Low voltage fault protection

In existing designs, most systems operate from standard supply voltages (unipolar 3.3V or 5V, or bipolar ±3.3V or ±5V), which is usually the highest voltage on the board.

In actual use, the input terminals of the circuit board may be exposed to a voltage higher than the power supply voltage. At the same time, after the circuit board power is cut off, the voltage on the input terminals may still exist. Often the first component affected by this overvoltage is a multiplexer or switch, which requires appropriate protection for the switching components and downstream circuitry.

The channel components within an analog switch typically include one or more MOSFETs, as well as parasitic clamping diodes (clamped to the supply voltage) for ESD protection. Figure 1 shows the equivalent circuit diagram of a closed analog switch. As long as V+ and V- are present, and the input voltage does not exceed: supply voltage + forward bias voltage of the clamping diode (typically 0.6V), the diode is reverse biased and no current flows through it.

Improper power supply sequencing can cause overvoltage faults, with many switches requiring the largest "positive" voltage to be turned on first and the lowest "negative" voltage last. It should be noted that when there is an input voltage when the power is turned off or when the input voltage exceeds the supply voltage, there will be current flowing through the clamping diode. These diodes have only a few milliwatts of power capability (depending on the IC's semiconductor process) and will permanently damage the switch when the heat generated by power dissipation exceeds a certain tolerance.

Lower currents can also cause latch-up, a fault condition that causes the switch to fail and draw excessive current from the power supply. In most cases you can remove the latch by simply removing all voltage from the switch without damaging the switch, but the entire board will not function properly until then.

Figure 1. Equivalent circuit of closed switch

　　external protection

A simple way to prevent the analog switch from going into lockup is to add a high-current Schottky diode (Figure 2) with a low forward bias (0.3V maximum). If the input voltage exceeds the supply voltage, the Schottky's low bias ensures that no current flows through the clamping diode, which has a typical forward bias of 0.6V.

Figure 2. Using an external Schottky diode to prevent latch-up

However, this latch-free circuit still has drawbacks, and not just because of the extra cost of the two protection diodes. Schottky diodes will pass any voltage more than 0.3V above the supply voltage. For devices connected to V+, V-, there is no problem when the power supply is not turned on (V+ and V- are at ground level) and the input voltage is always below the limit of each device connected to the power line.

However, this circuit does not provide overvoltage protection. For example, if V+ = 5V and the fault voltage at the switch input is 8V, V+ will be pulled up to nearly 7.7V - which is too high for most digital devices connected to V+. Even when the only thing on V+ is the switch itself, and the switch can withstand such a fault voltage, such high voltages can still endanger downstream devices through the closed switch. In addition, switches with multiple inputs require a Schottky diode connected to V+ at each input, which adds a lot of cost and board space.

The circuit in Figure 3 provides a better overvoltage protection scheme for applications where there will never be an input voltage until the switch is powered on. The forward bias voltage VD of a conventional silicon diode is generally 0.7V, so when selecting the zener tube breakdown voltage Vz1, it must satisfy VD + Vz1 < V+. The same is true for negative protection and Vz2: D + Vz2 < V-. The highest voltage rating of the diodes (zener tubes and standard silicon diodes) must be selected according to the highest possible fault voltage.

Figure 3. Overvoltage protection using external diodes

For a sustained overvoltage fault (rather than a glitch), a resistor must be connected between ground and the Zener diode to limit the current through the diode. The biggest disadvantage of this protection is that it limits the input voltage range of the switch. Since the bias voltage of the diode varies greatly, the min/max limits of the diode network will also vary greatly. If the network is designed based on the worst-case scenario, it is possible that the diode will begin to conduct at a voltage much lower than the supply voltage, thus causing the switch to lose its rail-to-rail characteristics.

Limiting the current flowing through the clamping diode in the switch by adding a series resistor (k stage) in the input channel can also provide some degree of protection. However, overvoltage can still threaten devices downstream of the switch. The series resistor significantly increases the channel resistance when the open tube is conducting. This resistance will introduce errors into the signal as it changes with temperature because leakage current from the switch will flow through this increased on-resistance.

　　internal protection

The method of integrating fault protection inside the analog switch was first used in a certain type of multiplexer, whose channel element contained three MOSFETs in series, sequentially n-channel, p-channel and n-channel. This structure can provide ±100V protection for each signal channel (Figure 4). As the input voltage approaches and exceeds the supply voltage, the on-resistance of the multiplexer increases rapidly, limiting the input current and protecting the multiplexer (and the devices before and after the multiplexer). Limiting the fault current also blocks the coupling of faults to other channels.

Figure 4. On-resistance of early fault protection switch as a function of signal voltage

The series MOSFET approach also provides protection in the absence of power. Early devices, such as MAX388 or HI509A, only operate from ±4.5V to ±18V, have larger packages, higher on-resistance (minimum 350, up to 3.5k), and can only pass comparison

The supply voltage is approximately 2V lower than the input signal voltage.

For devices operating in the 9V to 36V or ±4.5 to ±20V range, the first step in solving these problems is to develop a new switch structure, similar to the low-voltage fault protection scheme discussed below. Compared with three-FET series technology, the most prominent advantage of the new solution is that it allows rail-to-rail operation and lower on-resistance. When the internal circuit detects a fault, it automatically cuts off the switch to prevent the fault from passing through the switch or multiplexer and reaching other circuits.

