9.2.2.1 Oscillator

The UC184xA-SP oscillator is programmed as shown in Figure 15. Timing capacitor C_T is charged from VREF (5 V) through the timing resistor R_T, and discharged by an internal current source. The first step in selecting the oscillator components is to determine the required circuit dead time. Once obtained, Figure 16 is used to pinpoint the nearest standard value of C_T for a given dead time. Next, the appropriate R_T value is interpolated using the parameters for C_T and oscillator frequency. Figure 17 shows the R_T/C_T combinations versus oscillator frequency. The timing resistor can be calculated from the following formula.

Equation 1.

Figure 14. E/A Compensation Circuit for Continuous Boost and Flyback Topologies

The UC184xA-SP has an internal divide-by-two flip-flop driven by the oscillator for a 50% maximum duty cycle. Therefore, their oscillators must be set to run at twice the desired power supply switching frequency.

Figure 15. Oscillator Programming

Figure 16. Dead Time vs C_T (R_T > 5 kΩ)

Figure 17. Timing Resistance vs Frequency