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CBC34813-M5C-TR5 Datasheet(PDF) 2 Page - Cymbet Corporation |
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CBC34813-M5C-TR5 Datasheet(HTML) 2 Page - Cymbet Corporation |
2 / 5 page AN-1059: Extend Battery Life by Reducing System Power using the EnerChip RTC ©2014 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com Doc AN-72-1059 Rev A Page 2 of 5 Average Power Consumption when Mostly in Sleep State When the system that has a large sleep power compared to its active power is asleep, there is an opportunity to reduce power by placing the microcontroller in its lowest power mode while using the CBC348xx RTC timer functions to provide periodic wake-ups to the microcontroller and associated circuitry. This way the entire system is totally asleep for the majority of time and the microcontroller is only awakened for short periods to determine if it needs to service a sensor or switch. The CBC348xx is configured to automatically wake the system at regular periods for a finite time and then the microcontroller goes back to sleep. The CBC348xx can also be configured to completely turn off power to the sensors and/or microcontroller by using its internal 1Ω pull-down switch. If the microcontroller determines during one of its waking intervals that it needs to service something then it quickly commands the CBC348xx to not automatically shut it down until further commanded. This technique can reduce the system power greatly since the CBC348xx only requires 36nA of current to manage the timing functions and the rest of the system can go to its lowest power mode. The average system power is a function of the time the system needs to run compared to the time it is asleep. If the microcontroller/system is awakened too often the power savings will be minimal. A simple metric of the possible savings is to add the sleep current (since it is always present) to the active current times the ratio of the active time divided by the sleep time. Table 1 below shows some examples of different power savings that can be achieved with different sleep vs. run times. Column one is the Original Sleep Current of the system without using the CBC348xx. The Original Sleep Current includes the sleep power of the microcontroller with a timer running plus any sensor current. The Power Savings Ratio is the Active Current times the ratio of active/ sleep times plus the 36nA CBC348xx current compared to the Original Sleep Current in column one. The Number of Instructions column shows how many instructions the microcontroller can execute in the period of time listed in the Active Runtime column. For sake of reference it takes about 28 I2C clocks at 400kHz or about 70µs to write to a single register in the CBC348xx. Make sure to write to the register to disable the timer before the CBC348xx automatically switches the microcontroller/system power off. Table 1: Combining Sleep Power and Active Power to Compare Power Savings Notice that the Power Savings Ratio is only a benefit if it is over 1.0. This table shows that the longer sleep periods have the best ratios. With higher Original Sleep Currents the benefits are also magnified. The next to the last line shows a Power Savings Ratio of 6.30 with over 300 instructions executed per wake-up. The system in this example had an Original Sleep Current of 1.6µA for the microcontroller current with internal sleep timer and an external sensor. A 6.30 Power Savings Ratio means 6.3 times more battery life in a battery powered system. The last line shows a one-second sleep time associated with a slower, environmental sensor. Notice the large 14.04 Power Savings Ratio in this example. These examples show that a long battery life extension can easily be achieved using this technique. |
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