RC is the oscillator timing pin. For fixed frequency operation, set the timing-capacitor charging current by connecting a resistor from REF to RC. Set frequency by connecting a timing capacitor from RC to GND. For best performance, keep the timing capacitor lead to GND as short and direct as possible. If possible, use separate ground traces for the timing capacitor and all other functions.
The UCCx813-x’s oscillator allows for operation to 1 MHz versus 500 kHz with the UC3842 family. Both devices make use of an external resistor to set the charging current for the capacitor, which determines the oscillator frequency. For the UCCx813-[0,1,2,4], use Equation 1.
For the UCCx813-[3,5], use Equation 2.
The recommended timing resistance is from 10 kΩ to 200 kΩ and timing capacitance is from 100 pF to 1000 pF. Never use a timing resistor less than 10 kΩ.
The two equations are different due to different reference voltages. The peak-to-peak amplitude of the oscillator waveform is 2.45 V versus 1.7 V in UC3842 family. For best performance, keep the timing capacitor lead to GND as short as possible. TI recommends separate ground traces for the timing capacitor and all other pins. The maximum duty cycle for the UCCx813-[0,2,3] is approximately 99%; the maximum duty cycle for the UCCx813-[1,4,5] is approximately 49%. The duty cycle cannot be easily modified by adjusting RT and CT, unlike the UC3842A family. The maximum duty cycle limit is set by the ratio of the external oscillator charging resistor RT and the internal oscillator discharge transistor on-resistance, like the UC3842. However, maximum duty cycle limits less than 90% (for the UCCx813-[0,2,3]) and less than 45% (for the UCCx813-[1,4,5]) can not reliably be set in this manner. For better control of maximum duty cycle, consider using the UCCx807.