Because this reference design is targeted at automotive applications, there are several considerations that limit design choices. Additionally, the following list of system-level specifications helped shape the final overall design:

• The total solution size must be minimized to meet the size requirement of this design, which is equivalent to 20 mm × 20 mm. This means choosing parts that integrate FETs, diodes, compensation networks, and feedback resistor dividers to eliminate the need for external circuitry.
• To avoid interference with the AM radio band, all switching frequencies must be greater than 1700 kHz or lower than 540 kHz. Lower switching frequencies are less desirable in this case because they require large inductors and can still produce harmonics in the AM band. For this reason, this reference design looks at higher frequency switchers.
• All devices must be AEC Q100 (-Q1) rated.

Before parts are chosen, the input voltage range, required voltage rails, and required current per rail must be known. In this case, the input voltage is a pre-regulated 12-V supply coming in over the coaxial cable. This system has only two main devices, the imager and serializer, which are responsible for power consumption during operation. Table 2-1 shows the requirements of the supplies:

The 12-V supply over the coaxial cable is first stepped down to 3.3 V, which then supplies the rest of the system on the camera module. In this design, the 1.8-V rail supplies both the DS90UB953 supply, and the interface supply of the AR0820 imager. The AR0820 2.8-V analog rail requires 141 mA, the DS90UB953 serializer 1.8-V rail requires 225 mA, and the AR0820 digital 1.2-V rail requires 217 mA.

Assuming 85% efficiency to simplify calculations with the previous values, it is calculated that the 3.3-V supply will require 124 mA to successfully power the 1.2-V, 1.8-V, and 2.8-V rails. Because the input and output voltages, output current requirements, and total wattage consumption are known, calculate the input current using Equation 3.

This information provides a strong foundation in the selection of power topologies and inductive passives that are explained in later sections.

Due to the requirement of Q100, it is mandatory that the switching frequency is rated outside of the AM band and must satisfy the voltage and current requirements derived previously. As the input voltage is a regulated voltage that will always be greater than any of the power rails produced, the power topologies selected should either be step-down converters (bucks) or LDOs. Bucks are generally included in supplies where switching noise is not a significant concern, and power savings is the largest care about. Conversely, LDOs can be incorporated in establishing low-noise analog supplies that reduce inherent noise and are more robust against EMI interactions; however, this is at the expense of larger current consumption.

In this design, a single Power Management IC is responsible for powering the supply rails. This device, the TPS650330-Q1, was chosen as it incorporates three step-down converters (BUCKS) and an LDO in a single 4.0-mm x 4.0-mm VQFN package. The current requirements of the design also played an important role in the selection of the device, as the secondary BUCKS are capable of providing 1200 mA, while the LDO is capable of supplying a maximum current output of 300 mA. BUCK1 steps down the 12-V POC input to 3.3 V. The 3.3-V rail then supplies power to BUCK2, BUCK3, and the LDO input. BUCK 2 provides the interface and digital supply for both the AR0820 imager and DS90UB953 serializer, while the LDO output creates a clean, low-noise supply for the 2.8-V analog supply for the AR0820.