Application Note «Fault-Tolerant Analog Switches» (Maxim) - 5

fault voltages through the switch). When the power supplies are off with input voltages present, the architecture provides the same protection. In this case, the +15V and -15V supplies go to 0V (Figure 3 again). Once more, with -25V at the input, the N3 sense switch compares the -25V input to its present 0V reference. This causes the N3 sense switch to short the input to the gate of N1, again causing N1's Vgs to equal 0V; the MOSFET stays off. Neither the comparator circuit nor the clamp has power, and thus the output goes to 0V through its load. The P1 gate goes to 0V, and again no power supplies are available to power the P3 sense switch, the comparator, and the p-channel driver. With a -25V input and 0V at its gate, P1 is also not enhanced. So, both N1 and P1 are off, and only leakage currents flow. For the +25V fault condition, the P3 sense switch shorts the input to the P1 gate, causing P1's Vgs to equal 0V. Neither P1 nor N1 are enhanced in this case, so the architecture prevents the signal from coupling through, allowing only leakage currents to flow. Just like a -25V input with the supplies turned off, the output in this situation goes to 0V through its load. The design works equally well with a single supply or dual supplies, which can be asymmetrical with virtually no difference in fault protection, and no sequencing order is required. Using the Parts Using the parts properly requires little thought, as the product-family members are pin-for-pin replacements for many popular, non-fault-protected switches and muxes. Simply drop these devices into existing sockets to get the fault protection. Adding these devices protects the downstream circuits, because the faults cannot couple through the switches, as discussed above. The protection afforded by this product family extends beyond switches and multiplexers. This family includes another type of device called a "signal-line circuit protector." These devices provide the protection described above, but are used solely for protection. They do not function as switches or multiplexers; instead, they protect one or more signal lines, the number of lines depending on the particular device (they are offered as singles, triples, and octals). Each protector has essentially the same internal structure (Figure 3 again) and looks like a 65Ω resistor to the input signal. If a fault is sensed, the protector turns off, isolating the fault from the load, again only allowing leakages to flow at the input. As with the fault-protected switches and muxes, the protector is an open circuit when the supplies powering it are off. These devices can be used to protect op amps, ADCs, sample-and-holds, and other devices. A typical application of the MAX4507 eight-channel protector shows it connected in front of a non-fault- protected 1-of-8 multiplexer (
Figure 4
). (The MAX338 shown might just as easily have been a DG506 or any other non-fault-protected device.) The MAX338 channel inputs (N01 through N08) must never exceed the power-supply voltages (here, ±15V). The output of the multiplexer appears on the common pin (a resistive load is shown), while logic signals at A0, A1, and A2 control the mux selection. The enable pin, EN, is shown connected to +5V, but it too can be driven by logic when muxes are paralleled. Page 5 of 8 Document Outline maxim-ic.com Fault-Tolerant Analog Switches - Application Note - Maxim