Science, Technology and Development ISSN : 0950-0707 SoC POWER MANAGEMENT UNIT FOR IOT APPLICATIONS Annesha Dasgupta*, Manasa Potta*, Vaishaka N. Raj*, Meghana D. R*, Dr. Kendaganna Swamy S+ *UG Students, {name}[email protected] + Assistant Professor, {[email protected]} Department of Electronics & Instrumentation RV College of Engineering, Bangalore, India ABSTRACT Power management in IoT devices is a challenging task because these devices are powered on constantly and often, they are located remotely and must manage the power efficiently. Thus, managing power is one among the chief factors to be considered while developing a custom SOC. Considering the importance of power management in IOT applications, the authors have proposed a design of a power management unit that can be used in SOCs to manage power efficiently. The system consists of LDO, DC-DC Converters and Voltage reference in the design. Power management unit is a flexible system that can be integrated in any SOC’s for power management and additional blocks can be added to improve the power performance of SOC. Keywords: Power Management Unit, Low Power Regulator, DC-DC Converter, Converter, Serial Interface, Voltage Reference, IoT 1. Introduction Internet of Things (IoT) refers to the billions of devices that are embedded with multiple sensors and other top end electronics for the main purpose of automating tasks via the Internet. IoT devices are expected to grow at a lightning speed to a mass 1.3 trillion dollars by the end of this decade. This impact is because IoT has found use in daily objects; smart fridge, smart TV, smart homes etc. [1] Many of these new IoT applications will need an implementation on a single chip known as System on Chip (SoC). As there is a huge demand in the industry, many companies begin to face challenges with the SoC development.[2] Power consumption in IoT is an important topic, primarily due its portable nature and the requirements of the device to function for a longer period of time without being charged. There are three factors that propel the need for power management; the complexity of the System on chip, mobility and process technology. To increase the efficiency of a power management system in an IoT device it is vital to reduce the power usage and area of each segment [3]. Fig 1 shows how SoC chips are used for IoT Applications. And for the chip to have high efficiency and have better performance in the application used it is important to manage the power in the SoC. Volume X Issue VI JUNE 2021 Page No : 169 Science, Technology and Development ISSN : 0950-0707 Fig. 1 SoC Chip for IoT Application This paper is organized as follows: Section 2 describes the Power Management Unit, Section 3 briefly describes Low Power Regulators, section 4 describes DC- DC Converters, section 5 describes Voltage References, section 6 describes Serial Interfaces and the paper is concluded in section 7. 2. Power Management Unit The power management unit (PMU) is made up of several blocks for governing the power functions in IoT Devices. Fig. 2 gives the block diagram of the power management unit. The PMU consists of LDO used for supplying various voltage level rails used in SoC [3], DC-DC Converter to withstand high voltage drop in the SOC, battery chargers to provide internal power supply and voltage reference is used as external power supply, clock generator is used for generating timing sequence for synchronization, and the overall communication within the unit takes place through serial interface.[4] Fig. 2 PMU Block Diagram Volume X Issue VI JUNE 2021 Page No : 170 Science, Technology and Development ISSN : 0950-0707 3. Low Power Regulator A low dropout regulator is essentially a voltage regulator, which is essential in providing a balanced power voltage which is not dependent on load impedance. These LDO regulators are known for their capacity to maintain a control with negligible difference between the voltage supplied and load voltage [5]. With an increase in IoT enabled devices, has propelled an increase in such devices being portable and requiring a stable voltage supplied regardless of the state of the battery. An IoT device works in a discontinuous manner, with its respective ON and OFF duty cycling, during the sleep mode, power is saved and, in this state, the LDO is not in force but because of its unique characteristics is able to maintain a constant output voltage [4]. There is only quiescent current that flows through the LDO in the sleeping state but it is done to preserve the internal circuitry of the device [6]. An LDO structure resembles that of a linear regulator but has replaced the N type MOSFET with a P type MOSFET, thereby becoming a current source [7]. LDO regulators require the application of a capacitor, it is primarily done for two reasons; a capacitor aids