Passive Components around ADAS Applications By Ron Demcko, AVX Fellow, AVX Corporation

Transcription

1 Passive Components around ADAS Applications By Ron Demcko, AVX Fellow, AVX Corporation The importance of high reliability - high performance electronics is accelerating as Advanced Driver Assistance Systems (ADAS) and their associated sense and support modules become common place in automobiles. System speeds and complexity are increasing, bringing with them a concern over signal and power integrity within ADAS-related building blocks. While many ADAS modules such as blind spot detection and lane departure warning are already in production vehicles, true radar and lidar adaptive cruise control and collision avoidance systems are still being developed. With over 40 years of signal processing and signal conditioning experience, Analog Devices is at the forefront of ADAS system design. The ADF5604 is a 4-channel, 24 GHz receiver downconverter that achieves a 10 db noise figure which is 3 db better than competition devices. Together with the ADAR channel, 16-bit DAC, they represent the industry s best combination of low noise performance, high linearity, and low power consumption. This combined performance is important when used to build a radar sensor actuator where improved receiver sensitivity and detection range are critical. Many IC-based systems are receiver noise limited, resulting in lower overall signal-to-noise ratio (SNR). Higher system noise can mask or hide smaller objects (such as a child or small pole) or targets while in the presence of larger object such as a wall which can potentially cause a safety issue when used in an automotive detection application. Any factor that can reduce overall system noise is crucial for ADAS system design. Two relatively new AEC Q200 components from AVX that can address and simplify ADAS related designs are: 1) Miniature, wideband EMI filters 2) Low inductance decoupling capacitors Miniature Wideband EMI Filters AVX Surface Mount Technology (SMT) FeedThru filters are miniature wideband EMI filters in a small Surface Mount Device (SMD) package. (Figure 1) FeedThru filters are a single component solution that can be used to replace discrete LC T-type filters. They can also replace the use of multiple parallel capacitors that are typically selected for a wideband frequency response used on Vcc lines for power conditioning.

2 Automotive grade SMT FeedThru filters come in either 0805 or 1206 case sizes. Each FeedThru is the equivalent of two inductors and a capacitor connected in a T filter configuration (Figure 2). A key feature of each 0805 Feedthru is its ability to replace three 0603 or 0805 discrete components; an extremely attractive feature for designers. In addition to the board space savings, weight is improved by a factor of ~3 and reliability is improved since the FeedThru has a Failure In Time (FIT) Rate of better than 1. Manufacturing process throughput is also Figure 1 : FeedThru filters replace multiple parallel capacitors typically selected for a wideband frequency response used on Vcc lines for power conditioning. (Source: AVX.com) improved with just 1 pick place and solder operation. All X7R AVX W2F/W3F/W2H are RoHS compliant and AEC Q-200 qualified versions are available. Perhaps the biggest advantage of using an AVX SMT FeedThru is a combination of multiple electrical characteristics. 1) FeedThrus offer wideband attenuation of approximately -30dB across RF spectrums as wide as 750MHz. The wideband response results in a continuous elimination of emissions across a broad RF spectrum. Unlike the discrete solution no birdies (false signals) can radiate within the attenuation range due to the parasitic interactions of discrete components causing discontinuous filtering. Figure 2:SMT FeedThru configuration. The equivalent circuit for a FeedThru filter is a T- network of two inductors and one capacitor. (Source: AVX.com) The following Frequency response curves are representative of some of the various SMT FeedThru filters available:

3 Figure 3: Frequency response curves for AVX s v rated NPO & X7R filters. (Source: AVX.com) Figure 4: Frequency response curves for AVX's v rated X7R filters. (Source: AVX.com)

4 2) FeedThrus offer high current-carrying capability, which is advantageous for broad-band power line filtering. Furthermore, AVX SMT devices do not experience hysteresis due to increased temperature or high current loads. An example of typical through-put for current versus temperature rise is shown in Figure 5 below: Figure 5: An example of typical through-put for current versus temperature rise in a component with a 0805 case size. (Source: AVX.com) 3) FeedThru Filters offer a broad impedance vs. frequency advantage in replacing multiple capacitors on Vcc. Figure 6: Frequency response curves for 1206 case size 50v rated X7R filter. (Source: AVX.com)

