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29 littelfuse.com Designers Have Choices First, consider the case of the camera subsystem. Among other things, multiple cameras work together to provide depth perception and convert visual light through a charge coupled device/complementary metal oxide semiconductor (CCD/CMOS) image sensor into electronic signals sent to a communication and control circuit. Of the circuit blocks in a camera subsystem, those that require protection components connect with external circuitry (Figure 1). They typically include a Controller Area Network (CAN) transceiver, the power supply, and an Ethernet transceiver. The camera power-supply subsystem requires protection from over-currents, high-energy transients, and ESD. A fuse provides over-current protection. Design engineers can select either a conventional one- time blow, ceramic fuse, or a polymer-based, positive temperature coefficient (PPTC) resettable fuse. Both components can have wide temperature ratings for automotive requirements. Ceramic fuses can have operating temperature ratings of -55°C to +150°C, and PPTC resettable fuses can operate up to +125°C. The PPTC has the advantage of not needing replacement if it experiences an over-current. The PPTC substantially boosts resistance in response to the heat generated by an over-current. When the over-current is removed, Figure 1: Of the circuit blocks in a camera subsystem, those that require protection components connect with external circuitry. (Source: Littelfuse) the PPTC recovers to low resistance. Both component types come in surface-mount packages to save valuable printed circuit board (PCB) space. Besides over-currents, power-supply circuits need protection from high-energy transients caused by in- vehicle sources such as motors turning on and off. The circuitry must be capable of withstanding transients defined by ISO Standards 7637 and 16750. Compliant components include transient voltage suppressor (TVS) diodes that can safely absorb both low-energy transients and high-energy transients as specified in Pulses 1, 2, 3, and 5 that the above-referenced standards spell out. Design engineers can also consider a metal oxide varistor (MOV) for transient energy protection. MOVs can absorb transients with surge currents of 500A from 8×20-µsec pulses and up to 2.5J from 10×1,000-µsec pulses. These components comply with Electromagnetic Compliance Standard IEC 61000-4-2. MOVs can also safely withstand the automotive environment with an operating temperature range of -40°C to + 125°C. To avoid catastrophic failure if the polarity of the voltage to the power supply accidentally reverses, designers can insert a Schottky diode in series with the fuse. Although providing reverse polarity protection, the diode's low- forward voltage drop will have a minimal impact on power-supply performance.