 34.5 pF from Equation 8 using the inputs above. This makes C_TOTAL equal to 50.5 pF which meets the requirement of greater than 33 pF to be inside the design space.
3. Calculate the average active current:
   Equation 10.

   • where:

   • ƒ_SAMPLE is the number of samples per second given from step 1. In this example, ƒ_SAMPLE is equal to 1.
   • t_CONVERSION is calculated from Equation 2 to give a conversion time of 433 µs.
   • I_DD is the total active supply current given by Equation 5 to be 1.587 mA.
   • I_ON is the active current consumed by the LDC0851 which comes to be 1.37 µA.
4. Calculate the average ramp current:
   Equation 11.

   • where:

   • ƒ_SAMPLE is the number of samples per second given from step 1. In this example, ƒ_SAMPLE is equal to 1.
   • t_AMT is the active mode transition time given in the electrical table as typically 450µs.
   • I_DD is the total active supply current given by Equation 5 to be 1.587 mA.
   • I_RAMP is the current consumed by the LDC0851 before a conversion has started which comes to be 0.357 µA.
5. Calculate the average standby current:
   Equation 12.

   • where:

   • ƒ_SAMPLE is the number of samples per second given from step 1. In this example, ƒ_SAMPLE is equal to 1.
   • t_AMT is the active mode transition time given in the electrical table as typically 450µs.
   • t_CONVERSION is calculated from Equation 2 to give a conversion time of 433 µs.
   • I_SD is the shutdown current of the LDC0851 given in the electrical table as typically 140nA.
   • I_OFF is the standby current of the LDC0851 which comes to be 0.140 µA.
6. Calculate the total average supply current:
   Equation 13.

   • where:

   • I_ON is the active supply current given from Equation 10 to be 1.37 µA.
   • I_RAMP is the ramp current given by Equation 11 to be 0.357 µA.
   • I_OFF is the standby current given by Equation 12 to be 0.140 µA.
   • I_AVG is the average supply current consumed per second which comes to 1.867 µA.
7. Finally the lifetime of the battery can be calculated:
   Equation 14.

   • where:

   • Battery Capacity is the amount of charge x time that the battery can hold in mAh. This example uses a CR2032 battery with 220 mAh.
   • I_AVG is the value reported in Equation 13 to be 1.867 µA.
   • Battery Lifetime (years) is how long the battery will last reported in years which comes out to be 13.5 years with the inputs from above.

For example, using a sensor frequency of 10 MHz, sensor inductance of 10 µH, and 1 sample per second yields a lifetime of 13.5 years for a single CR2032 battery.