The TCAN455x family of devices uses a Pierce oscillator based design that operates in the inductive region between a crystal’s parallel and series resonant frequencies for a wide range of quartz crystals up to 40 MHz with a maximum ESR of 60 ohms and with load capacitance (CL1=CL2) requirements ranging between 8 pF and 24 pF. It is also capable of operating with a single-ended clock source instead of a quartz crystal supplied to the OSC1 pin, automatically detecting when a single-ended clock is used, and disabling the quartz crystal amplifier and biasing circuit.

This design creates flexibility for how the device is used in the end application, but results in a few additional design requirements that need to be understood to ensure stable operation. This document discusses these requirements and serves as a guide on optimizing the oscillator clock circuit components.

A crystal-based oscillator is formed by placing a crystal in the feedback loop of an oscillator circuit that provides sufficient gain and phase shift around the loop to start and sustain stable oscillations. A detailed explanation of crystal oscillator operation will not be covered here. However, to support the reader’s interpretation of the guidance and recommendations contained in this application note, some essential aspects of the crystal oscillator circuit model are presented and explained here. A simple model of a crystal is shown in Figure 2-1. The model has R-L-C series components, called motional resistance (Rm), motional capacitance (Cm), and motional inductance (Lm). The capacitor in parallel, C0, is called the shunt capacitance, and models the package capacitance. Figure 2-2 illustrates a simple oscillator model, consisting of an inverting amplifier and crystal, and its equivalent circuit model.

The circuit model in Figure 2-2 is useful for understanding the necessary conditions for oscillation. These are:

Where:

X_XTAL = the imaginary part of the impedance represented by the crystal.

R_XTAL = the real part of the impedance represented by the crystal.

X_OSC = the imaginary part of the impedance represented by the oscillator.

R_neg = the real part of the impedance represented by the oscillator.

Mathematically, R_neg is a negative resistance. It represents a circuit that supplies power rather than dissipating power, for example, an amplifier. Consequently, a simple interpretation is that the amplifier must have enough gain to compensate for the losses represented by the crystal and load capacitance. The concept of negative resistance is important to crystal oscillator design and will be revisited later in this app note.