As a commonly used component, capacitors have seen their applications expand, leading to a proliferation of subtypes, such as coupling capacitors, fixed capacitors, and variable capacitors. To enhance understanding of capacitors, this article will introduce load capacitors, their classification, their functions, and how to select them. If you are interested in capacitors, please continue reading.
I. What is a load capacitor?
Load capacitance refers to the sum of all effective capacitances inside and outside the IC block connected by the two leads of the crystal oscillator. It can be regarded as a series capacitor of the crystal oscillator chip in the circuit.
Different load frequencies determine different oscillation frequencies of oscillators. Crystal oscillators with the same nominal frequency may have different load capacitances. This is because a quartz crystal oscillator has two resonant frequencies: one is a low-load-capacitance crystal oscillator connected in series, and the other is a high-load-capacitance crystal oscillator connected in parallel. Therefore, when interchangeing crystal oscillators with the same nominal frequency, the load capacitance must also be consistent; otherwise, it will cause the electrical appliance to malfunction. A device that converts electrical energy into other forms of energy is called a load. An electric motor can convert electrical energy into mechanical energy, a resistor can convert electrical energy into heat energy, a light bulb can convert electrical energy into heat and light energy, and a loudspeaker can convert electrical energy into sound energy. Electric motors, resistors, light bulbs, loudspeakers, etc., are all called loads. A transistor can also be considered a load in relation to the signal source. The most basic requirements for a load are impedance matching and the power it can handle.
A load is an electronic component connected to the two ends of a power source in a circuit. A circuit should not have the two terminals of a power source directly connected without a load; this connection is called a short circuit. Common loads include power-consuming components such as resistors, motors, and light bulbs. Components that do not consume power, such as capacitors, can also be connected, but this constitutes an open circuit.
II. Load Classification
Inductive load: A load is considered inductive when its current lags behind its voltage by a phase difference compared to the power supply (e.g., a motor, a transformer).
Capacitive load: A load is capacitive when the load current leads the load voltage by a phase difference compared to the power supply (e.g., a compensation capacitor).
Resistive load: A load that is resistive when there is no phase difference between the load current and the load voltage compared to the power supply (e.g., an incandescent lamp; an electric furnace).
A capacitive load has the properties of a capacitor (the voltage cannot change abruptly during charging and discharging); an inductive load has the properties of an inductor (the current cannot change abruptly in the magnetic field). In a mixed circuit, if the capacitive reactance is greater than the inductive reactance, the circuit is capacitive; otherwise, it is inductive.
III. The function of load capacitor
A crystal exhibits properties similar to an inductor. When voltage is applied, it undergoes mechanical bending. When the power is turned off, the stress from this bending is released, generating electrical energy. This energy is stored in a capacitor. When the bending returns to its normal state, the energy in the capacitor is transferred to the crystal oscillator. The circuit captures this release time and provides positive feedback, feeding the energy back to the crystal oscillator with the same polarity as the capacitor, reinforcing its bending and repeating the process. In other words, the capacitor and crystal oscillator form a circuit similar to an LC circuit.
The external capacitor required for a crystal oscillator is one that makes the equivalent capacitance (distributed capacitance between circuits) across the crystal equal to the load capacitance. The load capacitance is the capacitor that enables the crystal to oscillate. At this point, the capacitor's role becomes clear: charging and starting the crystal to oscillate. The load capacitance is crucial, determining whether the crystal oscillator can oscillate correctly in the product.
When purchasing a crystal oscillator, it's best to include important parameters such as the required load capacitance in the purchase instructions. This will save the purchaser a lot of trouble in the selection process. If the load of the crystal oscillator cannot be confirmed, the capacitance will be difficult to match, the starting capacitor will not be able to charge and discharge, and the crystal oscillator will not oscillate. When the distributed capacitance and the crystal oscillator capacitance are equal, the crystal oscillator can emit its resonant frequency. The size of the capacitance affects the stability and phase of the crystal oscillator frequency; the smaller the capacitance, the higher the price. Therefore, the load also determines the price of the crystal oscillator itself.
IV. How to select load capacitors
The load capacitance in an oscillator circuit is one of the most crucial values for ensuring the accuracy of a quartz crystal. It is also one of the most common causes of errors in oscillator circuit design. Therefore, selecting a suitable load capacitance for the crystal oscillator is extremely important!
Most quartz crystals are used in Pierce oscillator circuits, therefore, two external capacitors are required. Now, a question may arise: how to choose the correct values for these two capacitors?
The first common misconception is that the load capacitance on the crystal's datasheet directly determines the required values for both capacitors. This means that if the crystal's load capacitance is 20pF, then both capacitors need to be 20pF. However, this is incorrect and will cause a frequency shift.
Another misconception is that the load capacitance on the crystal datasheet needs to equal the sum of the two capacitors. If we use the same example with a 20pF crystal, this would mean both capacitors need to be 10pF. However, this is also incorrect. In most cases, even with an incorrect load capacitance, the crystal will still work in the circuit, but the frequency will be off, which can cause other problems. This is because both of the above assumptions about the load capacitance are wrong.
The correct approach requires considering all capacitors in the circuit. Therefore, not only the two main capacitors (the microcontroller and the microcontroller) but also the microcontroller's input/output capacitors and all stray capacitances must be considered. All of these together form the load capacitance. The biggest problem now is that without the actual circuit, it's impossible to know or determine the stray capacitances.
Therefore, during PCB design, you must only guess at stray capacitance and then check if the final circuit frequency is within tolerance. Typical stray capacitance in a Pierce oscillator circuit is between 3pF and 7pF.
Another suggestion is to choose values for Ca and Cb that are similar, or at least not far apart. This will prevent unintended frequency shifts and other interference. If Ca and Cb should not be equal, then Ca should be less than Cb.
