Design Of DC-DC Power Chip XD1719 With Constant On-time Control Mode

Electronic products are closely related to daily life.Power supply is one of the core components of electronic products,whether its performance is good is crucial.The traditional power often uses the linear power supply,which has the disadvantages of large volume,low efficiency and narrow input range,so the switching power supply emerges as the times require.In the switching power supply,DC-DC power supply is widely used because of its flexible voltage conversion.However,many DC-DC power supplies have the disadvantages of slow response to input voltage and load,and the switching frequency will change with the change of input or output,which can increase the design difficulty of inductor.So,in order to overcome these disadvantages,we must design a new control mode of DC-DC power supply.Based on constant on-time control mode,a DC-DC power chip XD1719 with an adaptive constant on-time control mode is designed in this paper.In this kind of control mode,the on-time of the power transistor can be adjusted adaptively,and is proportional to the output voltage,inversely proportional to the input voltage.Finally,the system can guarantee a constant switching frequency,and has a fast response to input voltage.In the On_Timer module,a circuit that can stabilize the frequency is designed.By switching the switching frequency into a voltage signal and comparing with the preset voltage,the switching frequency of the system is kept in a preset range,thereby ensuring the constant switching frequency.When the load changes,the on-time of the power transistor is adjusted directly in the current working cycle without waiting for the next working cycle,so it has fast transient response.The voltage reference of the chip XD1719 does not use the typical bandgap reference,but uses the full MOS voltage reference.In the structure,the full MOS reference voltage is composed of MOS transistors and resistors,and does not need bipolar transistors,so the chip occupied area is greatly saved.In performance,the temperature drift coefficient of the full MOS voltage reference is very small.When the temperature changes in the range of-40~125oC,the difference between the maximum and the minimum of the reference voltage is only 996.8?V.At the same time,chip XD1719 has a perfect protection mechanism.The chip is integrated with OTP,UVLO and so on,so as to ensure that the chip works in a safe and reliable environment.When the system is started,the output voltage is low.If the reference voltage suddenly rises,it will result in the imbalance of the loop comparator.So,chip XD1719 also integrates the soft start module.The soft start module will generate a rising ramp voltage to replace the reference voltage,which can effectively avoid the inrush current and overshoot voltage.The simulation and verification of chip XD1719 is finished with 0.5?m BCD 18 V process model and Spectre simulation tools of Cadence.The simulation results show that the performance indexes of chip XD1719 meet the design requirements,the starting process of the system is stable,the transient response is fast,and the switching frequency is constant.