Study And Design Of COT Architecture Synchronous Buck Chip

Switching power converters are widely used in consumer electronics,industry,automotive electronics and other fields.In recent years,the entire application system has shown the following trends: smaller volume,the output capacitors of the converter gradually adopts the smaller ceramic capacitors,lower supply voltage,higher integration and more frequent transient transition.In this context,switching converters' stability when using low equivalent series resistance(ESR)output capacitors like ceramic capacitors,output DC voltage accuracy,EMC characteristics and transient response are the key factors limiting system performance.Therefore,designing a DC-DC converter whose stability does not depend on the ESR of the output capacitors and with high DC output accuracy,good EMC characteristics and fast transient response has great theoretical and commercial value,which is the main purpose of this paper.In this paper,a synchronous buck DC-DC converter is designed based on adaptive constant on time(ACOT)control mode.On the premise of using ACOT architecture to ensure the transient response speed,the ripple injection circuit is introduced to realize the inductor current ripple compensation in the chip to ensure the system can work stably without ESR ripple information.By introducing the integrator circuit,the error between the feedback voltage and the reference voltage is sampled which significantly improves the DC output accuracy.By introducing the PLL circuit,the current switching frequency of the converter is adjusted to be equal to the reference frequency to achieve a constant switching frequency under different input voltages when the load is heavy,MODE pin is offered to set if the converter works in force pulse width modulation(FPWM)mode when the load is light which ensures good EMC characteristics under any working condition.This paper firstly compares the advantages and disadvantages of traditional control modes such as voltage mode,peak current mode,hysteresis mode and ACOT mode.After the ACOT control mode is adopted,the defects of ACOT control mode are analyzed in depth.Then,the improvement measures are proposed and the small signal model of ACOT is derived.In this paper,CSMC 0.5um BCD process is used to complete the circuit design.Finally,the simulation and verification of the module-level circuit and the overall circuit are performed by Cadence Specter simulation tool.The results show that the converter can work stably at even zero ESR,the DC output voltage error is less than 0.5% within the input voltage range of 2.6V ~ 5.5V and the load range of 0A ~ 6A.The maximum switching frequency error under heavy load is 3.4% and the FPWM function under light load can keep the frequency the same as which under heavy load.When the load current soar from 0.3A to 3A in 1us,the output voltage recover in 8us and the undershoot voltage is 43 m V.When the load current sag from 3A to 0.3A in 1us,the output voltage recovers in 18 us and the overshoot is 46 m V.The performance of the converter reaches the expected specification thus it have a broad application prospects.