Do you know how to avoid electrical stress in chip design? When chip designers take a sensitive pin of an op amp out of the chip, they usually think about whether the user will take this pin seriously? Or just carelessly connect this pin directly to AC power Connected? We all want to design good products that can handle extreme user usage. So, how to prevent product failure caused by over-electrical stress in the design?
OPA320 is one of most typical operational amplifiers, and its maximum rated parameter table is shown in Figure 1, which describes the chip’s maximum allowable power supply voltage, and the maximum allowable input voltage and current of the pins. According to the additional note of the parameter table, if the pin input current is limited, then the input voltage does not need to be limited. Internal clamp diodes allow ±10mA input current. But in the case of input voltages that are much higher than normal, limiting the input current requires a larger input impedance, which increases noise, reduces bandwidth, and may produce other errors.
The clamping diodes begin to conduct when the input voltage exceeds the supply rails by approximately 0.6V. Typically, many devices can withstand higher currents, but when the voltage increases dramatically, the probability of device failure increases.
By adding an external diode, the ability of the device to withstand large currents can be greatly improved, and the protection level of the device can also be improved. Common transmit-signal diodes on the market, such as the ubiquitous 1N4148, have a very low on-voltage drop (lab tests show at least 100mV lower than the op amp’s internal diode). In parallel with the internal diodes of the op amp, most of the current will flow to the external diodes when input overcurrent is encountered.
Schottky diodes have lower turn-on voltages, a feature that improves protection. But the disadvantage is also obvious, its leakage current is too large. At room temperature, its reverse leakage current is usually in the order of microamps or more, while increasing with temperature.
In addition, you also need a powerful enough power supply. Clamping diodes, whether internal or external to the op amp, require a relatively stable power supply to release energy. If the fault pulse is large, sinking too much current into the power rails and raising (or pulling down the negative supply) the supply voltage, the pulse will put excessive voltage stress on the supply terminals, as shown in Figure 2. A typical linear power supply cannot sink current, so don’t expect how stable it is to use it as a power supply. Large bypass capacitors can be used to absorb large fault pulse currents. For continuous fault currents, zener diodes can be added to the input pins and power supply. The reverse breakdown voltage of the Zener diode should be just above the maximum supply voltage of the system, so that the Zener diode will be turned on only in the event of a fault. For positive and negative power supply systems, the same protection circuits need to be designed on both power rails.
Despite these measures, the pin input voltage may still exceed the value in the maximum ratings table, but the crux of the matter is: the values in the maximum ratings table are often too conservative; chip damage is almost impossible at this voltage or current . In general, it is unlikely (but not guaranteed) that the device will be damaged by substantially exceeding these parameters. It is easy to clamp to a few volts higher than the value in the Maximum Ratings table while obtaining a low failure rate. In many cases, the goal of the design is to reduce the failure rate at a cost and performance tradeoff.
There is no one solution for all situations, and no one protection circuit for all needs at the same time. Protection circuit schemes vary widely in different applications. The sensitivity of different op amps is different, and the required protection level is also very different. This may require some creativity on your part, and it’s best to be your own expert. While doing some testing in extreme environments will lose some op amps, it is necessary.
Do you know how to avoid electrical stress in chip design? When chip designers take a sensitive pin of an op amp out of the chip, they usually think about whether the user will take this pin seriously? Or just carelessly connect this pin directly to AC power Connected? We all want to design good products that can handle extreme user usage. So, how to prevent product failure caused by over-electrical stress in the design?
OPA320 is one of most typical operational amplifiers, and its maximum rated parameter table is shown in Figure 1, which describes the chip’s maximum allowable power supply voltage, and the maximum allowable input voltage and current of the pins. According to the additional note of the parameter table, if the pin input current is limited, then the input voltage does not need to be limited. Internal clamp diodes allow ±10mA input current. But in the case of input voltages that are much higher than normal, limiting the input current requires a larger input impedance, which increases noise, reduces bandwidth, and may produce other errors.
The clamping diodes begin to conduct when the input voltage exceeds the supply rails by approximately 0.6V. Typically, many devices can withstand higher currents, but when the voltage increases dramatically, the probability of device failure increases.
By adding an external diode, the ability of the device to withstand large currents can be greatly improved, and the protection level of the device can also be improved. Common transmit-signal diodes on the market, such as the ubiquitous 1N4148, have a very low on-voltage drop (lab tests show at least 100mV lower than the op amp’s internal diode). In parallel with the internal diodes of the op amp, most of the current will flow to the external diodes when input overcurrent is encountered.
Schottky diodes have lower turn-on voltages, a feature that improves protection. But the disadvantage is also obvious, its leakage current is too large. At room temperature, its reverse leakage current is usually in the order of microamps or more, while increasing with temperature.
In addition, you also need a powerful enough power supply. Clamping diodes, whether internal or external to the op amp, require a relatively stable power supply to release energy. If the fault pulse is large, sinking too much current into the power rails and raising (or pulling down the negative supply) the supply voltage, the pulse will put excessive voltage stress on the supply terminals, as shown in Figure 2. A typical linear power supply cannot sink current, so don’t expect how stable it is to use it as a power supply. Large bypass capacitors can be used to absorb large fault pulse currents. For continuous fault currents, zener diodes can be added to the input pins and power supply. The reverse breakdown voltage of the Zener diode should be just above the maximum supply voltage of the system, so that the Zener diode will be turned on only in the event of a fault. For positive and negative power supply systems, the same protection circuits need to be designed on both power rails.
Despite these measures, the pin input voltage may still exceed the value in the maximum ratings table, but the crux of the matter is: the values in the maximum ratings table are often too conservative; chip damage is almost impossible at this voltage or current . In general, it is unlikely (but not guaranteed) that the device will be damaged by substantially exceeding these parameters. It is easy to clamp to a few volts higher than the value in the Maximum Ratings table while obtaining a low failure rate. In many cases, the goal of the design is to reduce the failure rate at a cost and performance tradeoff.
There is no one solution for all situations, and no one protection circuit for all needs at the same time. Protection circuit schemes vary widely in different applications. The sensitivity of different op amps is different, and the required protection level is also very different. This may require some creativity on your part, and it’s best to be your own expert. While doing some testing in extreme environments will lose some op amps, it is necessary.
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