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The application and integration of the power IC chip is more and more important for electronic devices. In this study, we develop high-voltage buck-boost conversion chip. Based on auto mode switch, this chip can achieve a stable output voltage without the need of detecting input level. The proposed technique can automatically adjust the on-time value using discharged loop to rapidly shorten the transient response time. The system could always maintain steadily the output voltage of 24V as the range of input voltage is widely from 12V to 40V. The output load current is available from 20 ​mA to 1A, and the highest transform efficiency is 86.3%. As the load current is suddenly changed from high to low, the overshoot level can be reduced with a fast discharging loop, and to fasten the transient speed. As for the low-voltage conversion, it outputs 3.3V under the input from 2.5V to 5V, where the maximum efficiency of buck/boost can achieve 94%. The chip is implemented TSMC0.25 ​ μ m HV- process, and the size of the circuit layout is 2.18 ​mm2, where four power MOS had been integrated to the chip for buck-boost control. [ABSTRACT FROM AUTHOR]