The VI uses two heavy-duty low dropout linear regulators to convert the unpredictable +12V automotive power source to the +5V and +3.3V rails needed by the MCU and related ICs. Incoming power is protected from overvoltage by D3, a SMBJ16CA Transient Voltage Suppressor (TVS) diode. D3 will protect the interface from any voltage over 19.7V (Vbr). The power regulators can handle up to 40V. Reverse voltage is protected by D1. Both regulators do not require any additional current or temperature protection - they can safely drive into a short circuit at temperatures up to 150 degrees C, and will limit current if they become too hot. Plenty of bulk and bypass capacitance has been provided to absorb automotive power transients.
The regulators are cascaded such that the +5V rail supplies the +3.3V rail. The TLE4284DV33 has a maximum voltage drop of 1.4V, so it can be safely powered by any source above 4.7V. While this puts additional load on the +5V rail, it also allows the interface to be powered either by the OBD-II connector or the MicroUSB port. Both regulators can deliver current up to 1.6A. Current budget:
This yields a headroom of ~300mA when all peripherals are active and a Tablet is charging at 1A.
The VI has the following power connections:
The external accessory power connector is controlled by a TPS2421-2 power switch with internal MOSFET. This allows the external accessory power to be disabled when the vehicle is turned off, as well as providing current protection. The power switch has an active 5A limit, and will actively attempt to re-engage power when the limit is tripped (“retry on fault”).
The USB micro connector is directly connected to the +5V bus through a fuse. The lack of a diode means that the connected USB device can either consume or supply current. If a PC is connected to the USB micro connector, it will power the 3.3V regulator and the +5V regulator will be bypassed. If an Android tablet is connected that supports USB OTG charging, it will be allowed to consume +5V power. A resettable fuse (F1) has been added to protect the VI from a bad USB cable or USB host. The fuse is a Bourns MF-MSMF110/16-2. The fuse will automatically reset when power is removed and the fuse is allowed to cool off. This has an added downside of temperature dependence - the fuse can only pass the full 1A of current up to 40 degrees C. Above that temperature the limit slowly decreases - see the datasheet for a thermal derating chart.
VI Reference Design