The design rules for smaller semiconductors have also led to lower operating voltages, which can help simplify the design of battery-powered circuits and allow a single alkaline (1.5 V) or lithium (3.6 V) battery to support a long operating life. Improvements in power management integrated circuits (such as the energy-saving idle and standby modes provided by devices like microcontrollers) have also made it possible for new designs to achieve longer operating lives.
However, end-user expectations often conflict with designers' goals of increasing the time between battery recharges or replacements. Another element in the list of demands placed on the batteries of many modern portable devices is the trend toward large color screens with more features that put a heavy load on the processor (such as multiple touch controls). Likewise, there is a constant demand for added functionality and increased capabilities in products, from small industrial sensors to smartphones offering value-added features like media storage and playback, mobile internet, and digital cameras for both photos and videos.
These factors affect both the required battery capacity and its ability to deliver high discharge currents, necessary in applications such as digital media devices. Generally, designers must offer additional functionality in the same or smaller dimensions as the previous generation of products. This prevents them from simply using a larger battery with increased capacity.
Battery Selection:
The main types of batteries used in portable devices are lithium-manganese coin cell batteries, such as the 3V CR2032 (20 mm diameter x 3.2 mm); 1.5V batteries such as the LR44 (11.6 mm diameter x 5.4 mm); or alkaline or lithium batteries in standard sizes widely known as AA (51 mm x 14 mm diameter) or AAA (44.5 mm x 10.5 mm diameter).
Battery characteristics, such as energy density, discharge current, and cost, are defined by their chemical composition. Generally speaking, lithium-manganese batteries cannot withstand high current discharges but offer a relatively high energy capacity (mAh) in a small, thin coin cell. The self-discharge current can be as low as 250 nA.
Alkaline AA or AAA batteries can offer higher capacity and withstand higher current discharges, but they are much larger. Non-rechargeable lithium primary batteries are also available in AA or AAA sizes, offering comparable energy capacity with higher current discharges. Additionally, a lithium battery can be up to 30% lighter than its alkaline equivalent.
Coin cell or AA/AAA batteries are generally used in designs where the battery will be replaced by the user once discharged. Some types of equipment, such as appliances that are installed and require no maintenance for several years, can be discarded once the battery is discharged. Low self-discharge batteries, such as lithium batteries, are preferred for long-life or sealed-for-life applications, particularly in emergency equipment such as smoke detectors.
Rechargeable nickel-metal hydride (NiMH) AA or AAA batteries can offer mAh capacity comparable to that of alkaline batteries and are suitable for high-discharge applications. They can withstand hundreds of recharge cycles, but their self-discharge rate is relatively high, so they cannot operate for long periods without maintenance or recharging. Table I summarizes the merits of popular lithium, alkaline, and NiMH batteries.
In applications where particularly small or thin dimensions are required, and where market size permits, a custom-designed lithium-ion battery can offer high energy density with high discharge capacity and low self-discharge.
Power Structure:
If a lithium battery such as a CR2032 is chosen, the application's microcontroller should generally operate at less than 1 µA in idle modes to maximize battery life. Low-voltage devices, such as embedded microcontrollers, can be powered by a CR2032 when the battery voltage drops from 3.0 V to 2.5 V. However, this does not fully utilize the battery's modest energy capacity. A boost converter can use a larger proportion of the battery's energy, increasing the voltage to maintain the integrated circuit's supply voltage above the required level as the battery discharges.
An example of such a device is Microchip's MCP1624 synchronous boost regulator, designed to generate the necessary supply voltage for any Microchip PIC microcontroller from a battery voltage between 0.65 V and 1.8 V. This allows any PIC microcontroller to be powered using a single 1.5 V battery. The MCP1624 boosts the battery voltage to between 2.0 V and 5.5 V to operate the microcontroller. An initial voltage of 0.65 V is required, but the regulator can maintain operation even if the battery voltage drops to 0.35 V. The MCP1624 has a shutdown mode in which the current drawn is reduced to less than 19 µA. This helps reduce the load on the battery when the application is in idle or standby mode.
Power Management:
If the battery meets the application's maximum current draw, power management and careful selection of low-power devices, such as the main microcontroller, can allow designers to use the smallest battery to achieve the target operating life. Battery life is determined by the average current drawn over the entire operating period. For the main component, such as the system's microcontroller, the average current is a composite of the currents drawn in power-saving and idle modes. Therefore, the time spent in each mode has a significant impact on battery life. Thus, in addition to selecting devices with low current values in idle and standby modes, designers should take full advantage of the device's low-power modes.
Several manufacturers are creating microcontrollers that operate efficiently on batteries. These not only draw extremely low currents in active and standby modes, but often offer additional standby modes in which only certain functionalities remain operational. The microcontroller can resume operation more quickly from these modes than from a full standby, providing further energy-saving opportunities while the application is running. Other energy-saving features may include individual gates to peripherals, allowing certain circuits to be turned off when not in use.
Conclusion:
The desire to achieve more with less is fundamental to human nature, as demonstrated by the ever-increasing market demands on electronic products. Although Mother Nature limits battery performance based on its chemical composition, meticulous design and innovative features in integrated power management circuits make further improvements possible.
Author:
Jo Brook, Product Manager, Farnell.
