The oscillator design for the UCC28C5x controllers incorporates a trimmed discharge current to accurately program maximum duty cycle and operating frequency. In its basic operation, a timing capacitor (C_CT) is charged by a current source, formed by the timing resistor (R_RT) connected to the device reference voltage (VREF). The oscillator design incorporates comparators to monitor the amplitude of the timing capacitor voltage. The exponentially shaped waveform charges up to a specific amplitude representing the oscillator upper threshold of 3 V. After the controller reaches this level, an internal current sink to ground turns on and the capacitor begins to discharge. This discharge continues until the oscillator lower threshold has reached 0.7 V at which point the current sink is turned off. Next, the timing capacitor starts charging again and a new switching cycle begins.

Figure 8-4 Oscillator Circuit

While the device discharges the timing capacitor, resistor R_RT continues attempting to charge C_CT. It is the exact ratio of these two currents, the discharging versus the charging current, which specifies the maximum duty cycle. During the discharge time of C_CT, the device output is always off. This represents an ensured minimum off time of the switch, commonly referred to as dead-time. To program an accurate maximum duty cycle, use the information provided in Maximum Duty Cycle vs Oscillator Frequency for maximum duty cycle versus oscillator frequency. Any number of maximum duty cycles can be programmed for a given frequency by adjusting the values of R_RT and C_CT. After selecting the value of R_RT, find the oscillator timing capacitance using the curves in Oscillator Frequency vs Timing Resistance and Capacitance. However, because resistors are available in more precise increments, typically 1%, and capacitors are only available in 5% accuracy, it might be more practical to select the closest capacitor value first and then calculate the timing resistor value.