8.3.5 Oscillator

The UCCx813-x oscillator generates a sawtooth waveform on RC. The rise time is set by the time constant of R_T and C_T. The fall time is set by C_T and an internal transistor on-resistance of approximately 130 Ω. During the fall time, the output is OFF and the maximum duty cycle is reduced below 50% or 100%, depending on the part number. Larger values for the timing capacitor increase the discharge time and reduce the maximum duty cycle and frequency slightly, as seen in Figure 5 and Figure 6 .

Figure 16. Oscillator Equivalent Circuit

The oscillator section of the UCCx813-x BiCMOS family has few similarities to the UC3842 type — other than single-pin programming. It does still use a resistor to the reference voltage and capacitor to ground to program the oscillator frequency up to 1 MHz. Timing component values must be changed because a much lower charging current is desirable for low-power operation. Several characteristics of the oscillator have been optimized for high-speed, noise-immune operation. The oscillator peak-to-peak amplitude has been increased to 2.45 V typical versus 1.7 V on the UC3842 family. The lower oscillator threshold has been dropped to approximately 0.2 V while the upper threshold remains fairly close to the original 2.8 V at approximately 2.65 V.

Discharge current of the timing capacitor has been increased to nearly 20-mA peak as opposed to roughly 8 mA. This can be represented by approximately 130 Ω in series with the discharge switch to ground. The higher current is necessary to achieve brief dead times and high duty cycles with high-frequency operation. Practical applications can use these devices to a 1-MHz switching frequency.

Figure 17. Oscillator Waveform at RC

Figure 18. Oscillator Frequency vs R_T For Several C_T