Project of using ESD diodes as voltage limiters in input amplifiers
In many applications where input is not controlled by system but connected to outside world, such as test equipment, instrumentation, and some sensor equipment, input voltage may exceed input amplifier's maximum voltage rating. . In these applications, protection schemes must be implemented to ensure survivability and reliability of design.
The input amplifier's internal anti-static diodes are sometimes used to limit surge conditions, but many factors must be considered to ensure that these clamps provide adequate and reliable protection. Understanding various architectures of ESD diodes in input amplifiers, as well as understanding thermal and electromigration effects of a given protection circuit, can help designers avoid problems with their protection circuits and increase their lifespan in field.
ESD diode configuration
It is important to understand that not all ESD diodes are simple clamp diodes connected to power and ground. Various possible implementations can be used, such as multiple diodes in series, diodes and resistors, and back-to-back diodes. Some of more common implementations are detailed below.
Diode connected to power supply
In fig. Figure 1 shows an example of an amplifier with a diode connected between input pin and power supply. The diode is reverse biased under normal operating conditions, but becomes forward biased when input voltage goes above positive supply voltage or below negative supply voltage. When diode is forward biased, current flows through input of amplifier to corresponding power supply.
In case of circuit in Figure 1, when overvoltage exceeds +Vs, input current itself is not limited by amplifier itself and requires an external current limit in form of a series resistor. The 400 ohm resistor provides some current limiting below –Vs and should be considered in design.
Figure 1. AD8221 inlet ESD topology
In fig. Figure 2 shows an amplifier with a similar diode configuration, but in this case current is limited by an internal 2.2 kΩ series resistor. This differs from circuit shown in Figure 1 not only by limiting value of R, but also by fact that 2.2 kΩ resistance prevents voltage from exceeding +Vs. This is an example of complexities that must be fully understood in order to optimize protection when using ESD diodes.
Figure 2. AD8250 ESD input topology
JFET with current limit
Compared to implementations in fig. 1 and fig. 2, Current limiting JFET can be used as an alternative to clamp diodes in IC designs. On fig. Figure 3 shows an example of using a JFET to protect a device when input voltage exceeds device's specified operating range. The device is initially protected up to 40V from opposite power rail via JFET input. Since JFET is current limiting on input pin, ESD cell cannot be used as an additional overvoltage protection.
When voltage protection up to 40V is required, FET device protection provides well controlled, reliable and fully specified protection. This is usually different from use of ESD diodes for protection, where diode current limiting information is often listed as typical or may not be listed at all.
Figure 3: AD8226 input protection circuit
In applications where input voltage may exceed supply or ground voltage, a diode array can be used to protect input from ESD. On fig. 4 shows an amplifier implementing a multilayer diode protection circuit. In this configuration, a diode string is used for negative transient protection. A string of diodes is used to limit leakage current within usable input range, but provides protection beyond negative common mode range. Remember that only current limit is equivalent series resistance of diode string. An external series resistor can be used to reduce input current at a given voltage level.
Figure 4: AD8417 low-side input protection circuit
Anti-parallel diodes are also used when input voltage range may exceed supply voltage. On fig. Figure 4 shows an amplifier that uses back-to-back diodes to provide ESD protection for a device capable of up to 70V from a 3.3V supply. D4 and D5 are high voltage diodes to isolate possible high voltages on input pins, and D1 and D2 are used to prevent leakage current when input voltage is within normal operating range. In this configuration, it is not recommended to use these ESD elements for surge protection, as exceeding maximum reverse bias voltage of high voltage diode can easily cause permanent damage.
Figure 5: AD8418 high-side input protection circuit
Without ESD clamp
Some devices do not have ESD devices on external interface. While it is clear that designers cannot use ESD diodes to clip if they are not present, mention of this architecture is a case of caution when researching overvoltage protection (OVP) options. On fig. Figure 6 shows a device that uses only high value resistors to protect amplifier.
Figure 6: AD8479 input protection circuit
ESD unit as a fixture
In addition to understanding how to implement an ESD cell, it's also important to understand how to use structures for protection. A typical application uses a series resistor to limit current within a specified voltage range.
If amplifier is configured as shown in fig. 7, or input is protected by supply diodes, input current is limited by following equation.
Figure 7. Using an ESD cell as a mount
Equation 1 uses assumption: Vstress>Vsupply. If this is not case, a more accurate diode voltage should be measured and used in calculations instead of approximation of 0.7 V.
The following is an example of a protection calculation for an amplifier using +/-15V power, from input voltage to +/-120V, with input current limited to 1mA. Using Equation 1, we can use these inputs to compute following.
Given these requirements, R>105kΩ protection will limit diode current to <1mA.
Learn about current restrictions
The maximum Idiode value will vary from part to part and also depends on specific application in which load is being applied. For a one-time event lasting a few milliseconds, maximum current will be different than if current is applied continuously for entire 20-year application mission profile life cycle. Recommendations for specific values can be found in "Absolute Maximums" section or in amplifier data sheet in application notes, typically in 1-10 mA range.
The maximum current rating for a given protection circuit will ultimately be limited by two factors: thermal effects of power dissipated in diode and maximum current path rating. The power dissipation must be kept below a threshold value to keep operating temperature within allowable range, and current must be chosen within specified maximum value to avoid reliability issues due to electromigration.
When current flows through ESD diode, temperature rises due to power dissipation in diode. Most amplifier datasheets list thermal resistance (commonly referred to as Ө JA), which indicates how much junction temperature increases as power is dissipated. Considering worst case temperature of application and worst case temperature rise due to power dissipation, viability of protection circuit can be determined.
Even if current does not cause thermal problems, diode current can still create reliability problems. Due to electromigration, any electrical signal path has a maximum current rating over its lifetime. The electromigration current limit of a diode's current path is usually limited by thickness of internal traces in series with diode. This information is not always provided for amplifiers, but it should be taken into account if diode is active for a long period of time and not in transient mode.
An example where electromigration can be a problem is when an amplifier is controlled and thus connected to a voltage rail that is separate from its own power rail. In presence of multiple power domains, power sequence can cause voltages to temporarily exceed absolute maximum conditions. Reliability issues due to electromigration can be avoided by considering worst-case current path, duration that this current can be active over its lifetime, and knowing maximum allowable current for electromigration.
Knowing how amplifier's internal anti-static diodes are activated during electrical overload caneasy to improve reliability of structure. Examining protection circuit for thermal and electromigration effects can reveal potential problems and indicate where additional protection may be needed. Considering conditions listed here, designers can make informed choices and avoid potential reliability issues in field.
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