A Fixed Frequency COT Controlled BOOST Converter

With the rapid development of electronic products,such as mobile phones,tablet computers,wearable devices and so on,the requirement of power supply performance is becoming higher.These portable electronic products are developing in the direction of low voltage,low power consumption and miniaturization.In order to reduce the area,these portable devices are usually powered by single power supply.What’s more,the battery terminal voltage is also gradually decreasing to reduce power consumption,such as Li and Ni MH batteries.However,many mixed-signal are still powered by 3.3 or 5 V,and the general battery cannot provide enough supply voltage.Therefore,boost converter is necessary to be integrated in the system on chip to step up the battery voltage to an expected value.In this paper,a boost converter for portable devices is designed.The input voltage range is 1.5~4.5 V,the output voltage is 5V,the maximum load current is 1 A,and the switching frequency is 1 MHz.Based on traditional COT modulation,a fix frequency COT modulation mode is designed.The on time is no longer a fixed value,but varies with the input/output voltage.The switching frequency is constant under CCM and decreases with the decrease of load current in DCM.While retaining the advantages of traditional COT modulation,such as high light load efficiency and fast transient response speed,it solves the EMI problem introduced by COT modulation due to the change of switching frequency,which would facilitate the design of subsequent filter circuit.This paper first introduces the basic principle of the boost converter.Based on the COT modulation,the input voltage feedforward loop and the output voltage feedback loop are introduced,achieving a fix frequency in the CCM.The behavioral level modeling of fix frequency COT asynchronous voltage and current mode control are carried out in Simulink respectively to verify the correctness of their functions.Subsequently,some key modules such as comparator,on-time generator and inductor current sensing circuit are discussed in detail.The proposed boost converter is implemented by using a 0.18 um BCD process,and simulated on the Cadence Spectre platform.Simulation results show that the COT voltage mode control has the fastest transient response speed,but the switching frequency varies by 10 % and the maximum conversion efficiency is 80 %.The maximum overshoot voltage of the COT synchronous current mode is 80 m V,the recovery time is 50 μs,the switching frequency variation of the entire operating range is only 3.5 %,and the maximum conversion efficiency is 93.89 %.The fix frequency COT modulation technology designed in this paper has the advantages of high light-load efficiency,such as low power consumption,fast transient response and fast response in different working environments.It has reference value for the design and application of power management chips for portable devices.