Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 4555
Maxim > Design Support > Technical Documents > Application Notes > Audio Circuits > APP 4555
Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 4555
Keywords: automotive voltage protection, dual n-channel MOSFETs, shunt regulators, class AB amplifiers
Circuit Guards Amplifier Outputs Against
Jan 06, 2011
Abstract: To protect audio amplifiers and other automotive electronic circuits in the event of an accidental
direct connection to the car battery voltage, add the circuitry shown (a shunt regulator, diodes, and an nchannel MOSFET in series with each audio path).
A similar version of this article appeared in the July 7, 2007 issue of
EDN magazine.
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
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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 (Figure 1) can be used for other automotive circuits as well.
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Figure 1. This output-protection circuit provides continuous protection against overvoltage faults.
A dual n-channel MOSFET disconnects the amplifier outputs from the wire harness in response to a highvoltage 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).
During normal operation, the amplifier outputs' dc components are at one half of the VCC supply (2.5V in
this case, for which VCC = 5V). The MOSFETs are fully enhanced by the 11V gate drive, and the shuntregulator output is OFF because its feedback input (pin 5) is below the internal 0.6V threshold. If either
output exceeds 5V, current flows through D3 into the R5/R6 divider, pulling the feedback terminal above its
threshold. The shunt regulator output then pulls the MOSFET gate voltage from 11V almost to ground,
which blocks high voltage from the amplifier by turning off the MOSFETs. The MOSFETs easily withstand
the continuous output voltage, and the circuit returns to normal operation when the short is removed.
Because the circuit does not respond instantaneously, two zener diodes are included (D1 and D2) to provide
protection at the beginning of a fault condition.
The waveforms of Figure 2 represent an operating circuit. One of the amplifier outputs (top trace, dark blue)
is a 1kHz sine wave biased at 2.5V DC. The second trace (cyan) is the signal seen on the wire harness. It
also starts as a 1kHz sinewave biased at 2.5V DC, but at 200µs it is shorted to an 18V supply. The third
trace (magenta) is the shunt regulator output, initially biased at 11V, but pulled to ground in response to the
overvoltage condition. The fourth trace (green) is current in the wire harness. Initially a sine wave, this
current drops to zero in response to the overvoltage condition.
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Figure 2. In Figure 1, one of U1's two audio outputs (top trace) is protected when its external terminal
accidentally contacts an 18V supply voltage (2 nd trace).
The components shown optimize this circuit for 5V operation. For other voltages, you can adjust the R5/R6
resistor values. The shunt regulator must be able to function in saturation, and therefore requires a separate
supply pin in addition to the shunt output pin. The circuit withstands 28V shorts repeatedly without damage.
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Application Note 4555:
APPLICATION NOTE 4555, AN4555, AN 4555, APP4555, Appnote4555, Appnote 4555
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