SLLA549 July 2021 TCAN4550 , TCAN4550-Q1 , TCAN4551-Q1
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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:
XXTAL = the imaginary part of the impedance represented by the crystal.
RXTAL = the real part of the impedance represented by the crystal.
XOSC = the imaginary part of the impedance represented by the oscillator.
Rneg = the real part of the impedance represented by the oscillator.
Mathematically, Rneg 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.
The TCAN455x clock circuit contains three main blocks, an Amplifier and Bias Control block, a Filter and Comparator block, and a Clock Input Detection block as shown in Figure 1-1. A Comparator is used to compare the voltage difference between the OSC1 and OSC2 pins and generate the Clock Output signal used by the device when either a quartz crystal or single-ended clock input is used. The positive input of the comparator is connected to the OSC1 pin, and the negative input is connected to the OSC2 pin. When a single-ended clock is used, the clock signal is applied to the positive input of the comparator through the OSC1 pin, and the negative input of the comparator is held low by connecting the OSC2 pin to ground.
The Amplifier and Bias Control block provides the source current needed to start and sustain the crystal oscillation through the OSC1 pin when a quartz crystal is used, but it must be disabled when a single-ended clock input is used instead of a crystal. This is handled by the Clock Input Detection block that monitors the voltage level on the OSC2 pin and disables the Amplifier block if the voltage is below the detection threshold which is typically between 90mV to 150mV, such as when the pin is connected to ground. If the OSC2 pin is not connected to ground, the current supplied by the Amplifier and Bias block through the crystal will ideally keep the minimum voltage level of the oscillation waveform above 400mV and includes a peak detector circuit that will sense the oscillation envelope formed by the voltage difference between the OSC1 and OSC2 pins.
The crystal oscillator transconductance amplifier adjusts the amplifier bias current to maintain a consistent and stable oscillation when the voltage amplitude of the OSC1 and OSC2 waveforms are nominally 1 Vpp for each of the individual OSC1 and OSC2 signals, or 2 Vpp between the differential OSC1 and OSC2 signal resulting from their 180° phase shift. However, the actual voltage levels will depend on the total load connected to the TCAN455x oscillator circuit.
When the total external load on the amplifier created by the reactance of the load capacitance and the crystal equivalent series resistance is too small, the amplifier's minimum output current may cause the oscillation voltage amplitude levels to become too large by allowing too much power to flow through the crystal. If the voltage amplitude on the OSC2 pin becomes too large, the minimum peak voltage level of the signal could drop below the single-ended clock input detection threshold causing the amplifier and bias block to become disabled and stop the oscillation. When the external load is configured properly, the amplifier will be able to adjust the bias current to maintain a smaller voltage amplitude level in the nominal range and ensure the single-ended clock detection circuit will not disable the amplifier and bias control block.
Establishing the proper balance in the oscillation circuit is critical to stable reliable operation, preventing mechanical damage to the crystal, and guarantee several other factors not previously discussed. Ensuring the frequency of oscillation and the gain in the transconductance amplifier has enough margin to create the negative resistance needed to reliably establish the crystal oscillation must also be considered when selecting the values of the components used in the circuit. The remainder of this document will discuss how to avoid unreliable operation and determine the proper component values needed to meet all parametric concerns through a combination of measurements and calculations.
Common schematic representations of the Basic Pierce Oscillator circuit are shown in the following figure with the key components identified.
Without a TCAN455x device, crystal, and load capacitors mounted on the board, measure the parasitic capacitance between the crystal footprint pads. If series resistors are used, include those on the board when making the measurement. This becomes the PCB stray capacitance that will be used in calculations CPCB_Stray.
The TCAN455x OSC1 Output Pin Capacitance Cout is typically 10 pF
The TCAN455x OSC2 Input Pin Capacitance Cin is typically 9 pF
Calculate the load capacitance presented to the crystal by the following formula:
Where the OSC1 pin capacitance Cout is in parallel with the CL1 external load capacitance since they are both with respect to ground on the amplifier output side of the crystal and therefore added together.
Where the OSC2 pin capacitance Cin is in parallel with the CL2 external load capacitance since they are both with respect to ground on the amplifier input side of the crystal and therefore added together.
