| Nowadays,Lithium batteries have become the most economical and widely used energy storage among the portable electronic devices.With the miniaturization of electronic equipment,the integration level of power management modules is getting higher and higher.Correspondingly,the parasitic parameters of small-size off-chip devices also change due to special manufacturing processes.At the same time,with the increasing diversification of application,the power management modules are required to provide higher output voltage DC accuracy,better electromagnetic compatibility,faster transient response speed and better loop stability.In this case,it is of commercial value and theoretical research significance to design a DC-DC converter with good electromagnetic compatibility,high output voltage DC accuracy,fast transient response and perfect stability that does not depend on parasitic parameters of off-chip devices.This is the main research purpose of this article.Based on the adaptive on-time(AOT)control mode,a step-down DC-DC converter is designed for Lithium battery powered applications in this paper.Compared to the traditional control mode,the transient response of AOT control mode will be faster because the control loop does not need to be compensated.In addition,in order to prevent the parasitic parameter of the off-chip output capacitor from affecting the loop stability,This paper has adopted ripple injection to enhance the stability of the converter.As a result,the converter can work normally with low ESR output capacitor.Then,in order to overcome the problem of the poor output voltage DC accuracy caused by valley voltage detection and ripple injection in this control mode,an accuracy correction module is used to compensate for this error,and it effectively improves the output voltage DC accuracy.Afterwards,in order to suppress the shift of the switching frequency and improve the EMI performance of the converter,we take it into consideration that the influence on the switching frequency made by non-ideal factors such as on-resistance of power transistors.The sampling module is adopted to make on-time proportional to actual duty cycle,which ensures the consistent switching frequency.Finally,the chip also has a soft-start circuit and current limit protection to ensure that the chip can work stably and safely.At the beginning of this paper,this paper compares the advantages and disadvantages of various control modes,and then analyzes the existing problems in the adaptive on-time control mode.The corresponding methods are put forward for improvement,and the small-signal model of the converter is also derived.Among these methods,the ripple injection reduces the effect on the loop stability made by the parasitic parameters of off-chip components.The accuracy calibration module is used to improve the output voltage accuracy and the adaptive compensated on-time suppresses the switching frequency deviation and reduces EMI.The converter is designed in the SMIC 0.18 μm standard CMOS process.The Cadence Spectre simulation tools are used to simulate and verify the circuit module and the converter.The results show that the converter can work normally when the low ESR capacitor is used.When the input voltage is 2.7V-4.5V,the shift of the switching frequency in CCM can be less than 1.9% compared with the predetermined switching frequency of 2MHz.The shift of output voltage in CCM can be less than 1% compared with the predetermined voltage of 1.8V.When the load current changes from 0.2A to 0.5m A within 1μs or from 0.5m A to 0.2A within 1μs,the corresponding settling time is 7μs and 19μs,respectively.As for the output voltage,the overshoot voltage and undershoot voltage are 39 m V and 42 m V,respectively. |
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