The voltage monitoring module determines whether to adopt the dynamic mode according to the sampling result Vo, and the voltage monitoring module comprises three comparators and a logic unit to determine whether to adopt the dynamic mode. The three comparators determine the relationship between the sampling voltage Vo and the minimum voltage Vomin, the relationship between the sampling voltage Vo and the maximum voltage Vomax, and the relationship between the sampling voltage Vo and the reference voltage Vref respectively. The logic cell outputs a mode selection result according to the results of the comparators, wherein the output result mode_F of the logic cell is one of the HTL mode (mode_LTH), the LTH mode (mode_HTL) and the normal working mode (mode_normal); when Vo is greater than Vomax, the output mode_F is mode_HTL, i.e., the HTL mode is started, when Vo is less than Vomin, the output mode_F is mode_LTH, i.e., the LTH mode is started, and when Vo is between Vomin and Vref, if the output of the logical unit is the LTH mode in last period, then the output of the logical unit is the LTH mode in the period; if the output of the logical unit is the HTL mode in last period, then the output of the logical unit is the normal mode in the period; if the output of the logical unit is the normal mode in last period, then the output of the logical unit is the normal mode in the period; when Vo is between Vref and Vomax, if the output of the logical unit is the LTH mode in last period, then the output of the logical unit is the normal mode in the period; if the output of the logical unit is the HTL mode in last period, then the output of the logical unit is the HTL mode in the period; and if the output of the logical unit is the normal working mode in last period, then the output of the logical unit is the normal working mode in the period. The mode selection result is inputted to the slope calculation module and the mode control module in the dynamic modules.