Sawtooth oscillator

摘要

A sawtooth oscillator includes a current source having an output coupled to a first capacitor for supplying it with a charge current (I). A discharge circuit discharges the first capacitor during a discharge period (TDS) in response to a voltage on the first capacitor. A second capacitor is coupled to the output of the current source. A switch interrupts the supply of charge current to the first capacitor during an interrupt period (TIS). Part of the charge current occurs during the discharge period (TDS). Subsequent to the interrupt period, the first capacitor receives a charge surplus built up by the charge current in the second capacitor. The current supply to the first capacitor is temporarily interrupted at least during a part of the discharge period. The second capacitor operates as an auxiliary capacitor in which a charge is stored which would otherwise have been supplied to the first capacitor. After the interrupt period the charge stored in the second capacitor is supplied back to the first capacitor. In this way substantially no charge current is lost during a total time period of a charge and discharge cycle. In the total time period the whole charge current of the current source ends up in the first capacitor so that tolerances in the discharge period do not affect the oscillation frequency of the sawtooth oscillator.