AN607

Rev. 1.7

5

3. System Design Considerations

3.1. Power Requirements

The Si70xx devices are inherently low power if the heater is not used. For example, Si7013 power consumption

during a conversion is 150 µA typical, and the power consumption in idle mode is less than 100 nA. This low power

consumption means that there is no significant self-heating in the conversion process. Because the Si7007,

Si7022, and Si7023 PWM output parts do humidity and temperature conversions twice a second to update the

PWM output the power consumption if these parts is higher at 180 µA typical. Generally this still does not cause

significant heating.

3.1.1. Battery Life Considerations

approximately 2500 mAh for light loads when allowed to discharge to 0.9 V.

3.0 V when charged and 1.8 V when fully depleted, so this is acceptable.

Again, for the case of an Si7013, a humidity and temperature conversion typically consumes 150 µA. In normal

mode, the total time for an RH and temperature conversion is 8.4 msec, and, in fast mode, this is reduced to

5.5 msec. A temperature conversion is performed every time an RH conversion is done (to allow for temperature

correction).

hours is as follows:

Plugging in values, this translates to 11.1 mA-hours for one year in normal mode and 7.2 mA-hours for fast mode.

The 100 nA quiescent current of the Si7013 will consume an additional number of milliamp hours, as shown below:

At 100 nA quiescent current, this is 0.876 mA-hours per year.

Even when targeting a 10-year battery life, the sensor itself would consume less than 120 mA hours of charge in

normal mode. This means that only about 5% of the available 2500 mAh is consumed by the sensor.

Of course, in a practical system, there are many other drains on a battery, and, often, coin cells, which have much

less capacity than AA cells, are used. On the other hand, it is generally acceptable to reduce the conversion rate

well below once per second, meaning that a sensor, such as the Si7013, will generally consume a small portion of

the available battery life.

milliamp hours (mAh)

number of conversions

conversion time (msec)



conversion current (mA)

1000msec/sec

3600sec/hour





--------------------------------------------------------------------------------------



=

milliamp hours (mAh)

quiescent current (nA)

8760 (hours/year)

10

6

nA/mA

-----------------------------------------------



=