Coaxial Cable Power: Automotive Application Design Guide

Automakers use more cameras and sensors to achieve automotive safety requirements, while Coaxial Cable Power (PoC) provides automotive designers with a compact solution to reduce body weight. However, there is nothing perfect in the world that can cause problems when delivering power and front and rear channel signals over the same cable. In addition, the on-board battery used to power the system produces a wide voltage offset as low as 3V during cold-start operation and up to 42V under clamp load dump or other transient conditions. In order to ensure that important systems such as the Advanced Driver Assistance System (ADAS) can operate in any car condition, a well-designed power supply is essential.

Figure 1 is an example of an ADAS system equipped with a widely used flat panel display (FPD) link III digital video interface. The deserializer transmits and controls signals over the coaxial cable, and the serializer sends back the video signal over the same cable. The system has four significant power modules: the deserializer power supply, the camera power supply on the deserializer side, the serializer power supply, and the camera image sensor power supply.

Figure 1: Block diagram of the megapixel camera system module

The biggest challenge of the megapixel camera system is the potential voltage drop of the coaxial cable. To avoid signal integrity issues due to voltage drops, the deserializer voltage must be increased to at least 9V before transmitting the PoC. Once power is delivered to the serializer side, the voltage must be adjusted back to the required operating voltage of the serializer and image sensor.

We take a look at these module diagrams. Starting from the serializer side, size is a major consideration (and noise and power supply rejection ratio [PSRR]), I recommend using the LM53600-Q1 buck regulator, as well as for serializer power and image sensor power supplies. LP5912-Q1 Low Dropout Linear Regulator (LDO). The LM53600-Q1 features a wide input voltage range of 3.55V - 36V, a transient voltage of up to 42V, 23μA quiescent current (IQ) and 3mm &TImes; 3mm package size. It is also recommended that you use the LP5912-Q1 with a power supply rejection ratio of up to 75dB at 1 kHz, an output noise of 12μVRMS, a quiescent current of 30μA, and a package size of 2mm &TImes; 2mm. Because power supply ripple and noise directly affect image quality, high power supply rejection ratios and low output noise are critical for camera applications. Both devices operate with very few external components, thereby enabling miniaturization of the overall solution size.

On the deserializer side, the deserializer power module is similar to the serializer power module. I also recommend using the LM53600-Q1 and LP5912-Q1 to reduce bill of materials (BOM) costs. In this case, you don't have to start with the size to save costs, you can use other solutions. However, these solutions may require the use of more external components, which increases overall design costs. To avoid using additional components, you can visit TI.com.cn to see other solutions that might be right for your design.

Finally, the camera power module delivers power from the deserializer to the serializer over a coaxial cable. The power from this module must be boosted to 9V and matched with the signal path. According to Section 8.5 of the DS90UB91x datasheet, the differential output voltage |VOD| (DOUT+ and DOUT-) is only between 269mV and 412mV. This means that the output of the module must be clean enough to avoid signal-to-noise ratio (SNR) degradation. Fortunately, the PoC filter (used to ensure serializer and deserializer 50? impedance matching) includes an inductor that helps block high frequency ripple from the boost converter. The higher the switching frequency, the greater the attenuation. For this application, I recommend a boost adjustment with a TPS61093-Q1 with a 1.2MHz switching frequency. As shown in Figure 2, after installing an inductor with a typical value of about 100μH in the PoC filter, the output ripple is attenuated by about -57dB, or 0.2% of the original ripple voltage from the boost converter.

Figure 2: Ripple attenuation produced by a PoC inductor

The megapixel camera has become one of the indispensable sensors in the automotive industry. As cars are used more and more sensors for safety and comfort, the use of well-designed systems for coaxial power transmission saves cost and space.

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