Here's a look at an automotive-protection circuit that withstands repeated insults of direct short circuits to the full battery voltage.
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 an audio amplifier that produces indicator noises in the automotive interior. Though operating from a voltage of 3.3 or 5V, which is lower than the battery voltage, the amplifier must be able to stand off the full battery voltage. You can also use a protection network appropriate for these amplifiers for other automotive circuits (figure 1). A dual N-channel MOSFET disconnects the amplifier's outputs from the wiring harness in response to a high-voltage condition on either output. The MOSFETs, Q1A and Q1B, are normally on; zener diode D4 and its bias components drive the MOSFETs' gates to approximately 11V. 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 IC2. The circuitry protects IC1, a 1.4W Class AB amplifier suitable for audible warnings and indications for the automotive electronics.
Figure 1: This output-protection circuit provides continuous protection against over-voltage faults.
During normal operation, the amplifier outputs' dc components are at one-half of the VCC supply—2.5V in this case, for which VCC is 5V. The 11V gate drive fully enhances the MOSFETs, and the shunt-regulator output is off because its feedback input, Pin 5, is below its 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 you remove the short. Because the circuit does not respond instantaneously, zener diodes D1 and D2 provide protection at the beginning of a fault condition.
The waveforms of figure 2 represent an operating circuit. One of the amplifier's outputs (Trace 1) is a 1kHz sine wave biased at a dc voltage of 2.5V. Trace 2 is the signal on the wire harness. It also starts as a 1kHz sine wave biased at a 2.5V-dc voltage, but, at 200µsec, it shorts to an 18V supply. Trace 3 is the shunt regulator's output, initially biased at 11V but pulled to ground in response to the over-voltage condition. Trace 4 is current in the wire harness. Initially a sine wave, this current drops to zero in response to the over-voltage condition.
Figure 2: In figure 1, one of IC1's two audio outputs (Trace 1) has protection when its external terminal accidentally contacts an 18V supply voltage (Trace 2).
The components in figure 1 optimise 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 repeatedly withstands 28V shorts without damage.
This article is a Design Idea selected for re-publication by the editors. It was first published on July 5, 2007 in EDN.com.