Analog techniques for longer battery life

Achieving maximum battery life requires an understanding of three key factors: battery technologies, digital power management, and low-power analog techniques. While many designers are well aware of the strengths and weaknesses of different battery chemistries and digital power control,  They may not be as familiar with the role low-power analog electronics play in extending battery life.

battery chemistries

One of the key decisions for portable electronics designers is the choice of battery technology. The four main battery chemistries are: alkaline; nickel-cadmium (NiCd); nickel metal hydride (NiMH); and lithium ion (Li-Ion) and each has its own advantages and disadvantages.

As a rule of thumb, a fully charged alkaline battery will provide a voltage of around 1,5V. This voltage will drop as the battery power is used, so that by the time 90% is consumed the voltage will have dropped to around 0,9V. The combination of relatively high capacity and high internal resistance makes alkaline batteries inefficient for high current drawing applications such as remote control cars, camera flashes and power tools.

For those high current applications, NiCd battery cells provide a very durable and low cost option, offering a nominal voltage of 1,2V that drops to around 0,9V when the battery runs out. The drawbacks are its relatively low energy density and the presence of toxic metals. In addition, it is necessary to carry out a complete periodic discharge to prevent the formation of large crystals on the cell plates, which affect the autonomy and performance of the battery.

Instead, NiMH cells are environmentally friendly and provide 40% higher energy density than NiCd battery cells. Its nominal voltage of the order of 1,25V falls below 1,0V when the autonomy of the battery comes to an end. The drawbacks of NiMH batteries are their significantly higher self-discharge rate and poor durability compared to NiCd batteries, due to operation at high loads and extreme temperatures.

For most consumer electronics, lithium ion (Li-ion) is currently the dominant battery chemistry. A single fully charged Li-ion cell has an open circuit voltage of about 3,6V which drops to about 2,7V when fully depleted. Among the advantages of Li-ion cells are their lower weight, higher cell voltages and in the Li-polymer versions, the possibility of being shaped.

Other advantages are that the energy density of Li-ion and Li-polymer batteries continues to increase and currently doubles the energy of a standard NiCd cell, while their costs are decreasing. The main drawback of this chemistry is the risk that it can explode if overcharged. This safety concern has led some manufacturers to favor NiMH chemistry, especially when size and weight are not critical factors.

DC/DC converters

It is essential to know the DC/DC converter architectures to optimize the performance of a design and generally the choice will be made between linear regulators, switched regulators and charge pumps. Although there are several types of linear regulators, the most widely used in battery-powered applications is the LDO (low dropout regulator). These use a P-channel pass transistor as a variable resistor with feedback to regulate a given output voltage.

In contrast, a switched regulator uses a diode, inductor, and switch to transfer power from the input and provides a given output that is configured as a buck, boost, or buck/boost topology. A buck regulator provides a regulated output voltage lower than the input voltage, similar to the function of an LDO; a boost switched regulator provides a higher output voltage than the input; For its part, a buck/boost regulator provides a regulated output within a range of input voltages higher and/or lower than the output voltage.

The third type of regulator, a charge pump, uses a capacitor as an energy storage device and has switches to connect the capacitor plates to the input voltage. Depending on the circuit topology, a charge pump can double, triple, invert, halve, or even create an arbitrary regulated output voltage. Using charge and discharge capacitors to transfer power makes a charge pump provide relatively low output current and no more than a couple hundred milliamps.

Table 1 highlights the advantages and disadvantages of each of these DC/DC converter topologies and the choice of the optimal topology will depend on the parameters of each application. For applications where battery runtime is a priority, a high-efficiency switching regulator should be the best choice, while in applications with a high level of electrical noise, the choice should be a linear regulator. However, each application must focus on the power management circuitry if the goals set for system performance are to be achieved.

 Table 1: Analysis of different DC/DC converter topologies.

DC/DC conversion offers a number of techniques to extend battery operating time. Figure, for example, shows the placement of an input and output capacitor in relation to the DC/DC converter. In this configuration, the switched regulator used to open and close an input switch can generate surge currents on the input pin that can be minimized by using a large input capacitor as a charging buffer.

This can affect battery run time because, depending on the battery chemistry, internal resistance can be significant and the pulsing current from the battery can cause a noticeable voltage drop across the battery cell. A larger input capacitor, located between the battery and the switch, will reduce the instantaneous current draw and this will result in a voltage drop across the battery. To minimize these voltage drops, the battery operating time can be extended before the minimum battery cell voltage is reached. In low power applications that spend much of their time in standby or sleep mode, it may not be necessary for the controller to work at all times. In this case, using a larger output capacitor to supply the low current required by the load may be more energy efficient. Cycling the regulator on and off increases the charge on the capacitor as needed.

digital power management

Dynamic voltage variation is another common technique for maximizing battery runtime. When running on a lower voltage on a digital load, such as a microcontroller, it draws less current and therefore consumes less power. The drawback, however, is that running a microcontroller with low voltage can limit its processing speed and output capabilities. Dynamic voltage variation allows the microcontroller to combine lower voltage and lower consumption, in standby or sleep mode, with a boost converter to a higher voltage level to process or transfer information. This technique is often used in battery-powered computing and other types of computing, where the microcontroller works in different modes.

The relationship between operating time and standby or sleeping time for each application will also influence battery operating time. While applications such as carbon dioxide detectors generally need to run in continuous mode, others can remain in standby or asleep mode until needed.

Other examples of applications with intermittent operation are smart water meters, remote controls and smoke detectors based on photodetection.

Analog power management

 There are a wealth of online resources dedicated to helping designers understand and manage digital power through the different modes of operation offered by microcontrollers, as well as connecting and disconnecting built-in peripherals as needed. The impact of running the microcontroller in a continuously active state or putting it into sleep mode and then waking it up for active operation is also well documented. When managing the power allocated to analog components, the alternatives may not be so clear cut. While it is still critical to use analog ICs with the lowest possible active current for systems that are continuously in active mode of operation, applications that are subject to duty cycles will also need to weigh settling time against current draw. A faster device with a higher current may provide greater long-term efficiency than a lower current alternative with a slower response time. The choice of appropriate battery technology and digital power management techniques are well-known issues for designers trying to extend battery life. Low power analog implementation techniques are generally not as well known but they can play an important role in prolonging battery life and ensuring optimal system performance.