EDNE December 2012 - (Page 22)

The typical operating voltage required for any microcontroller is at least 3.3V, though 1.8V is sufficient for the core in any microcontroller to be functional. Because an AA or AAA battery will deliver 1.3 to 1.5V when fully charged, systems need at least two battery cells to operate. As the battery decay gradually falls below 0.9V, it becomes impossible to operate the system even with two batteries. With the use of a boost converter, however, a microcontroller can operate on a single battery cell that has been boosted to deliver 1.8V or higher. The boost converter not only lets systems operate from just one battery cell but also lets them sustain operation even when the voltage drops to 0.5V. Alternatively, solar-cell-powered devices—typically consumer-oriented products that require a small form factor—can use boost conversion to enable operation from a single 0.5V solar cell rather than the three 0.5V cells that otherwise would be required. Developers can also protect system data when voltages drop too low to boost by employing such techniques as low-power mode with RAM retention, which lets a user replace the battery and resume operation with no interruption. Squeezing the battery Figure 1 shows the discharge graph of an AA battery of 2500-mAhr capacity. Consider an application comprising a controller or SOC that operates at 1.8V and consumes an average current of 10 mA. The battery is expected to last for 2500 mAhr/10 mA, or 250 hours. As the graph shows, once the battery voltage drops to 0.9V, it has discharged about 2200 mAhr. Beyond this point, even with two batteries (assuming the microcontroller works at 1.8V), the features available in the controller may not operate normally. That means the remaining 300 mAhr, or more than 10%, of battery power cannot be used. A switched-mode pump, if available on the microcontroller, can boost the battery voltage to an appropriate usable voltage. Microcontroller at a g L a n c e ↘ The use of a boost converter can enable a microcontroller to operate on a single battery cell or a 0.5V solar cell. ↘ An SMP (switched-mode pump) can also boost battery voltage to access stored capacity that the system would otherwise be unable to extract. ↘ The efficiency of an SMP is limited by the losses in its passive components. ↘ An integrated SMP can enable a microcontroller to supply subsystems such as RF ICs with the required higher voltage. manufacturers provide an option to select this usable voltage, allowing the voltage to be boosted to 1.8V or higher for powering the application, even when battery voltage drops below 1V. The system thus is able to extract a part of the remaining 300-mAhr capacity still available in the battery cell. Below a certain input voltage, however, the boost circuitry may not be able to operate, thus limiting the system from extracting all of the remnant power. Note that the battery should be able to source enough current for the boost to 3.5 3 2.5 2 1.5 1 0.5 0 0 0.5 1 function. The input current to the boost circuitry is a function of the input battery voltage and the output boosted voltage. This current increases as the difference between the input voltage and the output voltage increases—that is, as the battery voltage drops. For example, consider an SMP being used to boost to a constant 3V output. In any system, the power is always constant—that is, output power is equal to the input power. The output power from a boost converter is slightly lower than the input power because of losses in the components used for conversion, but for our purposes let us assume an ideal boost system, with no loss. With the initial, 1.5V battery input boosted to 3V, in order to supply 50 mA to a load, the input current would be (3×50)/1.5=100 mA. Once the battery voltage drops to 1V, to maintain the output voltage, the required input current would increase (power is constant); the input current in this case would be (3×50)/1=150 mA. Thus, the boost converter provides a constant voltage-output regulation. architecture Figure 2 shows the architecture of an SMP boost converter in an SOC compared with an external boost- BATTERY VOLTAGE BOOST VOLTAGE VOLTAGE (V) BOOST WILL NOT OPERATE BELOW 0.5V. 1.5 2 2.5 CAPACITY (Ahr) Figure 1 This discharge graph of an AA battery with 2500-mAhr capacity indicates that when the battery voltage drops to 0.9V, approximately 2200 mAhr has been discharged. 22 eDN europe | december 2012 www.edn-europe.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE December 2012

Cover
Contents
Texas Instruments Europe
Microchip
Digi-Key
Masthead
EDN comment
Pulse
Digi-Key
Baker’s Best
Embedded World 2013
Test & Measurement World
Rohde & Schwarz
Digi-Key
Squeezing the most from battery cells with a switched-mode pump
Signal integrity
Processor architectures : one to rule them all ?
Digi-Key
Mechatronics in Design
Advances in wireless speaker performance and technology
Design Ideas
Teardown; the ultimate Consumer Product ?
Product roundup
Tales from the Cube

EDNE December 2012