Switches
Figure 3 Nexperia’s analog switches feature injection current control
Safe and efficient switching An analog switch is a device which uses a digital control signal to connect or disconnect an analog (or digital) signal between a single input and output terminal. A CMOS analog switch consisting of a single NMOS and a single PMOS transistor whose drain and source terminals are connected is shown in Figure 1. A multiplexer performs a similar function but has multiple input channels and can connect an analog signal connected to any of these channels to a single output terminal. While the function these devices perform is straightforward, they require several additional protection features to ensure they can perform safe and efficient switching in the latest electronic systems.
The application benefits of analog switches are summarized in Table 1.
Protecting against injected currents
Figure 4 Legacy analog switches can cause unintentional back-powering
Overvoltage spikes on unselected analog signal pins can potentially affect the analog signal on the selected channel, a scenario which arises in some legacy analog multiplexers which feature internal ESD clamping diodes (Figure 2). These conduct an overvoltage event to the positive supply voltage (VCC) and this has the effect of unintentionally activating the PMOS FET in the transmission gate. This in turn impacts the signal at the output and if the analog input source impedance is large, the signal here can also potentially be impacted. Nexperia’s NMUX130x devices incorporate injection current control circuitry (Figure 3) which isolates overvoltage spikes on the deselected channels to prevent them from affecting the pathway of the selected analog signal. This helps to preserve measurement accuracy of the sensor signal on the selected channel. This control circuitry brings the added advantage of eliminating the need for external overvoltage components like large series resistors or supply clamping Schottky diodes, which would typically be required to prevent the above scenario from occurring.
Figure 5 Legacy analog switches can back-power VCC supply if analog inputs are applied before VCC is activated
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Fail-safe logic prevents unintentional back-powering With some multiplexers back-powering of VCC (through the internal ESD clamping diode (that connects the analog input to the supply) can potentially lead to system issues in devices like microcontrollers which have independent core voltages
(Figure 4). Preventing this requires controlled power sequencing which takes precious design time to implement.
However, Fail-Safe Logic (FSL) in the NMUX130x prevents back-powering of digital I/Os to VCC, allowing devices with active digital inputs to be initialized first, even while the NMUX130X supply (VCC) is in a high-impedance or unpowered state.
In legacy analog switch devices, biasing of analog I/Os, when VCC is at GND or in a high impedance, could back-power the analog switch and generate voltage on the VCC supply, potentially causing partially powered subsystems. Initializing active circuitry before the analog multiplexer can cause the analog inputs to draw excessive current, particularly in devices with multiple channels, raising the risk of system issues (Figure 5). However, the NMUX130X includes internal circuitry that isolates the connection between the analog inputs and VCC while maintaining the analog switch output in a high-impedance state. This power-off- protection (PoP) feature is crucial in systems where active circuitry driving analog inputs have independent supply voltages which could be activated before VCC.
switches from Nexperia Nexperia’s NMUX1308 and NMUX1309 are flexible CMOS bi-directional analog switches. The NMUX1308 has eight single- ended (8:1) channels while the NMUX1309 features a dual four channel analog switch, SP4T. Both devices include the range of protection features discussed previously to help safeguard the device itself and other externally connected components from potential issues. Both devices have also been qualified for the Automotive Electronics Council (AEC) standard Q100 (Grade 1), meaning they can be used in automotive applications. Also suitable for a broad range of general-purpose applications, these devices operate within a voltage range of 1.5 V to 5.5 V. They additionally feature 5.5 V overvoltage-tolerant digital inputs and are compatible with 1.8 V CMOS levels, which eliminates the need for level shifting. All of the features and operational performance of the NMUX1308 and NMUX1309 can be further explored using the NMUX1308/NMUX1309 evaluation board, which will also facilitate accelerated development.
https://www.nexperia.com/ analogswitches
Components in Electronics October 2024 27
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