5 Applications Common applications for AVX SMT filters are: - Critical control & signal line filtering - Vcc power filtering - Mixed signal isolation - Multiple module power isolation General specifications by case size are shown in Table 1. (More details are available at: Parameter W2H High Current W2F W3F Case Size Voltage Rating 25/50/100v DC 50/100v DC 50/100v DC Current Rating < /= 2000ma </= 300ma </=300ma Capacitance Loading 22pf to 100nf 22pf to 47nf 22pf to 47nf Capacitor Dielectric NPO / X7R NPO / X7R NPO / X7R Temperature Range -55c to 125c -55c to 125c -55c to 125c Wideband SMT FeedThru Product Types Table 1 Low Inductance Decoupling Capacitors Low inductance decoupling capacitors can improve high frequency noise filtering and decoupling of the Vcc lines for ICs due to their extended frequency response and improved impedance vs frequency characteristics. The illustration in Figure 7 shows a photo of some 0508 reverse-terminated Low Inductance Ceramic Capacitors (LICC). The corresponding graph in Figure 8 shows a comparison between standard EIA case size MLCCs and reverse geometry capacitors. Capacitors with reverse geometry LICC type capacitors have a higher resonant point and lower Equivalent Series Resistance (ESR) than standard MLCCs. Figure 7: 0508 reverse-terminated Low Inductance Ceramic Capacitors (LICC). (Source: AVX.com)

6 In many cases, the use of low inductance capacitors allows one to reduce the number of decoupling capacitors, saving board space, reducing weight, and improving reliability by eliminating components and solder joints. Theory of Operation Figure 8: An Impedance (Ohms) vs. Frequency (MHz) comparison for 0805 vs 0508 LICC. (Source: AVX.com) An ideal capacitor can transfer all of its stored energy to a load instantly. A real world capacitor has parasitic elements that prevent instantaneous transfer of a capacitor s stored energy. However, the true nature of a capacitor can be modeled as an RLC equivalent circuit. For most simulation purposes, it is possible to model the characteristics of a real capacitor with one capacitor, one resistor, and one inductor. The RLC values in this model are commonly referred to as equivalent series capacitance (ESC), equivalent series resistance (ESR), and equivalent series inductance (ESL). The ESL of a capacitor determines the speed of energy transfer to a load. The lower the ESL of a capacitor, the faster that energy can be transferred to a load. Lower MLCC parasitic inductance (ESL) plays a major role in creating a high efficiency, low inductance decoupling capacitor. Furthermore, it s the geometry of the capacitor that affects the capacitor s inductance. The Evolution of Low Inductance Capacitors There are many types of low inductance MLCCs available to designers. The below graph is a comparison of performance by MLCC type. The device to the far left in Figure 9 is that of a traditional EIA case size MLCC that is terminated on the ends. This traditional case size MLCC is used as a base line to compare against reduced inductance devices. The second illustration from the left in Figure 9 (the LICC) shows the same case size capacitor, but now is terminated on the long side of the body. This is the simplest of the reverse geometry MLCCs and ESL is reduced almost in proportion to the reduction in spacing or loop area of the opposing electrodes. In fact, the ESL is reduced by approximately 60% over a standard MLCC. The third illustration from the left, shown under the two terminal devices grouping, shows the effect that vertical electrodes have in reducing in the current loop area, thereby reducing inductance.

7 Figure 9: A comparison of performance by MLCC type. The traditional case size MLCC on the far left is used as a base line to compare against reduced inductance devices. LICC capacitors are AEC Q200 qualified and are currently the preferred MLCC with low inductance for automotive designs. A discussion of low inductance MLCCs would not be complete without a discussion of multiterminal devices. Currently these AVX devices are not AEC Q200 qualified, but can be qualified once end-use circuits call out their level of performance. The two terminal inductance loop reduction is limited by the practical limits of MLCC dimensions, therefore, once that limit is hit, multi-terminal MLCCs must be used. Multi-Terminal MLCCs allow loop inductance between the electrodes to be reduced well beyond that of most two terminal devices. The first of the multi terminal devices to arrive was the Inter-Digitated Capacitor, or IDC. Terminals of alternate polarity are brought out of the capacitor s body in such a way that the loop inductance is approximately 80% lower than a standard MLCC. The next devices in the multi-terminal family are the Low Inductance Capacitor Array (LICA) and their successor, the multi terminal LGA. These components incorporate vertical electrodes to further reduce loop area. Multi-terminal, low inductance capacitors provide outstanding performance for the most advanced IC types. As automotive requirements dictate, they will become AEC Q-200 qualified.

8 General specifications by case size are shown in Table 2. Parameter Dielectric X7R X7R X7R Voltage Range 6.3v - 25v 6.3v 50v v Capacitance Range 1nf - 68nf 1nf - 1uf 1nf 2.2uf Table 2: General specifications by case size of AVX LGA Low Inductance Capacitors. More details are available at: Over the lifetime of a car, systems will experience transient surges including overvoltage. Protecting sensitive systems through various methods of circuit protection, including simple but very effective EMI filtering, is a critical to the longevity of automobiles. Novel advanced ceramic capacitors can be utilized to provide high reliability high performance systems.