To generate these two outputs, two buck converters are used. This works well under normal conditions when the battery voltage is around 12.0 V but especially at cold temperatures when the battery voltage drops significantly when the engine is started (due to the high current needed by the cranking motor). With the start-stop systems in modern cars, this might not only happen at the first start of the engine but also during traffic in a city. There are different specifications regarding these battery voltage profiles and some even go down to 3.2 V. However, it’s evident that with such a low input voltage, the two output voltages cannot be maintained by only using buck converters. That’s why a ‘pre-boost converter’ is needed to supply the bucks with an input voltage always high enough to maintain the programmed output voltages. This pre-boost only needs to be active when the battery voltage falls below a certain level – otherwise bypassing the battery voltage.
From a technical perspective, these two blocks can provide the power for an infotainment system but an additional block is necessary. A reverse polarity protection is mandatory for all automotive electronics to avoid damage if the car’s battery is connected incorrectly. Due to the relatively high input current typically seen in a lower battery voltage, a simple diode is an inefficient solution. An intelligent solution is to use a smart diode controller which emulates an ideal diode with a FET to keep the losses and voltage drop low. This block is directly connected to the car’s battery and followed by a low pass filter to reduce the noise caused by the following switch mode power supplies.
Figure 1 shows the block diagram of the reference design which fulfills all requirements as described in the previous section.
The smart diode controller LM74700-Q1 uses an N-FET to emulate an ideal diode. The main advantage of an N-FET based approach is the lower cost and variety of FETs available on the market. The disadvantage compared to a P-FET solution is that a voltage higher than the input voltage is needed to switch the FET on. Therefore LM74700-Q1 incorporates a highly efficient charge pump circuit to generate this voltage for driving the gate of the N-FET. If the controller is switched off by the enable input, the current consumption is reduced to only 3 µA. It is important to mention that by disabling the controller, the body diode of the FET is still conducting as it only disables the internal circuit of the device.