Datasheet
I t +I t +I t +I t
PU ACQ CONV SLEEPPU ACQ CONV SLEEP
t
CYCLE
I =
AVERAGE
0+(270 A)(4)(6 s)+(400 A)(4)(4 s)+(1.25 A)(4)(2.5ms)mm m m m
10.04ms
I =
AVERAGE
=2.5 Am
ADS7924
SBAS482A –JANUARY 2010–REVISED MAY 2010
www.ti.com
APPLICATION INFORMATION
AVERAGE POWER CONSUMPTION
With its fast conversion time and programmable sleep time with near-zero power, the ADS7924 allows periodic
monitoring of the inputs with a very low average power dissipation, especially as the monitoring interval
increases. The average current required can be calculated as the weighed average of the currents consumed
during the power-up, acquisition, converting, and sleep periods using Equation 3.
(3)
As an example, calculate the average current in the following configuration:
• Mode programmed to Auto-Scan with Sleep
• Power-up time (t
PU
) programmed to '0'
• Acquisition time (t
ACQ
) programmed to 6ms
• Sleep time (t
SLEEP
) programmed to 2.5ms
• AVDD = 2.2V
Looking at Figure 28, the cycle time is seen to equal t
CYCLE
= 4t
PU
+ 4t
ACQ
+ 4t
CONV
+ 4t
SLEEP
= 4(0) + 4(6ms) +
4(4ms) + 4(2.5ms) = 10.04ms.
Table 6 lists the supply current for different supply voltages and operating conditions. Using the data for 2.2V
with the calculated cycle time in Equation 3 gives the following average current:
(4)
Table 6. Supply Current for Various Operating Conditions
AVDD
STATUS 5V 3.3V 2.7V 2.2V
Idle 1µA 1µA 1µA 1µA
Awake 45µA 25µA 20µA 15µA
Acquiring 315µA 285µA 275µA 270µA
Converting 730µA 520µA 450µA 400µA
Sleeping 3µA 2µA 1.5µA 1.25µA
Note the acquisition, conversion, and sleep times are multiplied by 4 because these are repeated four times in
one cycle when in auto-scan with sleep mode.
Average power dissipation for the above configuration where all four inputs are monitored every 10ms is
(2.2V)(2.5mA) = 5.5mW.
Figure 3 and Figure 4 plot Equation 3 to help illustrate the relationship between cycle time and average power
dissipation.
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