in attaining loop stability because LDO follows a closed loop design. The other reason being, a capacitor assists the regulator in maintaining better noise performance. The newer generation LDO regulators are highly efficient with a lower magnitude of dropout voltage which makes it more reliable to be used in IoT devices. Initially analog regulators were heavily used, but because of a lack of adequate gain at lower voltage and poor noise parameters, there has been significant research in digital LDO regulators [7]. Therefore, digital LDO regulators are preferred over analog, because of its higher stability and having significantly lesser area requirements. In other words, the newer LDO regulators have a lower quiescent current, better transient response and a more stabilized load loop gain [8]. Therefore, with a more substantial presence of IoT, it has become imperative to design systems that have higher efficiency in both sleep and active state and the newer LDO regulators are making it possible by bridging the gap in efficiencies and simultaneously improving the noise parameters. Moreover, because of its cost effectiveness and good area considerations it is considered strongly for IoT applications [9]. Volume X Issue VI JUNE 2021 Page No : 171 Science, Technology and Development ISSN : 0950-0707 4. DC - DC Converter DC to DC Converters is a type of SMPS (switched mode power supply) that stores the input DC supply and outputs the stored energy at different voltages [10]. They are made up of two semiconductors (diode and transistor) or two transistors, as well as two storage devices (capacitor and inductor) [11]. When compared to traditional voltage regulators, they are noted for their high efficiency, which reach nearly 95% as their switching conversion is much more efficient than linear voltage regulators which dissipates heat.[12] 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, are the two types of dc-to-dc converter technologies.[13] DC to DC Converters is mainly of three types: Buck Converters: As the name suggests, buck converters/step down converter is a DC-DC converter used to step down voltage i.e., the output voltage is much lower than the input voltage [14]. Boost Converters: These converters are mainly used to step the voltage. i.e., the output voltage is much higher than the input voltage [15]. Buck Boost Converters: In these converters, the voltage output can be either increased or decreased compared to voltage input., i.e., it can act as both buck or boost converter. This type of switching mechanism of reversing the polarity is mostly used in battery applications [16]. 5. Voltage Reference Voltage reference is used to provide a continuous voltage supply to the SoC in spite of loads in the circuit, varying physical parameters and varying time [18]. They provide supply to various power management unit blocks like comparators, LDOs and other analog functions. In addition, it makes sure that the system is consuming low power and provides a good power supply rejection ratio.[19] Volume X Issue VI JUNE 2021 Page No : 172 Science, Technology and Development ISSN : 0950-0707 6. Serial Interface Serial interface is used to communicate within a system effectively. Advantages of serial interface is communication can be performed with lower number of pins and most of the embedded systems support serial interface. It can also communicate without the use of shared memory and semaphores, hence eliminating the problems caused by them. Thus, in a power management unit serial interface is used by the microcontroller/microprocessor to control the logic state machines. The commonly used serial communication are I2C, SPI and UART [20]. 6.1. I2C I2C which translates to Inter-Integrated Circuit is a serial, half-duplex, synchronous, short distance communication protocol. I2C uses only 2 pins for communication, i.e., SCL and SDA. The clock signal is sent through SCL for synchronization and data is sent through the SDA line. I2C supports multi-master and multi-slave to communicate with each other. I2C uses slave addressable protocol where the master sends a 7-bit address to access a slave. And when the slave acknowledges the master, communication is established [21]. Fig 3. I2C Master-Slave Connection 6.2. SPI Serial Peripheral Interface (SPI), is a full-duplex, serial, synchronous communication protocol which uses 4 pins for data transmission. SPI is used where extremely high-speed data transmission is required. SPI supports a single master to communicate with multiple slaves. The selection of slaves by the master takes place through pulling the active low chip select (CS) pin from high to low. The master contains multiple CS pins which are connected to each individual slave, whichever CS is asserted that respective slave responds and transmission is initiated [22].