Power supply meets automotive-transient-voltage specs

Figure 1 shows a power supply that delivers 5 V from a 12 V battery. With only a few components, the supply copes with all the automotive transients that ISO (International Organization for Standardization) 7637-1 lists without the need for a bulky transient-voltage suppressor. In normal operation, R₃ connects to the common through a microcontroller port. In standby mode, R₃ stays open, and the quiescent current of the supply decreases from approximately 2.8 mA to approximately 160 µA, and the output voltage then drops to approximately 3.5 V. If your application doesn’t require a standby mode, suppress R₃ and set R₅ to 220 Ω. With most common zener diodes, you would then set R₅ to 120 Ω and D₁ to 4.3 V. You can use the circuit in 24 V systems if D₂ is 36 V.

Figure 1.	This automotive regulator withstands overvoltages that ISO 7637-1 specifies.

If the voltage increases, the current through D₁ and the base of Q₃ increases, so Q₃ increases the current of Q₂, which lowers the gate-to-source voltage of Q₁. If the input voltage surpasses 19 V, D₂ starts to conduct and makes Q₂ switch off Q₁, so permanent overvoltages as high as 200 V cannot damage the circuit. The Miller capacitance of Q₁ makes it act as a fast integrator, which keeps the system stable. If you remove D₂, you must replace Q₃ with a high-voltage transistor, such as an MMBTA42.

If you omit D₂, the circuit cannot withstand permanent overvoltages without Q₁’s overheating. In this case, however, the circuit can cope with all the impulses, including the load-dump pulse, of ISO 7637-1. You should remove D₂ only if C₁ cannot maintain the voltage during long overvoltages, such as the load-dump pulse, and keeping the voltage is critical.

An added advantage of this circuit over most IC-voltage regulators is that it can sink current through D₁ and Q₃. This feature allows the use of diodes to fully protect the microprocessor’s inputs. Soldering the D-Pack package to a couple of 1-cm² copper pads allows the circuit to source 300 mA at 10 to 16 V or 150 mA at 20 to 32 V. More dissipation area allows for higher currents.

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