Low-voltage Low-power Integrated Circuit Voltage Bootstrap Circuit Analysis And Design

Voltage bootstrap circuit is a kind of circuit to generate a higher voltage than the supply voltage by using boot capacitors and switches. It is widely used in modern integrated circuits, such as in the memory circuit, the voltage bootstrap circuit is implemented to generate high voltage to write and erase the floating gate device; in analog circuit, it is used to generate high voltage to drive analog switches. However,with the scaling down of supply voltage, MOS threshold voltage consideration becomes more and more important, traditional bootstrap circuit will face many problems.This thesis analyzes the traditional voltage bootstrap circuits and find out the problems they face in low voltage integrated circuit, including the threshold voltage impact and process compatibility and so on. Then some solutions will be given to solve the corresponding problems. Besides, some critical properties of the voltage bootstrap circuit are analyzed in detail, including the voltage gain, power, efficiency, driving capability, ripple voltage and chip area.Two voltage bootstrap circuits are designed in this thesis. One is composed of four cascaded improved voltage doublers. The circuit operates at a frequency of 145MHz, and generates an output voltage of 8.2V when the input is 1.8V. When a 10k ohm resistor load is connected at the output, the output power is 4.4mW. Another combines three stages improved CTS circuit and a cross-coupled voltage doubler skillfully. This circuit can give a 7V output when the input is 1.8V, and can deliver a power of 2.2mW to the load circuit when load resistor is 10k ohm. All the designed circuits are simulated in Cadence with IBM 0.18μm model files. Simulation results show that the designed circuit can effectively overcome the MOS threshold voltage problems and improve the circuit's voltage output capacity and the drive capability.The designed voltage bootstrap circuits have high voltage gain and less threshold voltage affection, they can work down to 0.9V power supply voltage. Therefore they are very suitable for low voltage application.