Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
61
Figure 3-33. Instantaneous Current
I(t)
t
I
Act
I
Slp
t
Act
t
Slp
T
Where:
I(t) = Instantaneous current
I
Act
= Active current
I
Slp
= Sleep current
t
Act
= Active time
t
Slp
= Sleep time
T = Time period of a cycle
The average current consumed by the device over a long period can be calculated by using the following equation.
𝐼
𝐴𝑉𝐸
=
(𝐼
𝐴𝑐𝑡
×𝑡
𝐴𝑐𝑡
)+(𝐼
𝑆𝑙𝑝
×𝑡
𝑆𝑙𝑝
)
𝑇
Equation 23
The average power consumed by the device can be calculated as follows:
𝑃
𝐴𝑉𝐸
= 𝑉
𝐷𝐷
× 𝐼
𝐴𝑉𝐸
Equation 24
3.5.3 Response Time Versus Power Consumption
As illustrated in Equation 24, the average power consumption can be reduced by decreasing I
AVE
or V
DD
. I
AVE
may be
decreased by increasing sleep time. Increasing sleep time to a very high value leads to poor response time of the
CapSense button. Because of this tradeoff between response time and power consumption, the application developer
must carefully select the sleep time based on system requirements.
In any application, if both power consumption and response time are important parameters to be considered, then, an
optimized method can be used that incorporates both continuous-scan and sleep-scan modes. In this method, the
device spends most of its time in sleep-scan mode where it scans the sensors and goes to sleep periodically as
explained in the previous section and thereby consuming less power. When you touch a sensor to operate the system,
the device jumps to continuous-scan mode where the sensors are scanned continuously without invoking sleep and
thereby giving very good response time. The device remains in continuous-scan mode for a specified
time-out period. If you do not operate any sensor within this time-out period, the device returns to the sleep-scan mode.
