The automotive environment is extremely demanding for electronics: any circuit connected to a 12V power supply must operate within the nominal voltage range of 9V to 16V. Other pressing issues include load drops, cold starts, reverse battery operation, dual-battery boosting, spikes, noise, and an extremely wide temperature range. During a load drop, the alternator output voltage rapidly rises to 60V or higher; a cold start refers to starting the car in cold weather, which causes the battery voltage to drop to 6V or lower; reverse battery operation occurs when activating a dead battery due to careless reversal of cable polarity. Many tow trucks are equipped with two 12V batteries connected in series to help start a car with a dead battery in cold weather. This raises the electrical system's voltage range to 28V until the car starts and the tow truck driver disconnects the jumper cable. Considering that automotive electrical systems consist of high-current motors, relays, solenoids, lights, and constantly fluctuating switch contacts, spikes and noise are not surprising. Additionally, alternators are three-phase motors with chopper excitation regulation, which sometimes charge the battery with very high currents. Therefore, circuit designs operating in automotive environments especially need to accommodate the high input voltages generated under load drops and dual-battery boost conditions. Passive Protection Circuits A passive protection network for automotive electronics is shown in Figure 1. Similar or identical circuits are widely used to protect various systems connected to the automotive 12V bus. This network protects against damage caused by high-voltage spikes, sustained overvoltage, reverse battery connection, and excessive current consumption. The current protection in Figure 1 is evident; if the load current exceeds 1A for an extended period, fuse F1 will melt. D1, combined with F1, prevents damage caused by reverse battery connection; a large current flows through the forward-biased D1 and blows the fuse. Electrolytic capacitors exhibit an interesting characteristic at approximately 150% of their rated voltage: as the terminal voltage increases, the current consumed by this capacitor also increases. In the case of C1, it acts as a clamp when the input voltage continues to rise (ultimately blowing the fuse). The voltage during dual-battery boost is around 28V, which won't blow the fuse because C1's 25V rating is high enough that the extra current draw is minimal. A small resistance is added to the inductor to limit peak fault current and input transient slew rate, thus helping C1 clamp in the presence of spikes. A major drawback of passive networks is that they rely on blown fuses to prevent damage from overcurrent, overvoltage, and reverse battery connections. Another drawback is their reliance on electrolytic capacitors for clamping. As these capacitors age, the electrolyte dries out, and the increased equivalent series resistance (ESR) disappears, impairing clamping effectiveness. Sometimes a large Zener diode is used for D1 to help this capacitor function. Active circuits have been designed to overcome these drawbacks. Active Circuits Figure 2 shows an active solution used to shield sensitive circuitry from the fluctuating 12V automotive system. The LT1641 is used to drive the input N-channel MOSFET, a feature lacking in the passive solutions described above. First, the LT1641 disconnects the load when the input voltage is below 9V to prevent system failure at low input voltages and reduce the chance of the system supplying valuable current to non-critical loads during startup or charging system failures. Second, the LT1641 gradually increases the output voltage upon initial power-up, providing a soft-start for the load. Third, current limiting and timed circuit breakers protect the output from overload and short-circuit effects. In the event of a current fault, the circuit breaker automatically re-attempts to reconnect at a rate of 1 to 2Hz. The protection circuitry's upstream fuse tolerance can be set to prevent it from melting in the event of a current fault on the LT1641's downstream circuitry. Finally, the circuit shown in Figure 2 isolates overvoltage conditions at the input and provides a clamped output so that the load circuitry can continue to operate normally in the event of an overvoltage. Under normal 12V input conditions, the LT1641 charges the MOSFET gate to approximately 20V to adequately boost the MOSFET voltage and supply power to the load. The 27V Zener diode D1 is connected to the gate and ground respectively, but it does not function within the 9 to 16V operating voltage range. When the input rises above 16V, the LT1641 continues to charge the MOSFET gate, attempting to keep the MOSFET fully on. If the input rises too high, the Zener diode clamps the MOSFET gate, limiting the output voltage to approximately 24V. The LT1641 itself can handle voltages up to 100V