Application Note «Fault-Tolerant Analog Switches» (Maxim) - 3

15V supplies powered on, the body of P1 (n-substrate) forms a forward-biased diode with the P1 input (p-material), and the resulting diode current is virtually unlimited. This usually leads to serious circuit- level problems. N1 exhibits a similar, albeit reverse, action with a -25V input. Series-Switch Structure A partial solution to the fault problems (see
Figure 2
) is to use a series configuration of two n-channel MOSFETs and one p-channel MOSFET with the gates driven, as before, by internal driver circuitry such that the n-gates are out of phase with the p-gates. If the switch is supposed to be on, then the N1 and N2 gates are driven to +15V, and the P1 gate to -15V; the opposite occurs if the switch is supposed to be off. Figure 2. This series configuration solves the fault problems of the parallel configuration, as fault signals can neither get through the switch nor cause damaging diode currents to flow. The disadvantage of the series configuration is its higher on-resistance, relatively large chip size, and limited signal-handling capacity; this circuit switches inputs only from 0V to 15V minus the threshold voltage (this usually means 0V to ±13V or so). The series arrangement does a good job of protecting the chip, but at the expense of switch on-resistance and signal-handling capability. Maxim Parallel-Sense Structure The basic switching element of the new Maxim fault-protected family is the same parallel n- and p- channel MOSFET configuration found in Figure 1. Within the simplified drawing (see
Figure 3
) of the basic switching element (which forms the core of the switches, muxes, and signal-line circuit protectors within this product family), the parallel FETs are represented by N1 and P1. What's new is the sensing circuitry that surrounds N1 and P1, as well as the clamping blocks at the output. The sensing circuitry is the heart of the fault-protected architecture, whereas the clamps are a useful feature for certain applications (see below). Page 3 of 8 Document Outline maxim-ic.com Fault-Tolerant Analog Switches - Application Note - Maxim