Power supply meets automotive-transient-voltage specs
Diodes » MMBTA42
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.
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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