Power for Automotive Infotainment Systems: Page 2 of 5

November 27, 2018 // By Markus Zehendner, Texas Instruments
In today’s cars, the In-vehicle infortainment (IVI) systems are rather complex compared to earlier generations. This article describes requirements and solution for such systems.

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.


Figure 1 – Block Diagram

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.

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