Battery apps benefit from MCU clocking, mode control and low-power peripherals

We've come to expect and require more from our battery-powered devices. In tandem with this increase in system sophistication, the battery-power requirements are increasing while battery chemistries are not keeping up. This increase of functionality is only accomplished if a savvy firmware/software engineer understands the tools available in the microcontroller as well as hardware options. As a result, firmware designers are forced to use what is on hand today in order to optimize power consumption. This can be done by using power efficient hardware combined with microcontroller mode controls.

A simple battery-operated system with a microcontroller (such as Microchip's PIC18F1320) provides a useful example. The central focus for a successful low power design is a microcontroller of this type. The attributes of the microcontroller by itself include idle modes and sleep modes where system power can be conserved. The idle modes of the microcontroller power down the CPU while allowing peripheral functions such as the 10-bit A/D converter to continue to operate. The sleep mode implements a complete shutdown of the controller.

The microcontroller's clock power dissipation, and more importantly, start up time will determine what type of clock is selected for the application. The choices include an R/C loop, a crystal oscillator or a resonator. For example, the start up time of a 32kHz crystal oscillator is 400msec to 900msec. During this start-up time the microcontroller is drawing power from the battery. In contrast, the typical start-up time of internal R/C oscillators is in microseconds. With the internal R/C oscillator, code can be executed immediately after the microcontroller comes out of its sleep mode, but the R/C oscillator is not as accurate as an external crystal if critical timing events are required.

The clock source's power dissipation during start-up can be reduced by using a two-clock strategy with the microcontroller. A microcontroller two-clock function is implemented by using the clock with a faster start-up time, in this case the internal R/C oscillator, to execute code while the more accurate clock is starting up in the background. If it is determined by the microcontroller that the second more accurate clock is not required, it is turned off. On the other hand, if the second, higher accuracy clock is required to execute critical time events, it is allowed to complete its start-up time cycle and the first clock is turned off.

Microcontroller manufacturers are adding modes, such idle modes and sleep modes, that save average power over long periods of time. The combination of lower power peripherals and microcontroller modes enhance the chances of having a low power battery-powered solution.

Optimizing these independent peripheral power trade-offs device by device is important, but the real power savings can be found when the external and internal peripherals are used in concert with the microcontroller mode programming capability. For instance, the microcontroller can be used to control the power supply voltage by switching a new configuration into the resistive feedback system of the regulated adjustable output charge pump.

A higher output voltage may be needed from the charge pump to insure that the analog circuitry performs at its optimum level. A lower voltage may be desirable when only digital events are occurring, such as code being executed by the microcontroller. For instance, the power-supply specifications for the microcontroller are rated from 2V to 5.5V. The power savings for this type of change could be calculated as a direct ratio of the two voltages from the charge pump. As an added benefit, if the external peripherals are powered down with the lower-power supply voltage using the I/O ports, the power saving could be further improved.

Individual power savings from each device in a battery-powered application is extremely important if battery life is a critical issue. But, true value is achieved when the microcontroller's programmable capability is exploited. A few areas where this could be done would be to change the power supply voltage at the output of a regulated charge pump, power down non-critical peripherals when not in use and controlling the clocking strategy in order to optimize power versus functionality. Integrated circuit manufacturers are continuing to improve the dynamic performance of their peripheral devices while reducing the quiescent current and supply voltage requirements.

Lower power is achieved by reducing power supply requirements and quiescent currents, by optimizing topologies for the lower power jobs. In the example shown here, an operational amplifier, A/D converter are the managed peripherals, while power is applied by a regulated, adjustable charge pump is examined.

Power push

The operational amplifiers are designed using CMOS processes. These types of op amps are continuing to push minimum power supply voltage requirements down. For handheld data acquisition, a 14kHz, 600nA amplifier can function on single-supply voltages as low as 1.4V and up to 5.5V. The combination of reduced supply voltage and lower quiescent provides a good solution for power management concerns in battery-operated equipment.

With internal or external integrated A/D converters, the amount of power dissipated is more dependent on the converter topology than on IC design innovation. For instance, the ratio of conversion time to current consumption of the Successive Approximation Register (SAR) converter is considerably lower than a common alternative, the delta sigma converter. In battery-powered applications the SAR converter is usually used unless high resolution and accuracy is an absolute necessity.

The power supply to the circuit may need to be adjustable. A higher voltage of 5V is best suited for analog circuitry and a lower voltage of 2V is acceptable for digital activities. If an adjustable converter is used, it should be optimized for high efficiency with low-output currents and Li-ion battery input voltages (4.2V down to 2.8V). The regulated, adjustable charge pump, DC/DC converter (such as the MCP1252-ADJ in this example) is selected for this application for these reasons, but alternatives can be used dependant of the particulars of the application's battery output voltage and power requirements.