Power Management for Internet of Things (IoT) System on a Chip (SoC) Development

By Stephen M. Nolan, Vidatronic, Inc.

ABSTRACT

There are many factors that must be considered when developing a custom System on a Chip (SoC) for Internet of Things (IoT) applications. Chief among these are the power management circuits on the die. Vidatronic offers this white paper to discuss these considerations and all of the various circuit blocks that can be found in this application. Vidatronic IP solutions are discussed and the benefits they bring to IoT SoC designers.

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INTRODUCTION & BACKGROUND

The Internet of Things (IoT) is the interconnection of millions of autonomous devices to the internet. These devices collect data on physical parameters and processes and report those to the network. Some autonomously, or by network command, provide outputs that control other equipment and systems.

There are a wide range of applications for IoT devices including infrastructure, utilities, home automation, personal medical, vehicle, industrial and more. The full range of potential applications has not been conceived. The market for IoT devices will explode as additional applications are addressed.

There will be many new ideas in this field, and a great number of these will come from companies without background or experience in developing high-tech devices and equipment.

Many of these new IoT inventions will be implemented with a single system on a chip (SoC) to provide the highest level of integration and conservation of area. As numerous new companies enter into the market with their own ideas for IoT, many will begin to face the challenges of IoT SoC development.

One of the most important aspects of developing a custom SoC for an IoT application is differentiating your product from other competitors in the market. Error! Reference source not found. is a block diagram showing many of the typical functions that can be included in an IoT SoC design. You can see in the bottom portion of the diagram that many of the functional blocks that are used in a typical SoC are either standardized or very highly commoditized. Utilizing commercially available IP blocks for these functions brings several benefits but no differentiation. In fact, for those blocks that are completely standards driven (for example a USB interface) it is nearly impossible for there to be any differentiation.

Figure 1: Example IoT SoC Block Diagram

The high-performance analog and mixed-signal blocks can be an area for customization and differentiation. Most IoT SoC designs are implemented in smallgeometry processes (55 nm and smaller) to take advantage of both power and die area savings. However, there are significant challenges of analog circuit design in small process linewidths due to transistor mismatch and leakage. Vidatronic has significant experience in overcoming the difficulties of designs in advanced-processes (down to 7 nm), and using our IP can help you get to market faster with lower risk and less cost than trying to design these circuits in house.

The power management blocks, in the upper portion of the block diagram, are analog and mixed-signal circuits specialized for the application. In this paper, we will look at the various blocks represented in the powermanagement section of the block diagram and how you can choose IP technology from Vidatronic that will bring substantial differentiation and value to your IoT project.

LOW-DROPOUT REGULATORS (LDOS)

The first power management block that we will explain is the low-dropout linear (LDO) voltage regulator. LDOs are frequently used for the purposes of supplying the many different voltage rails required on the SoC, and for isolating the internal SoC circuitry from external sources of noise on the circuit board power supply rails.

Most LDO circuits require an external capacitor on their output for a couple of reasons. The first reason is that an LDO is a closed-loop architecture circuit. A capacitor is required on the output to create a pole in the loop transfer function to achieve loop stability. Remove the capacitor, and the LDO can become unstable and even start to oscillate. The second reason to have a capacitor on the output is to help the LDO achieve its stated noise performance, both output noise voltage and power supply noise rejection (PSRR). The use of output capacitors for every LDO in an SoC system can consume significant amounts of board real estate, add cost to the system, and decrease reliability due to solder joints for the extra external components as well as due to capacitor failures on the PC board. Frequently, external LDO ICs are used because of the difficulty of integrating this analog function onto the die.

Vidatronic has an LDO technology, Noise Quencher®, which can be integrated onto the die and does not require an output capacitor. It is designed for unconditional loop stability across a wide range of output capacitances all the way down to zero output capacitance, and it will still achieve the required noise rejection. Also available is the reduced-powerconsumption version, called Power Quencher™. Vidatronic IP blocks are available in the process nodes typical for IoT SoC development and therefore they can be implemented on the IoT SoC die to supply all of the various internal power voltages necessary.

DC-TO-DC CONVERTERS

One problem with using LDOs to generate low voltages from an incoming power supply is that they burn power relative to the amount of voltage-drop from the input to the output. If you have a relatively high voltage drop, LDOs are not efficient. In this case, it is better to use a dc-to-dc converter that can achieve significantly higher efficiency than an LDO. The drawback to a dc-to-dc converter is that it is a switched-mode circuit and can induce switching noise into its output voltage. Extremely noise-sensitive circuits should not be supplied by dc-todc converters. One possible solution to overcome this is to cascade a dc-to-dc converter for efficient voltage drop followed by an LDO to clean up the supply noise.