at its input and is unaffected by gate clamping. Gate clamping circuitry is much more precise than that of passive solutions, and it can be easily adjusted to meet load requirements simply by selecting D1 with an appropriate breakdown voltage. The circuit shown in Figure 2 works well with load currents up to around 1A, but for higher load currents, the circuit shown in Figure 3 is recommended to prevent the MOSFET from excessively consuming power. Excessive power consumption is risky if overvoltage conditions persist, such as when the electrical system is powered by two series-connected batteries for longer than normally required, or when the current rises slowly after a load sag and the MOSFET is small. The output is sampled by D1 and D2. If the input exceeds 16.7V, a signal is fed back to the "SENSE" pin to stabilize the output at 16.7V. The regulation here is more precise than that shown in Figure 1 and can be easily customized to meet load requirements by selecting a suitable Zener diode. Total power consumption is limited by the "TIMER" pin, which records the total duration of MOSFET output regulation. If the overvoltage condition persists for more than 15ms, the LT1641 shuts down, allowing the MOSFET to stop output regulation. After approximately half a second, the circuit attempts to restart. This restart cycle continues until the overvoltage condition disappears and normal operation resumes. Overcurrent faults are handled in the same way as described in Figure 2. Battery Reverse Protection Battery reverse protection can be added to the circuits shown in Figure 2 or Figure 3 simply by adding a series diode. In most cases, a standard pn diode will suffice; if the forward voltage drop is critical, a Schottky diode can be chosen. In critical applications where power consumption in isolation diodes is unacceptable, the simple circuit shown in Figure 4 can solve this problem. Under normal operating conditions, the body diode of MOSFET Q2 is forward biased and delivers power to the LT1641. When the LT1641 is on, the gate of Q2 is driven, thus fully turning on. If the input is reversed, the emitter of Q3 is pulled low below ground, Q3 turns on, thereby pulling the gate of Q2 low and keeping it close to the source level of Q2. In this case, Q2 remains off, isolating the reverse input from reaching the LT1641 and the load circuit. A microamp-level current flows through a 1MΩ resistor to the "GATE" pin of the LT1641. High-voltage LDOs can extend battery life. LDOs used as voltage limiters with a maximum input voltage rating of 25V or lower (such as the LT1616) are generally not considered for automotive applications. However, this disadvantage in input voltage can be easily overcome when combined with low-dropout (LDO) linear regulators such as the LT3012B/LT3013B. This small, efficient combination, as shown in Figure 5, can provide a 3.3V output in automotive environments. The LT3013B has a wide input voltage range of 4V to 80V and integrates battery reverse protection, eliminating the need for special voltage limiting or clamping circuitry, thus saving cost and board space. When operating with a moderate load current, the efficiency of an LDO regulator is approximately equal to VOUT/VIN. If VOUT is much lower than VIN, the efficiency of the LDO decreases. For example, when stepping a 12V input to a 3.3V output, the efficiency is only 28%. In Figure 5, higher efficiency is achieved by allowing the LT3013B to operate in a low-dropout mode within the normal input voltage range. In this case, the LT3013B's output voltage is set to 24V. This LDO's output voltage is only 400mV lower than VIN, powering the LT1616 buck regulator with 97% efficiency, and the voltage is right in the middle of its normal operating voltage range. Under load sags, VIN may rise rapidly to as high as 80V, but when VIN exceeds 24.4V, the LT3013B will adjust its output and effectively "limit" it to 24V, which is just within the rated voltage range of the LT1616 switch. If VIN rises above 24.4V, the LDO's efficiency will decrease, but this situation is short-lived and has no adverse consequences. The LT1616 converts the limited output of the LT3013B to 3.3V. With a 12V input, the switch is approximately 80% efficient. During a cold start, the car's voltage can drop to 5V. In this case, the LT1616's input voltage is 4.6V, which is right within its operating voltage range. The LT3013B LDO regulator, combined with the LT1616 switch, provides a stable 3.3V output over the typical wide operating voltage range of a 12V automotive electrical system without sacrificing efficiency. A more integrated solution is the LT3437. The LT3437 is a 200kHz monolithic step-down regulator with an input voltage range of 3.3V to 80V. Its low quiescent current of 100uA under no-load conditions is essential for keeping systems constantly on today. A low-cost diode can be connected in series at the input of the LT3437 to provide battery reverse protection.