Design Of A PFM/PWM High Efficiency Synchronous Step-up DC-DC Converter

With the rapid development of semiconductor technology, the replacement rate of electronic products is faster and faster. Power supply as the heart of electronic products has also been considerable development, and its various index is required more stringent, domestic and international market demand for power management chip is growing. High performance, high efficiency, integrated and intelligent power management is the main direction of future chip development. Nowadays people advocate environment protection, energy conservation and low carbon economy, so high efficiency DC-DC converter has been widely concerned and researched.Firstly, the paper analysises the fundamental principles, loop control and modulation of synchronous step-up DC-DC converter. Then the stability and compensation method of Current and voltage loop were analyzed and researched. On the basis of this analysis designing a PFM/PWM high efficiency synchronous step-up DC-DC converter with 3.1V~4.2V input voltage and 0~2.1A load. An adaptive slope compensation dynamically adjusted based on the input and output voltage is presented, which ensure the current loop stability and effectively solve the problem of over compensation and reducing system load capacity brought by a linear slope compensation. The converter integrates synchronous rectifier technology, uses PMOS replaceing Schottky diode, which reduces conduction losses and improves the conversion efficiency of the system. For synchronous step-up DC-DC converter power-up process is easy overshoot, the converter designs a novel soft-start circuit, the whole process is divided into two phases open-loop precharge and closed-loop switching soft start, so that the converter can start up with heavy load and can smoothly start up within the range of 0~2.1A load without voltage overshoot. The converter designs PFM and PWM mode automatic switching circuit to ensure the process of switching between PFM and PWM can proceed smoothly with the change of load. With heavy load, the converter work in PWM mode, the system can quickly respond to the change of input voltage, load and internal circuit parameters. With light load, the converter automatically switches to PFM mode, skiping the part of the switching cycle and reducing driving and switching losses brought by the switching action, which improves light load efficiency. In order to make inductorcurrent does not occur backflow after the current zero-crossing and cause unnecessary power consumption, the converter designs zero-crossing comparator, when the inductor current zero-crossing, the system will shutdown the PMOS in time and prevent current backflow, making the chip have high efficiency over the entire load range, the efficiency is up to 93%. The converter integrates undervoltage protection, over temperature protection, overcurrent protection, short circuit protection and output overvoltage protection, they can effectively guarantee the converter work normally and will not be destroyed in unusual circumstances.The design of synchronous step-up DC-DC converter is based on CMOS process and uses Cadence Spectre simulation platform, the circuit simulation and design have been accomplished, verificated the whole circuit performance at different input and output voltage, load, temperature and Corner, and completed layout under Cadence Virtuoso, this converter is currently being taped.