7.3.1.6 RC

The oscillator programming pin. The oscillator of the UCC3808-x tracks VDD and GND internally, so that variations in power supply rails minimally affect frequency stability. Figure 10 shows the oscillator block diagram.

Only two components are required to program the oscillator: a resistor (tied to the VDD and RC), and a capacitor (tied to the RC and GND). The approximate oscillator frequency is determined by Equation 1.

Equation 1.

where

• frequency is in Hz
• resistance in Ω
• capacitance in Farads

The recommended range of timing resistors is between 10 kΩ and 200 kΩ and range of timing capacitors is between 100 pF and 1000 pF. Timing resistors less than 10 kΩ must be avoided.

For best performance, keep the timing capacitor lead to GND as short as possible, the timing resistor lead from VDD as short as possible, and the leads between timing components and RC as short as possible. Separate ground and VDD traces to the external timing network are encouraged.