The main reasons for connecting a capacitor across the power supply terminals of an integrated chip include decoupling, energy storage, filtering, and improving electromagnetic compatibility. Specifically:
Decoupling: Capacitors can provide a path for high-frequency signals, reduce the internal resistance of the power supply, eliminate the influence of long traces of power and ground lines on the copper-clad board, and prevent harmful cross-linking between different parts of the circuitry of a shared power supply. The main function of decoupling capacitors is to absorb the high-frequency AC components in the power supply voltage and guide them to the ground line, thereby enabling the chip to obtain a more stable and reliable DC voltage.
Energy Storage: When the power consumption of a circuit suddenly increases, without a capacitor, the power supply voltage may be pulled down, generating noise, or even causing the device to restart. Large-capacity capacitors can temporarily release stored electrical energy to stabilize the power supply voltage.
Filtering: Capacitors can filter out high-frequency noise in the power supply and stabilize the circuit's operating state. Power supply filter capacitors can filter out noise and AC components in the power supply, smooth pulsating DC voltage, and store electrical energy.
Improving electromagnetic compatibility (EMC): Capacitors can reduce the impact of high-frequency interference signals on circuits, suppress transient common-mode voltage differences between circuits and interference sources, and prevent high-frequency signals from radiating into the external environment.
Provide transient current: When the chip needs a large current momentarily, the capacitor can respond quickly and provide the required current, avoiding voltage fluctuations caused by power network delays.
Preventing DC short circuits: Use capacitors instead of conductors for short circuit connections, because conductors can cause DC short circuits and potentially blow fuses, while capacitors can block DC and only allow AC to pass through.
Reduce the impact of inductance: Capacitors used for high-frequency short circuits must have low lead and PCB trace inductance, so each power supply capacitor must be very close to the two pins of the IC it is decoupling.
Adding a capacitor between two chip pins serves several purposes: stabilizing the supply voltage, filtering noise, providing instantaneous current, and reducing power consumption. These designs all aim to improve circuit stability and performance. For example, stabilizing the supply voltage is crucial. Voltage fluctuations are inevitable on power lines, potentially stemming from inherent power supply instability or current variations in other components. Significant voltage fluctuations during chip operation can lead to malfunctions or even damage. Therefore, adding a bypass or decoupling capacitor between the chip's power and ground pins acts as a small energy storage unit. When the supply voltage temporarily drops, the capacitor releases stored energy, ensuring a stable supply voltage for the chip.
I. Stable power supply voltage
A capacitor is connected between the chip's power supply pin and ground. Its main function is to provide a relatively stable voltage to the power supply line under high load. Capacitors store electrical charge; when the power supply line voltage suddenly drops, the capacitor quickly releases the charge, preventing significant fluctuations in the chip's supply voltage. Conversely, when the circuit load suddenly decreases, the capacitor absorbs excess charge, thus preventing a sharp, short-term voltage spike.
II. Noise Filtering
No circuit system can completely escape external and internal electromagnetic interference, which manifests as noise on power lines. This noise can be caused by high-frequency alternating current or electromagnetic radiation from other electronic devices. Capacitors placed between two chip pins effectively filter out high-frequency noise. These capacitors act as low-pass filters, allowing low-frequency current to pass through while blocking high-frequency current, thus protecting the chip from high-frequency electromagnetic interference.
III. Providing instantaneous current
When a chip is operating, certain functional modules may require a large current for a very short period of time. In such cases, the power supply system may not be able to respond quickly enough to this sudden surge in current. At this point, a capacitor acts as a momentary power source, rapidly providing the necessary current to ensure the chip functions normally. The instantaneous current released by the capacitor prevents the chip from experiencing performance degradation or instability due to unmet instantaneous current demands.
IV. Reduce power consumption
In complex electronic systems, the power supply may undergo multiple voltage reduction stages before reaching the chip's operating voltage. During this process, the voltage reduction devices themselves incur some losses, which capacitors can mitigate. Capacitors connected in series in the power supply path can store some energy, releasing it during periods of inactivity in chip operation. This reduces the energy drawn from the power lines and lowers the overall system power consumption.
In electronic circuits, capacitors connected to the pins of integrated chips are primarily used for storing charge and energy, filtering and decoupling, and ensuring circuit stability and signal integrity. Specifically:
Power supply filtering: Large-capacity electrolytic capacitors are used for energy storage and replenishment under low-frequency conditions to ensure a stable power supply. Meanwhile, capacitors located near the chip's power pins filter high-frequency characteristics, reducing power supply noise.
Decoupling: Decoupling capacitors can alleviate coupling interference between circuits, especially when there is a large capacitance between the drive signal source and the load capacitor. Decoupling capacitors can provide a fast energy response and prevent noise caused by current fluctuations. Decoupling capacitors are usually large, such as 10uF, to meet circuit requirements.
High-frequency bypass: High-frequency bypass capacitors are mainly used to bypass high-frequency noise. The capacitance is generally small, such as 0.1uF or 0.01uF, which helps to reduce the interference of high-frequency signals to other parts of the circuit.
Power bypass: Power bypass capacitors are used to stabilize voltage, prevent parasitic oscillations, and provide a stable power source for devices.
Determining the capacitance:
Circuit characteristics: The selection of capacitance should first take into account the basic characteristics and requirements of the circuit, such as power supply filtering, decoupling, high-frequency bypass, etc.
Frequency requirements: The filtering effect and decoupling capability of capacitors vary at different frequencies. Therefore, it is necessary to select an appropriate capacitance based on the operating frequency of the circuit.
Signal quality: The choice of capacitor directly affects the signal transmission quality and circuit stability. To ensure accurate signal transmission, the capacitance needs to be determined based on the signal quality requirements.
Capacitor characteristics: The actual characteristics of a capacitor also affect the choice of capacitance. For example, capacitors with a larger ESR are suitable for board-level filtering, while capacitors with a lower ESR are more suitable for decoupling and high-frequency bypassing.
