Analog and digital - a symbiotic relationship

We often overlook the fact that most electronic components in a design's bill of materials today are analog. It doesn't matter whether it is an industrial IoT sensor or an automotive infotainment system; you'll find just a few digital ICs such as a microcontroller and other digital logic. The rest of them perform or support analog functions. Examples include power conversion, analog to digital converters, op amps and comparators, and wireless transceivers.

Here we investigate the diversity of analog ICs available, their functions, and some of the online design resources available to assist engineers in speeding through the design process. A selection of analog devices from ST are showcased together with some application examples designed with the help of ST's eDesignSuite.

Our world is analog

Since the advent of digital logic ICs and early microcontrollers, we have increasingly relied on digital processor circuits to crunch data. As their processing capabilities increased, so did our ability to find applications that needed immense amounts of data processed quickly. Processing data in the binary digital world is the norm, and over the past decades many legacy applications, such as voice communications, have become digital. As digital use cases evolved, various optimised processing devices advanced to embrace them. Examples include graphical processing units (GPUs), field-programmable gate arrays (FPGAs), and neural processing units (NPUs).

In the electronics industry, announcements about process node performance, the number of transistors, and the number of cores occur frequently. Despite the unique nature of these achievements, as engineers we know that all digital processors rely heavily on analog ICs and functions to support them.

The application use case dictates what analog functions a design requires. However, some analog requirements are common to most hardware circuits, of which power supply and signal conditioning are two popular functions.

Power management and conversion

Power is a vital aspect of any design and has many perspectives. One hot topic related to power in today's world is energy efficiency. 

Most of our consumer tech operates from DC batteries, while many other appliances use an AC line supply. The device's power consumption profile determines how long it will work on a single charge or how frequently the batteries need replacing. Creating a low power design helps prolong battery life and requires the careful selection of critical components.

Power supply functions may require the battery voltage to be converted to another level or multiple different voltages. Voltage conversion and regulation tasks may incur losses, impacting the overall energy efficiency.

Given the complex nature of architecting an energy-efficient design, here are some key topics to consider:

Achieving a low power design: This is a consideration for the whole device, although while focusing on the application-centric components that consume power, a display, and a microcontroller, for example, it is easy to overlook the power components. The power components used for conversion and regulation also consume power, so check the datasheet specifications and select the lowest power, energy-efficient items.

No-load power consumption: Some power conversion components consume power even if the end application is not operational. A device that is on standby, for example, still has some circuits operating to detect when the user activates it. Even a low no-load current can impact battery capacity over time.

Voltage conversion and regulation methods: This is a reasonably complicated topic, so this article will cover only some basics.

AC/DC is the default requirement for line-powered appliances and equipment. The AC line voltage is converted to a lower unrelated DC voltage. There are several ways of achieving this. A linear power supply typically uses a transformer and a bridge rectifier. A more popular, less bulky, and more efficient approach is to convert the AC, either with a bridge rectifier or an active 'Totem pole' PFC circuit in front of a 'switching' DC/DC converter.

DC/DC conversion takes many forms, with different conversion architectures, termed topologies, to convert the input voltage to an output voltage that may be lower or higher than the input voltage. In addition to conversion, the output is regulated, ensuring that despite variations in the input voltage and changes in demand from the load, the output voltage is maintained within a defined tolerance.

When faced with power design decisions, the ST eDesignSuite provides a specialist power management design resource that guides engineers through selecting and developing efficient power systems. It covers AC/DC and DC/DC conversion, solar battery charging, and LED lighting applications.

Popular switching conversion topologies include buck (lower output voltage than the input) boost (higher output voltage than the input).

Figure 1 illustrates an example buck converter design created with the ST eDesignSuite. The desired converter input and output parameters assist the eDesignSuite in identifying suitable converter devices. The example illustrates the proposed method using a low power L7983 60 V, 300 mA synchronous step-down (buck) switching regulator. The regulator's quiescent current is 10 µA, which significantly contributes to a solution's low power attributes. The eDesignSuite provides comprehensive design information, including efficiency, frequency response, and associated components. The resource also highlights suitable evaluation boards to prototype an initial design.

Figure 1 - An ST eDesignSuite design example specified to deliver a 5 V, 200 mA output from an 18 V to 48 V input - source ST (click image to enlarge)

Another more straightforward and often cheaper approach is using a linear regulator. Linear regulators, however, require a small overhead voltage known as a "dropout voltage" to maintain the correct output voltage. An example of an LDO regulator is the ST LD59150. Capable of delivering up to 1.5 A, an output voltage range of 0.8 V to 3.6 V, and with industrial and automotive-qualified variants, the LD59150 has a dropout voltage ranging from 65 to 125 mV at full load. Figure 2 highlights the internal architecture of the LD5190, highlighting the use of an NMOS pass transistor.

