What is fail-safe logic?
Gain an understanding of how systems may be back-powered during power-up. Learn to prevent back-powering and complications of power sequencing with fail-safe logic.
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Hello, and welcome to the TI Precision Labs series on analog multiplexers. This video will provide an introduction to fail-safe logic found in analog switches and multiplexers. The goal of this video is to learn the features of fail-safe logic and its advantage to your end equipment.
Before we jump into fail-safe logic, I will explain how an ESD diode in a traditional switch works. In the setup shown here, both the subsystems and the switch are powered with 5 volts. The ESD diode prohibits any current flow from VDD into select. The signal on the select pin will only flow into VDD if it surpasses the maximum ratings mentioned in the datasheet. This is usually 0.5 volts above VDD. Due to this, the device is protected from ESD events up to the datasheet's specification.
If the second power rail is empowered, as indicated in this picture, the diode is not operating as intended due to the lack of a potential on VDD. A current can flow from select into the VD rail. There it back power subsystem B. This unwanted voltage can damage the switch and subsystem B if their max ratings are violated. This is especially worrisome as the current might not be limited by the design.
Even if the max ratings are not exceeded, if subsystem B is a microcontroller, this may cause issues on boot-up as register settings are not set correctly. This is often the case when the switch and subsystem B is unpowered while the subsystem A is already operating. The digital logic is back powered as well and may turn on the switch. Unwanted currents can flow through the open switch and might damage to single ports on subsystem B.
ATI switch with fail-safe logic will maintain a high impedance state on the single path and digital logic pins when the switch supply is unpowered. The internal protection will prevent current to flow into VDD, thus the switch in subsystem B remains protected. This feature can be found in all TMAX devices. The fail-safe logic feature also eliminates the need for a voltage translator if different voltages are used within the design. For example, subsystem A powered with 5 volts can control a multiplexer powered with 3.3 volts. The signal levels still need to comply with the switch ratings.
Thank you for watching the TI Precision Labs video Fail-Safe Logic. To find more switches and multiplexers, technical resources, and search products, visit ti.com/switches.
This video is part of a series
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Precision labs series: Understanding switches and multiplexer's features and parameters
video-playlist (15 videos)