MENU

Power for Automotive Infotainment Systems

Technology News |
By Christoph Hammerschmidt

Today’s car infotainment systems share little similarity with the radios in early cars. When first used, car radios were simplistic; nothing more than just analog radios. Today, however, modern infotainment systems provide far more than just music and news to the driver and their passengers. Besides playing music from storage devices like SD cards and USB drives, radios can play various frequency bands, covering both analog and digital broadcasts. In addition, mobile devices can now connect to infotainment systems via Bluetooth to play music and make phone calls, providing seamless integration from your personal device to your car.

Powerful navigation systems, meanwhile, now come as standard in modern cars, using GPS and real-time data from the internet to find the fastest route for the driver. However, the use of an internet connection is not exclusively there for the navigation system. Some cars provide a Wi-Fi access point so passengers can browse on their phones during the trip. The number of features and functionalities is increasing for every new generation of infotainment systems, but one thing stays the same: a stable and reliable power source is needed under all operating conditions. This article focuses on the power tree which is needed to supply a state-of-the-art infotainment system. It presents different challenges when designing this kind of application and shows a built and tested power management reference design.

Requirements

A modern infotainment system has several loads which need to be powered with typically two different supply voltages needed as a main power source. The reference design discussed in this article provides one rail with +3.3 V @ 3.0 A (6.0 A peak) and +7.5 V @ 1.5 A (2.5 A peak). With the +3.3 V rail, loads like the tuner and microprocessors can be supplied either directly or by subsequent low-voltage point-of-load (POL) converters. The rail with +7.5 V for example is used for microphone phantom biasing of the hands-free set and active noise cancellation, active antennas (FM, AM, GPS, DAB), rear view camera and a CD player.


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.

Design

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.


The low pass filter reduces the differential mode noise which is caused by the following switch mode power supplies. It is built as a PI filter with a corner frequency of approximately one tenth of the switching frequency of the subsequent power supplies. This results in a theoretical ripple rejection of around 40 dB, which means it’s lower due to the interwinding capacitance of the inductor and other parasitic elements on the board.

As previously mentioned, the battery voltage can break down significantly when the engine is started, especially at colder temperatures. To maintain the power for the infotainment system, a pre-boost converter is used to provide a high enough input voltage for the dual buck converter. To reduce size and cost of the pre-boost, several loads like audio amplifiers are reduced or switched off during cranking. Therefore the output power of the pre-boost can be significantly lowered compared to the overall output power of the infotainment power tree. In this design, the pre-boost only needs to support around 40% of the regular power for the two output rails. The output voltage of the LM5150-Q1 boost controller can be set to 6.8, 7.5, 8.5 or 10.5 V. This design uses 10.5V output voltage to provide a high margin for the buck inputs. The device is optimized for pre-boost applications. As a result, it starts up quickly as soon as the battery voltage drops below the programmed output voltage.

Figure 2 and Figure 3 show the 10.5 V output voltage of the boost at a 1.5 A load with the Volkswagen E-11 start test pulse “severe” charged. Even though the boost reacts quickly, the output voltage drops down to slightly above 6.0 V before it recovers – as seen in Figure 2 (5 ms per div). The reason is that this design uses only ceramic capacitors for reliability reasons. The four 10 µF (50 V, X7R, 1210) ceramic capacitors on the boost output / buck input might be too low for some applications, but for this specific design the +7.5V rail does not need to be maintained during cranking. As a result, a relatively high break down of the buck input voltage can be tolerated. If, however, a significant break down of the output voltage is not acceptable, higher output capacitance is needed. A good choice for this kind of application is to use hybrid capacitors which offer high capacitance, high ripple current capability and low ESR at the same time. Choosing an output capacitance in the range of several hundred µF reduces the break down in the worst case to only a few hundreds of millivolts.


Figure 3 shows not only the critical beginning of the test pulse, but also the sinusoidal battery voltage as soon as the starter motors begin to spin. The output voltage is well regulated to 10.5 V without any disturbance.

Figure 2 – Cranking Pulse – 5ms per div

                                 

Figure 3 – Cranking Test Pulse – 500ms per div

When the battery voltage is above 10.5 V, the boost converter is not able to switch. It then makes sense to bypass the boost’s inductor and diode to reduce the conduction losses. In this design, the status pin of LM5150-Q1 is used to control a P-FET parallel to the boost’s inductor and diode. This pin is low while the device is not switching and can be used to drive the gate of a P-FET directly. When the boost converter is switching, this open drain output goes high and the switching node voltage is used to generate a voltage to turn off the P-FET.

Finally, the two output rails are generated by LM5140-Q1. For this synchronous dual current-mode buck controller, 440 kHz or 2.2 MHz can be selected for the switching frequency. To keep the switching losses low and achieve a high efficiency, 440 kHz was selected for this reference design. For accurate current sensing and overcurrent protection, a shunt resistor in series to the inductor is used. To minimize the losses of the shunt, the overcurrent threshold can be set to either 48 mV or 73 mV (typ.). The controller also offers the possibility to operate the buck converter in forced PWM mode for best transient response but with lower efficiency at light loads. If diode emulation mode is enabled, the efficiency is significantly higher under light load conditions. To minimize the overall footprint of the design, a fast switching and low resistive 60V dual N-FET (Vishay SJQ260EP) in PowerPAK SO-8L package is selected. This offers a moderate package size and a good thermal interface to distribute the losses on the board. Choosing appropriate inductors is also important for a compact solution. Therefore, Cyntec’s VCHA054T series has been chosen with a size of only 5.4 mm by 5.2 mm by 3.8 mm. The composite material has higher core losses compared to ferrite, but offers smaller size and higher saturation current, important for the pulsed loads. All capacitors are ceramic types to ensure high reliability as well as low height. Due to the high converter bandwidth of 30 to 40 kHz at 440 kHz switching frequency, 3x 47 µF (10 V, X7R, 1210) for the +3.3V rail and 2x 22 µF (16 V, X7R, 1210) for the +7.5V rail are sufficient.


Figure 4 shows the complete design. The total footprint is only 42 mm x 51 mm with almost all components mounted on the top side of the PCB. On the bottom side are only the two feedback voltage dividers of the dual buck.

Figure 4 – PMP30267 Board Image

 

Conclusion

A car’s infotainment system is a complex unit which typically needs several power rails. These rails have to be present all the time to ensure stable operation without interruption. Starting the engine on a cold day causes a significant break down of the battery voltage which can lead to a shutdown of the infotainment system. Therefore the power tree contains more blocks than only two buck converters to provide the needed supply voltages. This article describes a complete power solution for a state of the art infotainment system containing an active reverse polarity protection, a pre-boost converter and a synchronous dual buck converter to generate the two power rails. All information regarding this design (schematic, bill of material, layout, test report and design files) can be found on the TI website here.


Share:

Linked Articles
Smart2.0
10s