Automotive Power Tree Reference Design White Paper

In recent years, with the development of automobile accident prevention measures and autonomous driving technology, the demand for advanced driver assistance systems (ADAS) that support high safety requirements (ASIL) has also increased. Autonomous driving refers to driving in which a unit installed in the car performs the four elements of human driving ("cognition" through ears and eyes, "prediction" and "judgment" through the brain, and "operation" through the steering wheel and accelerator) instead of the driver. To achieve safe autonomous driving, accurate sensing and timely display and control are required. Therefore, the number of cameras and sensors used in hardware is increasing, and in order to accurately inform the situation, there is also a demand for multifunctionality in the infotainment system.

In this case, the unit used to implement the safety function also needs to monitor the internal operating status and pay attention to the loss of function caused by unit failure. This also requires electronic circuits to monitor the operating status inside each unit, which will make the electronic circuits more complicated and the time required for unit and system design will be longer.

Reference designs required by the market

As the number of on-board units increases and the number of functions that must be implemented by electronic circuits increases, the electronic circuits of ADAS/infotainment system peripheral units require the following complex designs:

As the number of cameras and sensors increases, the number of electronic components that need to be installed also increases, and the power rails that need to be supplied are becoming more and more complex, so an optimized combination of cost, size and features is required.

Since the driving range cannot be sacrificed, a high-efficiency power system is required.

Since there are other design elements in addition to functional design (such as the noise standard of CISPR25 Class5), it is necessary to design not only the product itself but also the entire on-board unit.

To improve the safety functions of units and systems, the ability to monitor power rails, detect electronic circuit failures and transmit the corresponding information to the CPU is required.

To meet such market demands, ROHM has developed reference designs that meet the design elements required for unit design and has begun to make the design data publicly available.

Overview of reference design "REFRPT001"

The outline of the reference design "REFRPT001" introduced this time is as follows:

Equipped with power functions for 8 systems, covering the power rails required for ADAS/infotainment functions.

The primary (*1) DC/DC converter IC adopts the "BD9P Series", which can stably supply power even when the input voltage is lower than the set output voltage (such as when starting with a battery).

The secondary (*1) DC/DC converter IC adopts the "BD9S series" which features ultra-small size and ultra-high efficiency.

The power monitoring IC "BD39040MUF-C" is adopted, which can monitor the output voltage of all 8 systems and has an IC self-diagnosis function, which helps to improve the functional safety level.

System-level verification has been completed

-Completed standard electrical characteristics test

-EMC test completed (without input filter, meets CISPR25 Class 5 requirements)

- Thermal testing completed (distributed configuration of high-efficiency DC/DC converter ICs to disperse heat)

The ICs and discrete components used are in compliance with the automotive AEC-Q100 and AEC-Q101 standards.

Major ICs all support functional safety "FS supportive(*2)".

The appearance diagram (Figure 1) and block diagram (Figure 2) of the reference board "REFRPT001-EVK-001" of the reference design "REFRPT001" are as follows. Assumptions: Four system outputs are branched from the primary DC/DC converter "BD9P series" of two systems, and power is supplied to SoC, MCU, and CAN devices. In addition, since the output power rails of the eight systems are monitored by the power monitoring IC, it helps to improve the level of the functional safety system.

Figure 1. Appearance of the reference board "REFRPT001-EVK-001"

Figure 2. Block diagram of reference design “REFRPT001”

Next are the EMC test results disclosed as evaluation data. From the test results, it can be seen that even if the entire reference board is operated, without an input filter, the EMC radiation noise (antenna vertical) (Figure 3), radiation noise (antenna horizontal) (Figure 4), and conduction noise (Figure 5) all meet the CISPR25 Class 5 standard. An input filter mounting pattern for increasing the noise characteristic margin is also prepared in advance on the PCB, so it is also possible to take measures to add an input filter for EMC issues that will have a significant impact on the improvement work of the entire unit.

Full support for customer-designed content and tools

For the reference design "REFRPT001", the following data has been released on the ROHM website as a development tool (content and tool) to support customer designs.

Reference block diagram/reference circuit diagram/parts list (BOM)

PCB information/Gerber data

Test report (electrical characteristics, EMC characteristics, thermal characteristics)

 Free online simulation tool (partial circuit of reference design)

 SPICE models of the products installed

CAD tool symbols and pin pads of the products installed

Thermal model for thermal simulation of the products installed

In addition, as mentioned above, you can also use the ROHM Solution Simulator (*3) to simulate some of the circuits in this reference design. The ROHM Solution Simulator is a free online simulation tool that also provides standard circuits including peripheral circuits, so you can easily simulate without preparing simulation circuits and models. Next, we will introduce a simulation example.

Figure 6 shows a simulation example of a power tree for two systems. The power tree is powered by a battery, and the primary DC/DC converter IC "BD9P105" is equipped with a secondary

DC/DC converter IC "BD9S201" and LDO "BD00IA5M". (Please click here to view the simulation circuit. Registration for "My ROHM" is required.)

Figure 7 is a simulation example of a power tree for three systems. The power tree is powered by a battery, and the primary DC/DC converter IC "BD9P205" is equipped with various

Secondary DC/DC converter IC. (Click here to view the simulation circuit. Registration for "My ROHM" is required.)

Products that incorporate advanced technologies create ROHM's unique reference designs

This featured reference design is made up of products that combine advanced technologies and features.

BD9P Series (products using NanoPulseControl™ technology (*4))

-42V withstand voltage, automotive primary DC/DC converter IC series (Table 1)

- Excellent high-speed response performance, stable power supply immediately after battery startup

-With spread spectrum function, low EMI (low noise)

-Support for functional safety "FS supportive"

BD9S series

-Automotive Secondary DC/DC Converter IC Series (Table 2)

- Equipped with an output voltage monitoring function that helps improve system reliability, and the soft start time can be set

-Ultra-efficient operation

-Switching frequency 2.2MHz (typ.), no interference with AM band

-Support for functional safety "FS supportive"

BD39040MUF-C

-Power supply monitoring IC with built-in self-diagnosis function (BIST) and functional safety support (Figure 8)

- Equipped with adjustable watchdog timer (Window type) and overvoltage monitoring, undervoltage monitoring, and reset functions

- Equipped with self-diagnostic function (BIST), which can not only monitor the power rails of the system, but also detect potential faults

-The internal reference voltage circuit and oscillation circuit are multiplexed, and the probability of failure is significantly reduced

-Support for functional safety "FS supportive"

- Small 3mm x 3mm package for easy system modification

RBR3LAM60BTF

Automotive grade (AEC-Q101 qualified), high reliability, 60V Schottky barrier diode. In this reference design, it is used as a backflow prevention diode at the battery input. In order to minimize the voltage drop caused by the forward voltage (Vf) of the diode, the RBR series with very low Vf is used.

RV4C020ZPHZG

Automotive grade (AEC-Q101 compliant), high reliability, 1.5V driven low on-resistance PchMOSFET. Bottom electrode package, but with a wettable flank shape for higher mounting reliability, visibility after mounting is better. In this reference design, it is used as a load switch to branch off a 3.3V system.