9.2.2.8 R_T and C_T

The internal oscillator uses a timing capacitor (C_T) and a timing resistor (R_T) to program operating frequency and maximum duty cycle. The operating frequency can be programmed based the curves in Figure 3, where the timing resistor can be found once the timing capacitor is selected. The selection of timing capacitor also affects the maximum duty cycle provided in Figure 5. It is best for the timing capacitor to have a flat temperature coefficient, typical of most COG or NPO type capacitors. For this converter, 1000 pF and 13.6 kΩ were selected for C_T and R_T to operate at 110-kHz switching frequency.