Linear building blocks for power management in portable systems

Electronic News, August 12, 1996 by Don Alfano

5V, Do-Do birds and other extinct species

Conventional wisdom says smaller is better where portable systems are concerned. Higher silicon integration and smaller package technology have shrunk system electronics to the point where system size is dictated by the sizes of the battery pack and user interface (keyboard and display).

Reducing the size of the user interface is largely an ergonomic issue. On the other hand, reducing battery size means reducing the number (or size) of the cells in the pack, or changing to a higher-energy-density battery chemistry (such as lithium ion). Both options result in battery size reduction at the expense of lower battery energy capacity and/or lower terminal voltage. Reduced battery terminal voltages is one factor hastening the departure from 5V to 3V (and lower) system supplies.

Another, more predominant factor has been the migration to 3V by the digital supply base at large (i.e., suppliers of processors, memory and logic). As these devices grew in complexity and speed, the transistor geometries used in fabricating these circuits had lower breakdown voltages and therefore required lower supply voltages.

The resulting industry-wide journey for the holy grail (3V) was established, and today low-voltage processors, logic and memory, are readily available from a number of different suppliers. These devices not only offer 3V (and lower) supply voltages, but also often have low-power standby modes that suspend operation and reduce supply current. For example, most low-power microcontrollers have a low-power "sleep" mode, where normal operation is suspended and supply current dramatically reduced.

Power management basics

While the very act of moving to a lower supply voltage helps conserve power, system power management circuits are still required to maximize running time between battery charges. They allocate power only to the system resources required to perform the task at hand, and place the rest of the system in a low-power (standby) mode. Whether implemented using a microcontroller or not, power management circuits often require analog inputs such as battery voltage and ambient temperature. This, in turn, creates a need for low-power, low-operating-voltage linear components (such as amplifiers and comparators).

Unlike digital circuitry, lower supply voltages mix with analog circuits about as well as the proverbial snake and mongoose. As the supply voltage to an analog circuit is reduced, the input and output dynamic range collapses, noise rejection is compromised and gains are reduced. In addition, operation at higher temperatures becomes strained and output drive capability falters.

There are two common ways to approach this problem. The first method uses a DC/DC converter (a switching regulator or capacitive charge pump) to power higher-voltage analog circuits. Dedicated charge pumps like the TCM680 can furnish 6V at 10 milliamperes (mA) from a single 3V input. This output is good enough to power the analog portion of the system in many cases. While this approach accommodates lower-cost analog circuits, the DC/DC converter itself takes up board space, consumes power and adds cost. Choosing single-supply, low-voltage linears is often a better approach. Such devices are now available from a variety of suppliers, with more coming every day.

Solutions in the form of linear building blocks

As previously stated, the preponderance of portable systems consists of digital hardware, with analog functions required "here and there" where needed. Common examples of such functions are battery low voltage detectors, power supply current sensors and specialized DC regulator controllers. The configuration of these circuits varies widely with the application. As a result, they are usually made of individual op-amps, voltage references and comparators (unless the end application can afford an analog or mixed-signal ASIC).

Various semiconductor vendors have responded to this need with an assortment of "linear building block" products that combine op-amps, comparators and voltage references in single-package configurations. These products allow the user to create any number of analog circuits with complete flexibility, while still enjoying the size and cost benefits of integration.

Newer linear building block products offer the lowest operating voltage and supply current in their price range. A product like the TC43 (Fig. 1) is one of a family of seven building block products, and operates from power supply voltages as low as 1.8V drawing only 15 microamperes (mA) maximum (7 mA typical) when fully enabled. Other members in this family have different combinations of amplifiers, comparators and references in SOT23-5, MSOP-8 and QSOP-16 packages. This allows the user to select the best functional combination for his application, thereby minimizing installed size.

The TC43, for example, can replace as many as three separate packages (dual op-amp, dual comparator and voltage reference), while reducing the installed circuit size by 20 percent and component count by 66 percent, saving assembly cost. Given lower supply voltage operation, a greater emphasis is placed on rail-to-rail amplifier and comparator inputs and outputs. In addition, these inputs are frequently used to monitor the battery or power supply voltage making low-input bias current a requirement. A product like the TC43, for example, has rail-to-rail inputs and outputs with input currents on the order of 100 picoamperes (pA); perfect for battery-level monitoring and other applications requiring low-input loading.