Figure 2 - The internal functions of the ST LD5190 low-dropout linear regulator - source ST

Voltage and current measurement: In battery-powered applications, an indication of the battery's state of charge, derived from its voltage, is helpful for the user. Also, measuring the supply current is desirable for application fault or status monitoring. Overcurrent conditions may cause hazards to the user, so some form of protection is always required.

An example of an innovative and reliable monitoring and protection device is the electronic fuse STEF05 from STMicroelectronics. The STEF05 combines an electronic fuse (E-fuse) with an output current and input voltage monitor. Designed for use on a 5 V rail and with a continuous current rating of 3.6 A, it protects from overcurrent and overvoltage. The output overvoltage is clamped at 6.65 V, and should an overload condition occur, the STEF05 restricts the output current to a safe level. In the event the overcurrent continues, the STEF05 disconnects the load. Once power is resupplied, the E-fuse reconnects the load, and the current is initially limited until normal operation is observed. Unlike a regular fuse, E-fuses have repeatable performance, and unlike resettable polyfuses their performance does not degrade after being triggered. Figure 3 illustrates a typical implementation of a STEF05 E-Fuse in an application circuit.

Figure 3 - The ST STEF05 in an example application circuit - source ST

Signal conditioning

Analog components are essential for signal conditioning, and their usage can include the reduction of unwanted spurious artifacts, small-signal amplification, isolation, and current limiting. Removing high-frequency artifacts on an analog signal that fall outside the required frequency range is a good example. To satisfy such a design criterion, an engineer will design a filter circuit. Passive filters use components such as inductors and capacitors, but a popular method is using an operational amplifier (op-amp) IC as an active filter.

Op-amps are extremely versatile analog ICs that form the basis of many circuit functions. Op-amps may be considered for a multistage, high gain signal amplification. They have an inverting and a non-inverting input and a single output. Typically, they are supplied with positive and negative voltage rails, although, today, a singled-ended supply is preferred. Despite an op-amp's ability to exhibit high gain, it can only produce an output within the limits of its supply voltage rails.

Key op-amp terminology includes:

Input offset voltage: This parameter specifies the differential voltage required at the input to achieve an output voltage precisely between the two supply rails. The offset results from minimal differences in transistor fabrication during the ICs manufacturing.

Gain-bandwidth product (GBP): This is the product of the op-amp's gain and bandwidth characteristics based on a 20 dB gain reduction across the desired frequency range. The metric indicates the op-amp's high-frequency response since an op-amp's gain characteristic tends to reduce (roll-off) with frequency.

Rail-to-rail output: An op-amp's rail-to-rail output characteristic indicates how close the output voltage can come to the supply voltage rails.

Zero drift: The performance of op-amps, like many components, are affected by temperature. A zero-drift op-amp incorporates a feature that automatically corrects any changes in the offset voltage and mitigates low frequency 1/f noise.

Many different types of op-amps are available that suit specific applications. ST provides a smartphone and tablet op-amp selection guide, ST-OPAMPS-APP, that helps the selection of op-amps and other signal conditioning components. Op-amps Quick Reference Guide is another smart way to select the right op-amp for your application.

Also, the ST eDesignSuite features a set of signal conditioning design tools for active filters, comparators, and current sensing applications.

Figure 4 highlights the design of a low pass filter using the ST eDesignSuite. The desired filter specifications were a 2 kHz cut-off frequency (-3dB), the use of a Bessel approximation to yield a flat passband, and a 5 V supply. The eDesignSuite output illustrates the filter's characteristics, including the response curve and the circuit's BOM. The selected op-amp, the ST TSV792, offers a high 50 MHz bandwidth, a low offset voltage of 200 µV, and has rail-to-rail inputs and output.

Figure 4 - A 2 kHz low pass filter designed using the ST eDesignSuite uses an ST TSV791 op-amp - source ST (click image to enlarge)

Analog is essential to any design

Analog components feature heavily in all electronics-based designs today. ST offers a comprehensive line-up of analog ICs, from power and signal conditioning to RF. ST supports engineers with extensive design resources such as engineering calculators, circuit design, and simulation tools.

Avnet Silica is an authorised ST distribution partner.

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