How to Prevent Product Failure Caused by Over Electric Stress in Design

When chip designers lead a sensitive pin of an operational amplifier out of the chip, they usually think whether the user will seriously deal with this pin? Or just carelessly connect this pin directly to AC? We all hope to design good products that can cope with the extreme use of users. So, how to prevent product failure caused by over electric stress in design?

Opa320 is one of most typical operational amplifiers. Its maximum rated parameter table is shown in Figure 1. It describes the maximum allowable supply voltage of the chip, the maximum allowable input voltage and current of the pin. According to the additional instructions in the parameter table, if the pin input current is limited, the input voltage does not need to be limited. The internal clamp diode allows an input current of ± 10mA. However, when the input voltage exceeds the normal value a lot, limiting the input current requires a large input impedance, which will increase the noise, reduce the bandwidth, and may produce other errors.

The clamp diode begins to turn on when the input voltage exceeds the power rail by about 0.6V. Generally, many equipment can withstand large current, but when the voltage increases sharply, the probability of equipment failure will increase.

By adding external diodes, the ability of the equipment to withstand large current can be greatly improved, and the protection level of the equipment can also be improved. Common signal transmission diodes on the market, such as the ubiquitous 1N4148, have a very low on voltage drop (laboratory tests show that it is at least 100mV lower than the internal diode of the operational amplifier). After being connected in parallel with the internal diode of the operational amplifier, when the input overcurrent is encountered, most of the current will flow to the external diode.

Schottky diode has lower on voltage, which can improve the protection performance. But the disadvantage is also obvious. Its leakage current is too large. At room temperature, its reverse leakage current is usually microampere or greater, and increases with the increase of temperature.

In addition, you also need a strong enough power supply. Clamp diodes, whether inside or outside the operational amplifier, need a relatively stable power supply to release energy. If the fault pulse is large, inject too much current into the power rail and increase (or pull down the negative power supply) the power supply voltage, the pulse will make the power supply terminal bear excessive voltage stress, as shown in Figure 2. A typical linear power supply cannot absorb current, so don't expect it to be more stable as a power supply. Large bypass capacitance can be used to absorb large fault pulse current. For continuous fault current, zener diode can be added to the input pin and power supply. The reverse breakdown voltage of Zener diode should be just higher than the maximum supply voltage of the system, so that the zener diode will be turned on only in case of fault. For the positive and negative power supply system, the same protection circuit needs to be designed on the two power rails.

Despite these measures, the pin input voltage may still exceed the value in the maximum rated parameter table, but the key problem is that the value in the maximum rated parameter table is usually too conservative; At this voltage or current, chip damage is almost impossible. Generally speaking, if these parameters are greatly exceeded, the device is unlikely to be damaged (but not guaranteed). It is easy to clamp to a voltage several volts higher than the value in the maximum rating table while obtaining a lower failure rate. In many cases, the goal of the design is to reduce inefficiency in the case of a compromise between cost and performance.

No scheme can deal with all situations, and no protection circuit can meet all requirements at the same time. In different applications, the protection circuit schemes vary greatly. The sensitivity of different operational amplifiers is different, and the required protection level is also very different. This may require you to be creative. You'd better be your own expert. Although some tests in extreme environments will lose some operational amplifiers, this is necessary.

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