The oscillator of the UC1825B-SP is a saw tooth (see Figure 8-1). The rising edge is governed by a current controlled by the RT pin and value of capacitance at the CT pin (C_CT). The falling edge of the sawtooth sets dead time for the outputs. Selection of RT must be done first, based on desired maximum duty cycle (see Figure 8-3). CT can then be chosen based on the desired frequency (RT) and D_MAX (see Figure 8-2). Equation 1 shows the design equations.

Equation 1.

Recommended values for R_T range from 1 kΩ to 100 kΩ. Control of D_MAX less than 70% is not recommended.

Figure 8-1 Oscillator

Figure 8-2 Oscillator Frequency vs Timing Resistance

Figure 8-3 Maximum Duty Cycle vs Timing Resistance