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Ask The Applications Engineer–40
In addition, manufacturers specify absolute maximum ratings for every integrated circuit; these ratings must be observed in order to maintain reliable operation and meet published specifications. When absolute maximum ratings are exceeded, operational parameters cannot be guaranteed; and even internal protections against ESD, overvoltage, or latch-up can fail, resulting in device (and potentially further) damage or failure.
This article describes challenges engineers face when designing analog switches and multiplexers into modules used in hostile environments, and provides suggestions for general solutions that circuit designers can use to protect vulnerable parts. It also introduces some new integrated switches and multiplexers that provide increased overvoltage protection, latch-up immunity, and fault protection to deal with common stress conditions.
Analog Switch Architecture
A standard CMOS switch (Figure 1) uses both N- and P-channel MOSFETs for the switch element, digital control logic, and driver circuitry. Connecting N- and P-channel MOSFETs in parallel permits bidirectional operation, allowing the analog input voltage to extend to the supply rails, while maintaining fairly constant on resistance over the signal range.
Figure 1. Standard analog switch circuitry.
The source, drain, and logic terminals include clamping diodes to the supplies to provide ESD protection, as illustrated in Figure 1. Reverse-biased in normal operation, the diodes do not pass current unless the signal exceeds the supply voltage. The diodes vary in size, depending on the process, but they are generally kept small to minimize leakage current in normal operation.
analog switch is controlled as follows: the N-channel device is on
for positive gate-to-source voltages and off for negative gate-to-source
voltages; the P-channel device is switched by the complementary signal,
so it is on at the same time as the
With a fixed voltage on the gate, the effective drive voltage for either transistor varies in proportion to the polarity and magnitude of the analog signal passing through the switch. The dashed lines in Figure 2 show that when the input signal approaches the supplies, the channel of one device or the other will begin to saturate, causing the on resistance of that device to increase sharply. The parallel devices compensate for one another in the vicinity of the rail voltages, however, so the result is a fully rail-to-rail switch, with relatively constant on resistance over the signal range.
Figure 2. Standard analog switch RON graph.
Whenever an analog switch input voltage exceeds the supplies, the internal ESD protection diodes become forward-biased, allowing large currents to flow, even if the supplies are turned off, causing ratings to be exceeded. When forward-biased, the diodes are not rated to pass currents greater than a few tens of milliamperes; they can be damaged if this current is not limited. Furthermore, the damage caused by a fault is not limited to the switch but can also affect downstream circuitry.
The Absolute Maximum Ratings section of a data sheet (Figure 3) describes the maximum stress conditions a device can tolerate; it is important to note that these are stress ratings only. Exposure to absolute maximum ratings conditions for extended periods may affect device reliability. The designer should always follow good engineering practice by building margin into the design. The example here is from a standard switch/multiplexer data sheet.
Figure 3. Absolute Maximum Ratings section of a data sheet.
In this example, the VDD to VSS parameter is rated at 18 V. The rating is determined by the switch’s manufacturing process and design architecture. Any voltage higher than 18 V must be completely isolated from the switch, or the intrinsic breakdown voltages of elements associated with the process will be exceeded, which may damage the device and lead to unreliable operation.
Voltage limitations that apply to the analog switch inputs—with and without power supplies—are often due to the ESD protection circuitry, which may fail as a result of fault conditions.
Figure 4. Analog switch—ESD protection diodes.
Analog and digital input voltage specifications are limited to 0.3 V beyond VDD and VSS, while digital input voltages are limited to 0.3 V beyond VDD and ground. When the analog inputs exceed the supplies, the internal ESD protection diodes become forward-biased and begin to conduct. As stated in the Absolute Maximum Ratings section, overvoltages at IN, S, or D are clamped by internal diodes. While currents exceeding 30 mA can be passed through the internal diodes without any obvious effects, device reliability and lifetime may be reduced, and the effects of electromigration, the gradual displacement of metal atoms in a conductor, may be seen over time. As heavy current flows through a metal path, the moving electrons interact with metal ions in the conductor, forcing atoms to move with the flow of electrons. Over time this can lead to open- or short circuits.
When designing a switch into a system, it is important to consider potential faults that may occur in the system due to component failure, user error, or environmental effects. The next section will discuss how fault conditions that exceed the absolute maximum ratings of a standard analog switch can damage the switch or cause it to malfunction.
Fault Conditions, System Stresses, and Protection Methods
Some stress may not be preventable. Regardless of the source of the stress, the more important issue is how to deal with its effects. The questions and answers below cover these fault conditions: overvoltages, latch-up, and ESD events—and some common methods of protection.
1. Loss of power with signals present on analog inputs (Figure 5).
In some applications, the power supply to a module is lost, while input signals from remote locations may still be present. When power is lost, the power supply rails may go to ground—or one or more may float. If the supplies go to ground, the input signals can forward-bias the internal diode, and current from the switch input will flow to ground—damaging the diode if the current is not limited.