There are two types of dc-to-dc converter technologies, standard inductor-based dc-to-dc converters which require an external inductor and capacitor to create the dc output level, and switched-capacitor dc-to-dc converters which can be implemented with on-die capacitors but cannot supply as much current as inductor-based solutions. Vidatronic has both types of technologies available as silicon proven IP.

Additionally, dc-to-dc converters can be used either in buck mode, for use where the output voltage is lower than the input voltage, or in boost mode where the output voltage is higher than the input voltage. There are also combination buck-boost mode dc-to-dc converters that automatically supply the correct output voltage regardless of the input voltage. This is particularly common in battery-powered applications. When the battery is connected to a charger or is fully charged, the battery voltage may be higher than the required voltage of the load circuitry. Then as the battery becomes discharged, the battery voltage is lower than the required voltage of the load circuitry.

Vidatronic has a switched capacitor buck-boost dc-to-dc converter IP block that does not require any external components. It can generate the required output voltage regardless of whether the input voltage is above or below the required output voltage.

This IP block is designed to be scalable based on the required output current (see Figure 2). If for example your IoT SoC is only going to draw 1 milliamp from the buck-boost converter, then you use our smallest size IP module on the die. If you were to require a higher load current, then the size of the module can scale up linearly to support the additional load. For example, a 10- milliamp load would require an area 10x the size of the area as for our 1 milliamp module. This means that in your lighter-load applications you don't pay a die-area penalty for current that you're not using.

Figure 2: Buck-Boost DC-to-DC Converter Application

VOLTAGE REFERENCES

Voltage references are a necessary part of the power management system in nearly any IoT SoC device. Voltage references are commonly used as a reference for various op amps, comparators, data converters and many other analog and mixed-signal functions. Three desired characteristics in a voltage reference include low power consumption, high accuracy and good power supply noise rejection. Vidatronic has a leading voltagereference IP technology that can achieve industry-best 0.3% accuracy (trimmed) with less than 10 microamps (μA) of current consumption and better than 80 dB of power supply noise rejection. We also have extremely low power (IQ < 100 nA) voltage references, suitable for always-on micropower applications, with very good accuracy and noise performance.

FOUNDATIONAL BLOCKS

There are many other foundational analog circuit blocks that are commonly used in power management applications. These include oscillator, voltage monitor, temperature sensor and power-on reset (POR) circuits. Although there are many ways to implement oscillators, for power-management functions they are generally of either the relaxation or the ring architecture. While neither are novel, implementing them to reliably start and oscillate at the correct frequency across process, voltage and temperature (PVT), with reasonable power consumption, can be challenging in advanced fab processes.

Voltage monitors and temperature sensors are frequently used for failsafe reasons, flagging the controller to shut down the system when a limit has been reached. Key parameters are accuracy and hysteresis.

Power-on reset (POR) circuits are usually a necessary part of any IoT SOC design, to hold off the start-up of logic circuits until the power supply voltages have reached an adequate level to achieve valid logic states in the system.

POWER MANAGEMENT UNIT (PMU)

The use of these various power management blocks on an IoT SoC generally is not independent. Frequently what is required are very carefully controlled power-up and power-down sequencing of the various supply rails and the ability to shut down certain power supplies during specified low-power operating states of the SoC. To control the various power-management blocks, usually a logic state machine is added to ensure the right timing and sequencing of power supply bring up and shut down and to establish the various low-power states. This logic state machine is often controlled by a microcontroller or microprocessor through a serial interface (e.g. I2C). The careful development and testing of these logic circuits and their interface with the analog power management blocks is not trivial.

Vidatronic has these various technologies available and has many years of experience in the correct implementation. Generally, grouping all of these various power management blocks together with some control circuitry and an interface for external control is what is known as a power management unit (PMU). One of the customization services that Vidatronic makes available is the integration of various power management blocks into a single PMU that provides all of the necessary power functions for your entire SoC design.

CONCLUSION/SUMMARY

Utilizing Vidatronic IP and custom circuitry for your IoT SoC development can help get you to market faster, with differentiated product features and performance. Our experience reduces your overall project risk.

Vidatronic IP products for IoT SoCs include:

• LDOs that can be integrated on-die and do not require any external components
• DC-to-DC converters with die area scalable to the load requirement
• Bandgap references featuring either industry’s best accuracy or incredibly low power for always-on applications
• Foundational blocks
• Control logic and serial interfaces.

Vidatronic can also provide customization of any IP block to meet specific application requirements, and integration of the required blocks together into a full Power Management Unit that provides all of the necessary power functions for your entire SoC design.

Vidatronic specializes in the most advanced-process nodes in CMOS Planar, FinFET, and SOI technologies, from 180 nm down to 7 nm.

We are not a component vendor – our strength is in partnership. Collaboration is the key to our Partners’ success, and it allows us to grow with you in parallel. Partnering with Vidatronic can free up your valuable design engineering resources to focus on those areas where they have the most expertise so that you can develop an SoC that has significant value in the market and will be successful.