On the premise of meeting the functional requirements of the circuit, components with high voltage sensitivity thresholds should be selected whenever possible. This is because the electrostatic discharge (ESD) sensitivity of a PCB is determined by the component with the lowest voltage threshold in the circuit. In addition to making the correct component selection, it’s important to note that the voltage sensitivity thresholds of components with the same function and model may vary significantly depending on the PCB manufacturer. Even components from the same manufacturer, but from different production batches, could exhibit different thresholds.
1: Limiting the output current can prevent the lock-in effect in CMOS circuits
The lock-in effect is a specific failure mode that occurs in CMOS circuits. This happens due to the presence of parasitic PNP and NPN transistors within the internal structure of the CMOS circuit. These parasitic transistors form a parasitic PNPN thyristor structure, which can lead to the lock-in effect in CMOS circuits under certain conditions.
1. **Thyristor Effect**
Also referred to as the thyristor effect, this type of interlocking positive feedback mechanism can be triggered by external factors, such as electrostatic discharge. This causes current to flow through the PNP transistor (or NPN transistor), which then passes through another parasitic NPN transistor (or PNP transistor), amplifying the current. Due to the positive feedback, the current increases progressively, potentially leading to a failure. It is clear that limiting the current to prevent it from reaching the threshold that sustains the locked state is a key design consideration for CMOS devices in printed circuit boards. A common solution involves isolating each output terminal from its associated trace using a resistor, and employing two high-speed switching diodes to clamp the line to VDD (drain power) and VSS (source power).
2. **Using a Filter Network**
In some cases, a long input cable is required between the CMOS circuit system and mechanical contacts, which increases the likelihood of electromagnetic interference (EMI). This calls for the integration of a filter network. Additionally, long input lines come with larger distributed capacitance and inductance, which can easily form an LC self-oscillation circuit. This is especially problematic when there is a negative oscillatory voltage at the input terminal, as it may cause the protection diode in the network to burn out. To mitigate this, a resistor can be placed in series at the input. The resistance value can be determined using the formula ( R = frac{V_{DD}}{1mA} ). For instance, when ( V_{DD} = 10V ), the required resistance would be 10 kΩ.
3. **RC Network**
Where applicable, an RC network composed of a higher-value resistor and a capacitor of at least 100pF can be used to mitigate the effects of electrostatic discharge on sensitive input terminals of bipolar devices. If the circuit design necessitates it, two parallel diodes that clamp to 0.5V in either polarity can be used to short the input to ground, further reducing interference and protecting the input characteristics.
4. **Avoid Floating Input Pins on CMOS Devices**
It is essential to avoid leaving the input terminals of CMOS devices floating after they have been soldered to the circuit board. Additionally, unused or redundant input leads on the CMOS device should never be left floating. This is because a floating input terminal results in an unstable input potential, which can disrupt the normal logical behavior of the circuit and make it susceptible to electrostatic breakdown and external noise. Acoustic interference and other undesirable phenomena may also occur. Redundant input terminals should be handled appropriately based on the circuit’s function. For example, the unused inputs of AND and NAND gates should be tied to VDD (high level), while those of OR and NOR gates should be connected to VSS (low level).
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1: Limiting the output current can prevent the lock-in effect in CMOS circuits
The lock-in effect is a specific failure mode that occurs in CMOS circuits. This happens due to the presence of parasitic PNP and NPN transistors within the internal structure of the CMOS circuit. These parasitic transistors form a parasitic PNPN thyristor structure, which can lead to the lock-in effect in CMOS circuits under certain conditions.
1. **Thyristor Effect**
Also referred to as the thyristor effect, this type of interlocking positive feedback mechanism can be triggered by external factors, such as electrostatic discharge. This causes current to flow through the PNP transistor (or NPN transistor), which then passes through another parasitic NPN transistor (or PNP transistor), amplifying the current. Due to the positive feedback, the current increases progressively, potentially leading to a failure. It is clear that limiting the current to prevent it from reaching the threshold that sustains the locked state is a key design consideration for CMOS devices in printed circuit boards. A common solution involves isolating each output terminal from its associated trace using a resistor, and employing two high-speed switching diodes to clamp the line to VDD (drain power) and VSS (source power).
2. **Using a Filter Network**
In some cases, a long input cable is required between the CMOS circuit system and mechanical contacts, which increases the likelihood of electromagnetic interference (EMI). This calls for the integration of a filter network. Additionally, long input lines come with larger distributed capacitance and inductance, which can easily form an LC self-oscillation circuit. This is especially problematic when there is a negative oscillatory voltage at the input terminal, as it may cause the protection diode in the network to burn out. To mitigate this, a resistor can be placed in series at the input. The resistance value can be determined using the formula ( R = frac{V_{DD}}{1mA} ). For instance, when ( V_{DD} = 10V ), the required resistance would be 10 kΩ.
3. **RC Network**
Where applicable, an RC network composed of a higher-value resistor and a capacitor of at least 100pF can be used to mitigate the effects of electrostatic discharge on sensitive input terminals of bipolar devices. If the circuit design necessitates it, two parallel diodes that clamp to 0.5V in either polarity can be used to short the input to ground, further reducing interference and protecting the input characteristics.
4. **Avoid Floating Input Pins on CMOS Devices**
It is essential to avoid leaving the input terminals of CMOS devices floating after they have been soldered to the circuit board. Additionally, unused or redundant input leads on the CMOS device should never be left floating. This is because a floating input terminal results in an unstable input potential, which can disrupt the normal logical behavior of the circuit and make it susceptible to electrostatic breakdown and external noise. Acoustic interference and other undesirable phenomena may also occur. Redundant input terminals should be handled appropriately based on the circuit’s function. For example, the unused inputs of AND and NAND gates should be tied to VDD (high level), while those of OR and NOR gates should be connected to VSS (low level).
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