Figure 5. Fault paths.
If loss of power causes the supplies to float, the input signals can power the part through the internal diodes. As a result, the switch—and possibly any other components running from its VDD supply—may be powered up.
2. Overvoltage conditions on analog inputs.
When analog signals exceed the power supplies (VDD and VSS), the supplies can be pulled to within a diode drop of the fault signal. Internal diodes become forward-biased and currents flow from the input signal to the supplies. The overvoltage signal can also pass through the switch and damage parts downstream. The explanation for this can be seen by considering the P-channel FET (Figure 6).
Figure 6. FET switch.
A P-channel FET requires a negative gate-to-source voltage to turn it on. With the switch gate equal to VDD, the gate-to-source voltage is positive, so the switch is off. In an unpowered circuit, with the switch gate at 0 V or where the input signal exceeds VDD, the signal will pass through the switch—as there is now a negative gate-to-source voltage.
3. Bipolar signals applied to a switch powered from a single supply.
This situation is similar to the previously described overvoltage condition. The fault occurs when the input signal goes below ground, causing the diode from the analog input to ground to forward-bias and current to flow. When an ac signal, biased at 0 V dc, is applied to the switch input, the parasitic diodes can be forward-biased for some portion of the negative half-cycle of the input waveform. This happens if the input sine wave goes below approximately –0.6 V, turning the diode on and clipping the input signal, as shown in Figure 7.
Figure 7. Clipping.
the Best Way to Deal with Overvoltage Conditions?
Resistors, to limit current, are placed in series with any switch channel that is exposed to external sources (Figure 8). The resistance must be high enough to limit the current to approximately 30 mA (or as specified by the absolute maximum ratings). The obvious downside is the increase in RON, ∆RON, per channel, and ultimately the overall system error. Also, for applications using multiplexers, faults on the source of an off channel can appear at the drain, creating errors on other channels.
Figure 8. Resistor-diode protection network.
Schottky diodes connected from the analog inputs to the supplies provide protection, but at the expense of leakage and capacitance. The diodes work by preventing the input signal from exceeding the supply voltage by more than 0.3 V to 0.4 V, ensuring that the internal diodes do not forward bias and current does not flow. Diverting the current through the Schottky diodes protects the device, but care must be taken not to overstress the external components.
A third method of protection involves placing blocking diodes in series with the supplies (Figure 9), blocking current flow through the internal diodes. Faults on the inputs cause the supplies to float, and the most positive and negative input signals become the supplies. As long as the supplies do not exceed the absolute maximum ratings of the process, the device should tolerate the fault. The downside to this method is the reduced analog signal range due to the diodes on the supplies. Also, signals applied to the inputs may pass through the device and affect downstream circuitry.
Figure 9. Blocking diodes in series with supplies.
While these protection methods have advantages and disadvantages, they all require external components, extra board area, and additional cost. This can be especially significant in applications with high channel count. To eliminate the need for external protection circuitry, designers should look for integrated protection solutions that can tolerate these faults. Analog Devices offers a number of switch/mux families with integrated protection against power off, overvoltage, and negative signals.
Prepackaged Solutions Are Available?
The ADG4612 family offers power-off protection, overvoltage protection, and negative-signal handling, all conditions a standard CMOS switch cannot handle.
When no power supplies are present, the switch remains in the off condition. The switch inputs present a high impedance, limiting current flow that could damage the switch or downstream circuitry. This is very useful in applications where analog signals may be present at the switch inputs before the power is turned on, or where the user has no control over the power supply sequence. In the off condition, signal levels up to 16 V are blocked. Also, the switch turns off if the analog input signal level exceeds VDD by VT.
Figure 10. ADG4612/ADG4613 switch architecture.
Figure 10 shows a block diagram of the family’s power-off protection architecture. Switch source- and drain inputs are constantly monitored and compared to the supply voltages, VDD and VSS. In normal operation the switch behaves as a standard CMOS switch with full rail-to-rail operation. However, during a fault condition where the source or drain input exceeds a supply by a threshold voltage, internal fault circuitry senses the overvoltage condition and puts the switch in isolation mode.
Analog Devices also offers multiplexers and channel protectors that can tolerate overvoltage conditions of +40 V/–25 V beyond the supplies with power (±15 V) applied to the device, and +55 V/–40 V unpowered. These devices are specifically designed to handle faults caused by power-off conditions.
Figure 11. High-voltage fault-protected switch architecture.
These devices comprise N-channel, P-channel, and N-channel MOSFETs in series, as illustrated in Figure 11. When one of the analog inputs or outputs exceeds the power supplies, one of the MOSFETs switches off, the multiplexer input (or output) appears as an open circuit, and the output is clamped to within the supply rail, thereby preventing the overvoltage from damaging any circuitry following the multiplexer. This protects the multiplexer, the circuitry it drives, and the sensors or signal sources that drive the multiplexer. When the power supplies are lost (through, for example, battery disconnection or power failure) or momentarily disconnected (rack system, for example), all transistors are off and the current is limited to subnanoampere levels. The ADG508F, ADG509F, and ADG528F include 8:1 and differential 4:1 multiplexers with such functionality.