Where the capacitance on both sides of the crystal are in series with each other relative to the crystal due to the common connection through the ground plane. Therefore they are calculated as capacitance in series.
Where the parasitic capacitance of the PCB between the OSC1 and OSC2 pins is in parallel with the capacitance of the components and is therefore added to component capacitance.
Adjust the value of the external capacitors CL1 and CL2 to achieve a total load capacitance CLoad of the recommended value specified in the crystal data sheet.
Check for the motional parameters C0, Cm, resonant frequency (fs) and ESR on the data sheet of the crystal manufacturer. Calculate the resonant frequency of the crystal and of the crystal with the load capacitance.
Where the resonant frequency and motional inductance of the crystal is:
Where the resonant frequency of the crystal and capacitive load is:
Because the oscillator frequency is determined by load capacitance which is a product of the crystal’s shunt capacitance C0, the crystal’s Motional capacitance Cm, and the load capacitance of the board and external capacitors CLoad, it will shift according to the following formula:
Measure the oscillation frequency using an E-Field probe, sometimes called either a sniffer probe or antennae probe, connected to a RF Amplifier to strengthen the signal detected by the oscillation of the crystal, and a Spectrum Analyzer or Frequency Counter to display the frequency of the signal detected. This method does not place a direct load on the oscillation circuit that will alter the frequency of oscillation through additional resistance or capacitance inserted into the circuit by a directly connected voltage probe.
If an E-Field probe is not available, one can be made by exposing the conductor on one end of a coax cable that will act as an antenna, insulating it with heat shrink tubing to prevent electrical contact with any components on the board, and placing the insulated conductor directly on top of the crystal. The shield of the coax cable should be electrically connected to a ground location near the crystal to improve the signal quality and provide a reference for the test equipment.
The signal detected will likely have low amplitude and may need to be amplified with an RF Amplifier for a more reliable measurement by the spectrum analyzer. The amplitude of the signal is not important and it simply needs to be large enough for the spectrum analyzer to measure the frequency.
Tune the spectrum analyzer with a center frequency at the nominal crystal frequency and with a fairly narrow span such as 100 kHz so that there is enough resolution to capture the oscillation signal and accurately measure the center frequency.
If a spectrum analyzer is not available, a High Resolution Frequency Counter with a minimum of 7 to 8 digits of precision may be used if the signal has enough amplitude and is free of enough noise to return an accurate frequency count.
An alternative measurement can be done with an oscilloscope through a very low capacitance and high impedance active probe. The probe will add an additional capacitive/resistive load on the circuit and will slightly alter the oscillation frequency. Less than 1pF of probe capacitance is needed to minimize the frequency shift caused by the probe.
Oscilloscope measurements may not have enough resolution to get a high precision measurement of the exact oscillation frequency. A more accurate measurement can be obtained by using a High Resolution Frequency Counter.
Verify the frequency of oscillation is within the required specification and adjust the external capacitors to shift the frequency if needed. Increasing the capacitance will lower the frequency, and likewise reducing the capacitors will increase the frequency.
Re-calculate the load capacitance if the external capacitor values have been adjusted.
Identify the crystal's shunt capacitance C0 and motional resistance Rm from the crystal’s data sheet or ask the crystal vendor if this is not specified.
Some crystal data sheets specify the Rm as the crystal’s maximum Equivalent Series Resistance or ESR. Other crystal data sheets use the term ESR to specify the total load resistance under the specified Capacitive Load which is greater than the motional resistance of the crystal itself as defined in the next step.
Calculate the combined load of the series resistance in the crystal and the reactance of the capacitance placed on the oscillator circuit. This will be to total Load Resistance RLoad which is different than the crystal’s motional resistance Rm. Both are sometimes referred to as the ESR and therefore there can be great confusion about what the term ESR is referring to. In calculations and for clarity the term ESR will not be used in this document. The crystal’s motional resistance will be designated by Rm and the total Load resistance will be designated by RLoad.
If the Motional Resistance Rm of the crystal is unknown, this can be approximated by using the maximum ESR listed in the data sheet (for example, 50 ohms), but this may not be accurate and is only a rough estimation that will cause the results to be larger than the actual or typical value (for example, 20 ohms).