Bandgap reference and low dropout voltage regulator desıgn for capsule endoscopy system

Abstract

For the last 40 years, there have been many advancements in biomedical systems as there is a huge demand for them. A capsule endoscopy device is one of the biomedical systems which is used for the imaging of the gastrointestinal system. Endoscopic procedures require two operations because when the procedure starts from the esophagus it can only reach until duodenum, however imaging of the small bowel can be only accessed from the anal cavity. This operation is highly uncomfortable for the patient since the diagnosis of the entire gastrointestinal system requires two endoscopic procedures. To make this operation more comfortable for the patient, capsule endoscopy is developed. The capsule endoscopy system includes a laser source and a laser driver to process the information coming from the source and a transmitter system to transmit the processed data. The transmitter involves an analog to digital converter, transimpedance amplifier, power amplifier, and a phase generator. A single battery is used to supply voltage for all of these mentioned circuits. The battery input voltage of the system decays with time and to increase the lifetime of the capsule it is essential to design a power management unit. This power management unit involves a regulator to create supply voltage for chip blocks and a reference generator to obtain process-voltage-temperature independent reference. For the regulator, a low dropout regulator is chosen as they do not have ripple at the output voltage as in switching regulators which makes them less noisy. Noise is an important parameter because the input signal is low and any input voltage may affect the operation of the circuits. Traditional LDOs require a large off-chip capacitor at the output to create a right half plane zero and stabilize the circuit. However, the capsule is strictly limited in the area hence a cap-less LDO is designed. To enhance the transient performance after removing the output capacitor, a dynamic bias circuitry is added to the design. Output voltage only changes 6 % with process-temperature-voltage corners. Load and line transient results show that even though the input voltage or output load changes with time, the circuit can still regulate the output voltage. To obtain reference voltages for the blocks, a bandgap reference voltage generator is designed. In this design, two different design structures are used to achieve temperature independence. Both MOS and bipolar transistors are employed for this purpose. Design with MOS transistors advantages from operating in the subthreshold region hence supporting lower supply voltages however it has a larger variation at the output due to threshold voltage change over corners. Bipolar transistors benefit from less PVT variation however their performance degrades with lower supply voltages and high temperatures. To use two different characteristics of both designs, a system that switches to better performing structure is built. A comparator is designed to detect temperature and supply voltage. In this system, reference with bipolar transistors operates when the input voltage is higher than 2.8 V and at lower temperatures, and reference with MOS transistors starts to work when the input voltage is lower than 2.8 V and at higher temperatures. To further reduce the variation of the design with MOS trimming structure is implemented to the output resistor and variation decreased from 15% to 5%. The temperature coefficient of the reference generator is calculated as 75C. A power management unit that involves a bandgap reference generator and a low dropout regulator is introduced and designed. Important performance parameters are extracted from the requirements of the capsule endoscopy transmitter. Hence, the layout area is designed to be small and output voltage variation is minimized over PVT corners. The layout of the system is designed and post-layout simulation results are reported in the study.