the first or second reference voltage to the comparator and controls the turning on and off of the discharge constant current circuit a control constant current source controls the current from the charge current source for charging the capacitor the frequency of the output signal from the comparator is controlled by regulating the control constant current source to switch the current capacity of the charge current source application_number US 74205285 A description a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings in fig1 the preferred embodiment of the invention comprises a capacitor 5 a charge current source 1 for charging capacitor 5 a discharge constant current circuit 2 for discharging the capacitor 5 a comparator 3 for comparing the voltage charge on the capacitor 5 with a reference voltage c a reference voltage switching control circuit 4 for supplying the reference voltage c which may be either a first reference voltage e 1 or a second reference voltage e 2 e 1 gt e 2 which is selected according to the output of the comparator 3 control circuit 4 controls the turning on and off of the discharge constant current circuit 2 and constant current sources 6 7 and 8 for controlling the oscillation frequency when the charge current source 1 charges capacitor 5 to the voltage of the first reference voltage e 1 as shown in fig2 a the output b fig2 b of the comparator 3 rises to a high level responsive thereto the reference voltage switching control circuit 4 switches the reference voltage c fig2 c.