A universal requirement for automotive electronics is that any device with direct connections to the wiring harness must be able to withstand shorts to the battery voltage. Though brutal, this requirement is necessary for reliability and for safety. One example of the need for this protection is the audio amplifier that produces indicator noises in the automotive interior. Though operating from a lower voltage (3.3V or 5V), it must be able to stand off the full battery voltage. A protection network appropriate for these amplifiers can be used for other automotive circuits as well.
A dual n-channel MOSFET disconnects the amplifier outputs from the wire harness in response to a high-voltage condition on either output. The MOSFETs (M1A and M1B) are normally on, with their gates driven to approximately 11V by the 11V zener diode (D4) and its bias components. The dual diode D3 provides a diode-OR connection to the dc voltage on each output, thereby producing a voltage that controls the output of shunt regulator U2 (MAX8515). The circuitry shown protects U1, a 1.4W Class AB amplifier (MAX9716) suitable for the application mentioned above (audible warnings and indications for the automobile's driver).
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