ADS7924 Datasheet, PDF (32/42 Pages) Texas Instruments – 2.2V, 12-Bit, 4-Channel, microPOWER™ ANALOG-TO-DIGITAL CONVERTER WITH I2C INTERFACE

ADS7924
SBAS482A – JANUARY 2010 – REVISED MAY 2010
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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.
I_AVERAGE =
I_PUt_PU + I_ACQt_ACQ + I_CONVt_CONV + I t _{SLEEP SLEEP}
t_CYCLE
(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:
0 + (270mA)(4)(6ms) + (400mA)(4)(4ms) + (1.25mA)(4)(2.5ms)
I_AVERAGE =
10.04ms
= 2.5mA
(4)
STATUS
Idle
Awake
Acquiring
Converting
Sleeping
Table 6. Supply Current for Various Operating Conditions
5V
1µA
45µA
315µA
730µA
3µA
3.3V
1µA
25µA
285µA
520µA
2µA
AVDD
2.7V
1µA
20µA
275µA
450µA
1.5µA
2.2V
1µA
15µA
270µA
400µA
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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