“The internal ESD diodes of front-end amplifiers are sometimes used to clamp overvoltage conditions, but many factors need to be considered to ensure that these clamps can provide adequate and strong protection. Understanding the various ESD diode architectures inside the front-end amplifier, as well as understanding the thermal and electromigration effects of a given protection circuit, can help designers avoid problems with their protection circuits and increase their service life in field applications.
In many applications where the input is not controlled by the system but is connected to the outside world, such as test equipment, instrumentation and some sensing equipment, the input voltage may exceed the ZD rated voltage of the front-end amplifier. In these applications, protection schemes must be implemented to maintain the viability and robustness of the design.
The internal ESD diodes of front-end amplifiers are sometimes used to clamp overvoltage conditions, but many factors need to be considered to ensure that these clamps can provide adequate and strong protection. Understanding the various ESD diode architectures inside the front-end amplifier, as well as understanding the thermal and electromigration effects of a given protection circuit, can help designers avoid problems with their protection circuits and increase their service life in field applications.
ESD diode configuration
It is important to understand that not all ESD diodes are simple diode clamps connected to power and ground. Many possible implementations can be used, such as multiple diodes in series, diodes and resistors, and back-to-back diodes. Some of the more common implementations are detailed below.
The diode is connected to the power supply
Figure 1 shows an example of an amplifier with a diode connected between the input pin and the power supply. The diode is reverse-biased under normal operating conditions, but becomes forward-biased when the input rises above the positive supply voltage or below the negative supply voltage. When the diode is forward biased, current flows through the input of the amplifier to the corresponding power supply.
In the case of the circuit in Figure 1, when the overvoltage exceeds Vs, the input current itself will not be limited by the amplifier itself, and an external current limit in the form of a series resistor is required. When the voltage is lower than CVs, a 400Ω resistor will provide some current limit, which should be taken into account in any design considerations.
Figure 1: Input ESD topology of AD8221
Figure 2 shows an amplifier with a diode-like configuration, but in this case, the current is limited by the internal 2.2kΩ series resistance. This is different from the circuit shown in Figure 1 not only in limiting the value of R, but also in that 2.2kΩ prevents the voltage from being higher than Vs. This is an example of the complexity that must be fully understood when using ESD diodes to optimize protection.
Figure 2: Input ESD topology of AD8250
Current limiting JFET
Compared with the implementation in Figure 1 and Figure 2, the current-limiting JFET can be used as an alternative to diode clamping in IC design. Figure 3 shows an example where JFET is used to protect the device when the input voltage exceeds the specified operating range of the device. The device obtains inherent protection of up to 40V from the opposite power rail through the JFET input. Because the JFET limits the current into the input pins, the ESD unit cannot be used as an additional overvoltage protection.
When voltage protection up to 40V is required, the device’s JFET protection provides a well-controlled, reliable, and fully specified protection option. This is often in contrast to the use of ESD diodes for protection, where information about diode current limits is usually specified as typical information, or may not be specified at all.
Figure 3: Input protection scheme of AD8226
In applications that allow the input voltage to exceed the supply voltage or ground, a set of diodes can be used to protect the input from ESD events. Figure 4 shows an amplifier implementing a stacked diode protection scheme. In this configuration, the diode string is used to prevent negative transients. The diode string is used to limit the leakage current within the usable input range, but to provide protection when the negative common mode range is exceeded. Remember, the current limit of WY is the equivalent series resistance of the diode string. External series resistors can be used to reduce the input current for a given voltage level.
Figure 4: Low-side input protection scheme of AD8417
When the input voltage range is allowed to exceed the power supply, back-to-back diodes are also used. Figure 4 shows an amplifier that uses back-to-back diodes to provide ESD protection for the device, which allows the use of a 3.3V power supply to provide voltages up to 70V. D4 and D5 are high voltage diodes, used to isolate the high voltage that may exist on the input pins, and D1 and D2 are used to prevent leakage current when the input voltage is within the normal operating range. In this configuration, it is not recommended to use these ESD units for overvoltage protection, because exceeding the ZD reverse bias of the high voltage diode can easily cause YJ damage.
No ESD clamp
Some devices do not include ESD devices on the front end. Although it is obvious that if the ESD diode does not exist, the designer cannot use the ESD diode for clamping, but when studying the over-voltage protection (OVP) option, it is mentioned that this architecture is a situation that needs attention.Figure 6 shows a device that uses only large value resistors to protect the amplifier
ESD unit as a fixture
In addition to understanding how to implement ESD cells, it is also important to understand how to use structures for protection. In typical applications, series resistors are used to limit the current within a specified voltage range.
When the amplifier is configured as shown in Figure 7 or the input is protected by a power diode, the input current will be limited using the formula in the following formula.
The assumption used in Equation 1 is Vstress>Vsupply. If this is not the case, you should measure a more JQ diode voltage and use it in the calculation instead of the 0.7V approximation.
Below is a calculation example to protect an amplifier using a /-15V power supply from input stress up to /-120V, while limiting the input current to 1mA. Using Equation 1, we can use these inputs to calculate the following.
In view of these requirements, Rprotection >105 kΩ will limit the diode current to understand the current limit
The ZD value of Idiode will vary from part to part and also depends on the specific application scenario where the stress is applied. For YC events that last a few milliseconds, the ZD current will be different from the situation where current is continuously applied throughout the life cycle of the application’s mission profile for more than 20 years. Guidance on specific values can be found in the JDZD value section or in the amplifier data sheet of the application note, usually in the range of 1mA -10mA.
The ZD rated current Z of a given protection scheme will eventually be limited by two factors, the thermal effect of the power dissipated in the diode and the ZD rated current of the current path. The power consumption should be kept below a threshold to keep the operating temperature within the effective range, and the current should be selected within the specified ZD value to avoid reliability problems due to electromigration.
When current flows into the ESD diode, the temperature rises due to the power dissipation in the diode. Most amplifier data sheets specify a thermal resistance (usually designated? JA), which will indicate how the junction temperature increases with power dissipation. Considering the application temperature under the Z-bad situation, and the temperature increase in the Z-bad situation due to power consumption, will show the feasibility of the protection circuit.
Even if the current does not cause thermal problems, the diode current can still cause reliability problems. Due to electromigration, any electrical signal path has a ZD lifetime current rating. The electromigration current limit of the diode current path is usually limited by the thickness of the internal trace in series with the diode. This information is not always released for amplifiers, but if the diode is active for a long time, rather than a transient event, it needs to be considered.
An example where electromigration can be a problem is when the amplifier is monitoring and therefore connected to a voltage rail independent of its own power supply rail. When there are multiple power domains, power sequencing may cause the voltage to temporarily exceed the JDZD condition. By considering the current path under Z bad conditions, the duration that the current may be active during the entire life cycle, and understanding the ZD allowable current of electromigration, reliability problems caused by electromigration can be avoided.
Understanding how the amplifier’s internal ESD diode is activated during an electrical overload event can simply improve the robustness of the design. Examining the thermal and electromigration effects of the protection circuit can highlight potential problems and point out areas where additional protection may be needed. Considering the conditions listed here, designers can make wise choices and avoid potential robustness issues in the field.
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