Application Note «Fault-Tolerant Analog Switches» (Maxim) - 7

The two arrangements have some subtle differences. In the two-device circuit (Figure 4 again), any overvoltages present at the MAX4507 inputs are shared between the MAX4507 and the MAX338. In the single-device circuit (Figure 5 again), the MAX4508 must handle these overvoltages by itself, which lowers the maximum overvoltage allowed at the device's inputs. To understand this, take the unlikely example where opposite polarity faults appear at inputs IN1 and IN2 in the circuit of Figure 4 and at inputs NO1 and NO2 in the circuit of Figure 5. Assume that +40V appears on IN1 and -40V on IN2 at the same instant. When channel 1 is selected (IN1 shorted to the common pin [COM]), the MAX4507 limits and clamps the +40V at IN1 to +15V on OUT1, which connects to an input of the MAX338. The voltage drop across the MAX4507 is 40 - 15 = 25V, and the voltage drop across the MAX338 is approximately 0V, as its resistance is usually small compared to the load resistance (the voltage-divider effect). The voltage on COM is thus +15V. Channel 2 has -40V at its input and -15V at its output (OUT2), because the MAX4507 limits and clamps the input voltage. Again, 40 - 15 = 25V is dropped across this channel. The MAX338 N02 channel isn't selected, so N02 to COM is -15V to +15V, or 30V. Therefore, of the range of voltage from -40V at the input to +15V at COM (the output), 55V total, the MAX4507 shares 25V of this total and the MAX338 shares 30V. If the MAX4508 is subjected to the same conditions, the full 55V has to be handled by the MAX4508 channel. This exceeds the recommended 40V maximum across the channel. Thus, the MAX4508 can handle +25V and -25V faults, with ±15V power supplies, whereas the series circuit (Figure 4) can easily handle +40V and -40V. Both arrangements can withstand +40V and -40V inputs when the power supplies are turned off. The process and the architecture used in creating this new family of products have certain limitations. The parts do not protect against 110V AC or lightning damage. They are, however, effective in protecting against normally expected fault conditions, such as power-supply sequencing problems as well as inadvertent connections to higher-than-expected voltages. Under these conditions, each member of this product family senses the presence of a fault, shuts down to protect downstream circuitry, and prevents the fault from damaging the part itself. All the product-family members automatically reset themselves to normal operation as soon as the fault condition goes away.
Related Parts
MAX338 8-Channel/Dual 4-Channel, Low-Leakage, CMOS Analog Free Samples Multiplexers MAX339 8-Channel/Dual 4-Channel, Low-Leakage, CMOS Analog Free Samples Multiplexers MAX4505 Fault-Protected, High-Voltage, Signal Line Protector Free Samples MAX4506 Fault-Protected, High-Voltage Signal-Line Protectors Free Samples MAX4507 Fault-Protected, High-Voltage Signal-Line Protectors Free Samples MAX4508 Fault-Protected, High-Voltage Single 8-to-1/Dual 4-to-1 Free Samples Multiplexers with Output Clamps MAX4509 Fault-Protected, High-Voltage Single 8-to-1/Dual 4-to-1 Free Samples Multiplexers with Output Clamps Page 7 of 8 Document Outline maxim-ic.com Fault-Tolerant Analog Switches - Application Note - Maxim