Electronic Design

Don't Sacrifice Performance For Low Sleep-Mode Power

The accuracy and performance of battery-powered systems need not be sacrificed to achieve long battery life. Designers can use techniques that provide both high performance and low sleep-mode power consumption.

Applications such as keyless entry, climate control, and security systems are inactive almost all the time, waking up periodically for just a few microseconds to poll sensors or respond to an interrupt. In these predominantly sleep-mode applications, sleep-mode current is the most important power-consumption parameter. In fact, total system power consumption actually approaches sleep-mode power consumption.

One available 8-bit microcontroller consumes 340 µA in active mode and 650 nA in power-save mode. If it's designed into a system that wakes up once a second for 10 µs to poll a peripheral or update its real-time clock, it will be active 0.001% of the time and asleep 99.999% of the time. Total system power consumption will be:

active-mode consumption: 0.00001 × 340 µA = 0.0034 µA, plus power-save-mode consumption: 0.99999 × 0.65 µA = 0.6500 µA = total power consumption: 0.6534 µA

The key to long battery life in such applications is to minimize sleep-mode power consumption. Two factors have a big effect on it: brownout detection and the real-time clock.

Brownout detection is vital to these applications because it prevents the controller from operating below its minimum operating voltage, which can cause unpredictable behavior that can render the system inoperable. Brownout detectors (BODs) monitor the supply voltage of the system and shut it down in an orderly fashion before the supply voltage falls too low.

Since there is no way of knowing if the supply voltage will be high enough when the controller wakes up from sleep mode, most applications keep the BOD turned on during sleep mode. This makes it a major source of sleep-mode power consumption that can take years off the battery's life.

A common solution is to use a low- or "zero-power" BOD that consumes just a few nanoamps. This minimizes power consumption, but the controller may not be protected because the BOD is an analog module that requires a minimal level of current to function properly. Zero-power BODs are edge-triggered, and their detection thresholds often are below the required supply voltage. For instance, a BOD that triggers at 1.4 V and requires up to 1 ms to respond isn't acceptable for a controller that needs 1.8 or 2.2 V to function properly.

Instead, increase the current to the BOD by 15 or 25 µA for better accuracy, as well as to turn it off when the controller-is in sleep mode. Controllers are available with circuitry that restarts the BOD and checks the supply voltage before the microcontroller is restarted. If the supply voltage is below the threshold, the BOD resets the controller. This protects the controller, and the power consumption can be kept as low as 650 nA.

The other main source of sleep-mode power consumption is the clock source for the real-time counters. A very low-power oscillator isn't accurate enough to keep track of the time for applications such as security or climate control systems. A low-power, 32-kHz crystal oscillator (real-time clock) will provide the required accuracy while drawing very little current.

BOD and timer/counter accuracy should not—and need not— be sacrificed. A higher-current BOD and crystal oscillator may use more power. But this can be offset by putting the BOD to sleep when the controller is asleep and by using a low-power crystal oscillator for the real-time clock. This achieves the lowest possible power consumption with the highest possible performance.

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