On-Chip Inductor Based Step Down Converter with Fast Transient Response

Motivation

The number of smart appliances is increasing at a rapid rate. Compact size and high-power efficiency are critical requirements for smart systems like the Internet of Things (IoT). The trend of system miniaturization necessitates the need for an on-chip system solution. System power supplies are critical in determining system area and efficiency. For efficient operation, an electronic system requires dynamically adjustable voltage levels depending upon the mode of operation.

Low Drop-Out (LDO) regulators, switch capacitor-based converters and inductor-based DC-DC converters are used for converting one DC potential into another DC potential. LDO is usually used where the current consumption is very small otherwise the I²R losses affect the efficiency of the system. The inductor-based DC-DC converters are preferred over switch capacitor converters because of their fine resolution control, and high-power efficiency at heavy loads. That’s why we will be designing an inductor-based DC-DC converter [1].

Circuits operating over a high frequency demands different loading currents under heavy and light load. This dynamic behaviour causes the current change in the system, abruptly. The sudden load changes at high-frequency cause overshoot and undershoot of voltage and current for hundreds of the cycles of switching frequency, causing the performance degradation. So the control loop needs to be designed to improve the transient response of the system with minimum overshoot and undershoot.

For the circuits operating over a wide current range, the operating frequency needs to be adjusted over a wide range for optimal operation under heavy and light loads. The sudden change in load current causes voltage overshoot or undershoot. The control loop needs to adjust frequency to overcome the overshoot/undershoot, and provide efficient operation under new operating conditions. Thus, for fast convergence, and limiting overshoot/undershoot the control loop needs to be fast to improve the transient response of the system with minimum overshoot and undershoot.

The approach to implementation:

In this work, our area of interest is focused on the design of a fully integrated on-chip DC-DC step-down converter with a fast transient response. DC-DC Converters usually consist of a power stage and a control stage.

The power stage controls the efficiency of the converter. The power stage usually consists of switches, inductor, capacitor and load. Fig. 1 shows the circuit diagram of the DC-DC step-down converter. When switch S1 is on, the inductor stores the energy and the output voltage becomes equal to the input voltage. When switch S2 is on, the inductor is discharging through switch S2, which results in a lower voltage as compared to the input voltage[2].

Fig. 1:  Schematic of the conventional DC-DC step-down converter

The control stage usually consists of an error amplifier along with a compensation circuit for the stability of the system. The error signal is then compared with the saw-tooth signal to generate the respective duty cycle. The control stage helps us to mitigate the undershoot and overshoot of the system as well as to achieve the fast transient response of the system. Different loops are proposed over the year for precise control of the system. Control loops can be categorized into linearity. One is called a linear control loop while the other is called a non-linear control loop. Examples of linear control loops are voltage control loop and current control loop. The nonlinear control loops are hysteresis control loops, Constant ON Time (COT) control loop, Constant ON/OFF Time (COOT) control loop, Pseudo Constant ON/OFF Time (SCOOT) control loop and Ripple Based Constant ON/OFF Time (RBCOOT) control loop structure. The nonlinear control loops usually use the concept of Pulse Frequency Modulation (PFM) for generating desired duty cycle for switching while linear control usually uses Pulse Width Modulation (PWM) phenomena for voltage conversion [3].

The transient response of the system is limited by the control stage delay as well as the LC network of the power stage. A novel technique is proposed in [5] where a SAR ADC is utilized to control the current pump which eventually injects current or out to the system depending upon the undershoot or overshoot. This eventually improves the transient response. However, SAR ADC utilizes a precise clock as well as multiple clock cycles to generate the corresponding bits for the current pump. Our proposed architecture is to improve the transient response of the system by replacing SAR ADC with pair of hysteresis comparators. the proposed block level diagram is shown in Fig. 2. 

Fig. 2: Architecture of proposed monolithic voltage mode step down converter

It will not only reduce the area of the system but also helps us to achieve maximum efficiency with the fast transient response. The target specifications of the proposed design are summarized in Table 1.

Table 1. Target Specifications

Team Lead: 

Jawad Shakeel (i201316@nu.edu.pk) NUCES (FAST-NU)

Team Members: NUCES (FAST-NU)

Bilal Shabbir

i212434@nu.edu.pk

Hafiz Saleem Ullah
i212436@nu.edu.pk

Zohaib Ahmad
i212438@nu.edu.pk

Uzair Ahmad
i201317@nu.edu.pk

Dr. Rashad Ramzan
rashad.ramzan@nu.edu.pk

Dr. Hassan Saif
Hassan.saif@nu.edu.pk

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