AN2854| Application Note
CIRCUIT PROTECTION SWITCHES AND MULTIPLEXERS
Nov 17, 2003
Low-Voltage Fault Protection
This application note discusses the need for overvoltage protection in switches and muxes that connect to the rest of the system. The MAX4711/MAX4712/MAX4713 analog switches operate from 2.7V to 11V with integrated fault protection. Other methods of fault protection are discussed, along with their advantages and disadvantages. The standard supply voltages for today's systems (unipolar 3.3V or 5V, or bipolar 3.3V or 5V) are usually the highest voltages available on a board. The board's input terminals may be exposed to voltages higher than the supply voltages. When power to the board is off, voltages may remain at the input terminals. The first element to be affected by overvoltage is often a multiplexer or a switch, so they must protect downstream circuitry. The pass element in an analog switch contains one or more MOSFETs and has parasitic clamping diodes to the supply voltages for ESD protection. Figure 1 shows the equivalent circuit diagram for a closed analog switch. As long as V+ and V- are present an
d no input voltage exceeds those rails by the forward bias voltage of the clamping diodes (typically 0.6V), the diodes are reverse-biased and no current flows through them.
Figure 1. Equivalent circuit diagram for a closed switch. Because improper supply-voltage sequencing can cause an overvoltage fault, many switches require that the most positive voltage be applied first and the most negative last. An input voltage with supplies off or an input voltage that exceeds a supply voltage causes current to flow through the clamping diodes. Those diodes can handle only a couple of milliwatts (depending on the IC's semiconductor process) before heat due to power dissipation damages the switch permanently.
AN2854 Application Note maxim Download PDF
Add this permalink to your bookmarks for future download of AN2854 ApplicationNote
Permalink: http://application.emcelettronica.com/maxim/AN2854