Under fault conditions, since only a small leakage current flows into the switch or multiplexer, the switch will not be damaged by power dissipation. Like the earlier 3-FET solution, the switch/multiplexer based on this new process and structure will return to a high-impedance state in the event of a power outage, thus eliminating the problem of failure in the event of a power outage. This type of device (including the MAX4511 switch and MAX4508 multiplexer families) is suitable for high-voltage systems requiring ±40V fault protection, but is not suitable for common 3V and 5V systems. These devices have no specified characteristics in the low voltage range, and their Rds(on) at a 5V supply can be as high as several thousand ohms.

　　Low voltage fault protection

The newest members of the family of fault-protected switches are optimized to operate on unipolar 3.3V or 5V supplies, or bipolar (±3.3V or ±5V supplies). They require no external protection and feature up to 30 (±5V supplies) or Low on-resistance of 100 (+3V supply).

As shown in Figure 5, these switches consist of an n-channel FET (N1) and a p-channel FET (P1) connected in parallel to create a low-impedance input-to-output signal path. As long as the input signal is within the power supply range, or does not exceed 150mV beyond the power supply, it can reach the COM terminal through the switch, thus allowing the switch to operate at full rail.

Figure 5. Internal block diagram of low voltage fault protection switch

Two comparators inside the switch monitor the input voltage, which they compare with the supply voltages V+ and V-. When the signal on the NO (normally open) terminal or NC (normally closed) terminal is between V+ and V-, the switch operates normally. When the signal voltage exceeds the supply by approximately 150mV (fault condition), the output voltage (COM) is limited to the supply voltage - maintaining the same polarity and the input being high impedance. This is achieved under the control of a fault comparator, which switches off N1 and P1 in the event of a fault. The fault comparator also controls the clamping FETs (N2 and P2) according to the following rules: If a negative polarity fault occurs when the switch is closed, connect N2 to COM to V-. If a positive polarity fault occurs when the switch is closed, turn on P2 and connect COM to V+. If a fault occurs when the switch is open, the output will appear high impedance.

During a fault, the input always presents a high impedance, regardless of the switching state and load impedance. The maximum input fault voltage is limited by the switching element limit, which is ±12V for the MAX4711 series. For example, if the MAX4711 operates from a 5V supply, the maximum fault voltage it can withstand on the positive terminal is +12V, and on the negative terminal it is -7V (5V + -7V = 12V). The device is able to provide fault protection for the input pins (NO and NC) in the absence of supply voltage, and even provides more reliable protection during power outages, in which case fault voltages can approach ±12V.

The overvoltage protection of the logic input terminal (IN) can reach up to (V+)+12V in the forward direction, but can only exceed the negative power supply by one diode voltage drop in the negative direction. The output terminal (COM) is not protected. As mentioned above, the COM voltage should not exceed the supply voltage at either terminal by more than 0.3V.

Figure 6 shows the output of a closed, fault-protected switch during input fault voltages in both directions. Typically, after the input voltage is 150mV above V+ (or V-) for about 200ns, the output (COM) will equal the positive (or negative) supply voltage minus the voltage drop of one FET. When the input voltage returns to within the supply range, another 700ns (typ) delay is required before the output can recover and follow the input. This delay is related to the resistance and capacitance of the COM output terminal and has nothing to do with the amplitude of the fault voltage. The larger the resistance and capacitance at the COM terminal, the longer the recovery time.

Figure 6. Input and output voltages during fault conditions

　　application

In addition to typical applications such as protection of analog inputs in ATE and industrial equipment, these low-voltage, fault-protected switches can be used to simplify design and solve board space constraints in many other applications.

For example, many applications require the ability to plug expansion cards into a powered backplane in order to avoid cutting power to the entire equipment chassis. Although a hot-swappable controller like the MAX4271 can be used to limit the inrush current on the card, protecting its signal lines is not easy. When you insert the board card into the backplane, the data bus of the backplane uses 5V TTL level for communication. At this time, the digital IC (microcontroller, ASIC, etc.) on the card may "be at its input end before the 5V power supply is turned on." See "5V voltage. As mentioned earlier, this situation can lead to locking or card damage.

Connecting a low-voltage fault-protected switch between the sensitive device and the backplane (Figure 7) provides the necessary overvoltage protection. These switches keep the COM output in a high-impedance state until the power supply voltage on the card is turned on. When the power supply is ready, the switch is closed to connect the backplane. The protection input (NO) of the switch faces the base plate and provides ±12V protection when the power supply is not connected. After the power supply voltage is stable, it can also protect the board from overvoltage impact on the base plate. It should be noted that commonly used logic bus switches from other vendors do not provide this protection. They can provide higher locking current than standard CMOS devices

tolerance, but cannot withstand sustained overvoltage.

Figure 7. Hot-swappable backplane signals

In the circuit shown in Figure 8, the low-voltage fault protection switch cuts off the internal power supply (a 9V battery or two lithium batteries connected in series) when it detects that an external power source (such as a wall adapter) is connected. Normally, the switch is powered by the battery through pin 13. Low voltage Schottky diodes prevent non-rechargeable batteries from being charged by an external power source.

Figure 8. Battery cut off when external power is connected

VCC is taken out by the switch on pin 10. In most applications, this voltage is also regulated by the subsequent voltage regulator. Once the external supply voltage is detected, the microcontroller turns on switches 1 and 4 and turns off switch 3. The output capacitor C provides power to the system when VCC switches from switch 3 to switch 4. In order to protect the battery, when switch 4 is closed, switch 3 must always be kept in the open state. After the external power supply is removed, switches 4 and 1 open and switch 3 closes. When the external power supply voltage is higher than the battery voltage, or when the battery is deeply discharged and the external power supply is connected, or when C is charged and the battery is removed, the fault protection feature can ensure that the switch operates correctly and safely.