The ADG465 single- and ADG467 octal channel protectors have the same protective architecture as these fault-protected multiplexers, without the switch function. When powered, the channel is always in the on condition, but in the event of a fault, the output is clamped to within the supply voltages.
Figure 12. Parasitic SCR structure: a) device b) equivalent circuit.
The fault and overvoltage conditions described earlier are among the common causes of triggering a latch-up condition. If signals on the analog or digital inputs exceed the supplies, a parasitic transistor is turned on. The collector current of this transistor causes a voltage drop across the base emitter of a second parasitic transistor, which turns the transistor on, and results in a self-sustaining path between the supplies. Figure 12(b) clearly shows the SCR circuit structure formed between Q1 and Q2.
Events need not last long to trigger latch-up. Short-lived transients, spikes, or ESD events may be enough to cause a device to enter a latch-up state.
Latch-up can also occur when the supply voltages are stressed beyond the absolute maximum ratings of the device, causing internal junctions to break down and the SCR to trigger.
The second triggering mechanism occurs if a supply voltage is raised enough to break down an internal junction, injecting current into the SCR.
the Best Way to Deal with Latch-Up Conditions?
Switches with multiple supplies may have additional power-supply sequencing issues that may violate the absolute maximum ratings. Improper supply sequencing can lead to internal diodes turning on and triggering latch-up. External Schottky diodes, connected between supplies, will adequately prevent SCR conduction by ensuring that when multiple supplies are applied to the switch, VDD is always within a diode drop (0.3 V for Schottky) of these supplies, thereby preventing violation of the maximum ratings.
Prepackaged Solutions Are Available?
For applications requiring a latch-up proof solution, new trench-isolated switches and multiplexers guarantee latch-up prevention in high-voltage industrial applications operating at up to ±20 V. The ADG541x and ADG521x families are designed for instrumentation, automotive, avionics, and other harsh environments that are likely to foster latch-up. The process uses an insulating oxide layer (trench) placed between the N-channel and the P-channel transistors of each CMOS switch. The oxide layers, both horizontal and vertical, produce complete isolation between devices. Parasitic junctions between transistors in junction-isolated switches are eliminated, resulting in a completely latch-up proof switch.
Figure 13. Trench isolation in latch-up prevention.
The industry practice is to classify the susceptibility of inputs and outputs to latch-up in terms of the amount of excess current an I/O pin can source or sink in the overvoltage condition before the internal parasitic resistances develop enough voltage drop to sustain the latch-up condition.
A value of 100 mA is generally considered adequate. Devices in the ADG5412 latch-up proof family were stressed to ±500 mA with a 1-ms pulse without failure. Latch-up testing at Analog Devices is performed according to EIA/JEDEC-78 (IC Latch-Up Test).
ICs can be damaged by the high voltages and high peak currents generated by an ESD event. The effects of an ESD event on an analog switch can include reduced reliability over time, the degradation of switch performance, increased channel leakage, or complete device failure.
ESD events can occur at any stage of the life of an IC, from manufacturing through testing, handling, OEM user, and end-user operation. In order to evaluate an IC’s robustness to various ESD events, electrical pulse circuits modeling the following simulated stress environments were identified: human body model (HBM), field-induced charged device model (FICDM), and machine model (MM).
the Best Way to Deal with ESD Events?
Analog switch ESD protection is generally in the form of diodes from the analog and digital inputs to the supplies, as well as power supply protection in the form of diodes between the supplies—as illustrated in Figure 14.
Figure 14. Analog switch ESD protection.
The protection diodes clamp voltage transients and divert current to the supplies. The downside of these protection devices is that they add capacitance and leakage to the signal path in normal operation, which may be undesirable in some applications.
For applications that require greater protection against ESD events, discrete components such as Zener diodes, metal-oxide varistors (MOVs), transient voltage suppressors (TVS), and diodes are commonly used. However, they can lead to signal integrity issues due to the extra capacitance and leakage on the signal line; this means design engineers need to carefully consider the trade-off between performance and reliability.
Prepackaged Solutions Are Available?
Switch/mux products, like devices mentioned here, are available with integrated protection, allowing designers to eliminate external protection circuitry, reducing the number and cost of components in board designs. Savings are even more significant in applications with high channel count.
Ultimately, using switches with fault protection, overvoltage protection, immunity to latch-up, and a high ESD rating yields a robust product that meets industry regulations and enhances customer and end-user satisfaction.
High-Voltage Latch-Up Proof Switches
High-Voltage Latch-Up Proof Multiplexer
Low-Voltage Fault-Protected Multiplexers
High-Voltage Fault-Protected Multiplexers
High-Voltage Channel Protectors
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