【Introduction】In today’s vehicles, the importance of advanced driver assistance systems (ADAS) is increasing day by day. They can help minimize human error and improve driver and road safety. Early ADAS included only a single automatic driver assistance function, such as adaptive cruise control using a radar sensor. Now, more and more ADAS functions are being applied to cars, such as automatic emergency parking, blind spot monitoring, vehicle/pedestrian warning and avoidance, lane departure warning and assistance, etc.
The development of ADAS means that these vehicles will use more sensors and cameras, powerful real-time data processing and computing, and high-speed communications, so they will consume more power than before. For example, first-generation ADAS system-on-chips (SoCs) such as the Mobileye EyeQ from 2008 consume only 2 W to 3 W of power. Newly released ADAS SoCs, such as NVIDIA® Xavier™, consume 20W~30W or more power due to powerful data processing and computing power. Power for the ADAS is provided by a 12 V battery. It is first converted to a 5 V or 3.3 V intermediate supply rail and then to the different low voltages required by the SoC core, interfaces, peripherals, etc. After the power consumption of ADAS SoCs has increased, intermediate rail converters need to output 10 A or more to meet their requirements.
When designing a high-current intermediate power supply, the traditional approach is to use a buck controller. However, the overall solution size of this approach is larger due to the need to use external MOSFETs. This makes it difficult to fit controller power solutions into space-constrained locations, a common problem faced in automotive ADAS applications. Another problem with automotive switching power supplies: electromagnetic radiation. Power supply designers need to address the challenges of stringent radiated and conducted electromagnetic emissions limits that the automotive industry must adhere to. These electromagnetic emission standards are more difficult to meet after power consumption increases. To meet power, size, and electromagnetic emissions constraints, Analog Devices has developed two 42 V high-current monolithic Silent Switcher regulators: the LT8638S and the LT8648S.
Compact 10A/12A Peak Power Solution Using LT8638S
The LT8638S is a 42 V, 10 A, single-channel buck regulator in a 4 mm × 5 mm LQFN package that contains all control circuitry and MOSFETs. In a short time, its output current can reach 12 A. The LT8638S is a very suitable alternative for the compact 10 A intermediate supply rail. Figure 1 shows the schematic of a typical 5 V/10 A LT8638S. The switching frequency of the LT8638S regulator is adjustable from 200 kHz to 3 MHz. Table 1 lists the main components of the 400 kHz LT8638S circuit and the 2 MHz LT8638S circuit. Figure 2 shows the efficiency and temperature rise of the LT8638S at 400 kHz and 2 MHz on the demo board DC2929A.
Figure 1. 5 V/10 A power supply using the LT8638S.
Figure 2. Efficiency and temperature rise for the circuit shown in Figure 1.
Table 1. Components included in the schematic shown in Figure 1
Comparing the LT8638S 400 kHz circuit and the LT8638S 2 MHz circuit, it can be seen that the inductance size of the 400 kHz circuit is 2.5 times the size of the 2 MHz Inductor, and the output capacitance of the 400 kHz circuit is 3 times the size of the 2 MHz output capacitor. Therefore, for applications where size and cost are very important, 2 MHz switching frequency is more suitable. The main issues preventing power design engineers from using 2 MHz (switching frequency) are efficiency and thermal performance, as switching losses increase significantly at high switching frequencies. The LT8638S mitigates these issues by using fast switching edges to minimize switching losses, as shown in Figure 3. As shown in Figure 2, at a switching frequency of 2 MHz and an output power of 50 W, the temperature of the LT8638S rose by only 60°C. The efficiency difference between 2 MHz and 400 kHz switching frequencies is within 1.5% at a 10 A load.
Figure 3. Switching edges of the LT8638S with a 12 V input and a 10 A load.
At high switching frequencies, fast switching edges help improve efficiency, but can exacerbate electromagnetic emissions. The LT8638S uses the Silent Switcher architecture, which not only supports fast switching edges, but also achieves lower EMI and a smaller solution size. Figure 4 shows the 2 MHz LT8638S circuit with very low EMI. For excellent EMI performance, the regulator is connected to the SYNC/MODE pin and the INTVCC pin to use spread spectrum mode. Figure 5 shows the radiated emissions of the LT8638S using the circuit of Figure 4, the test setup is defined by the CISPR 25 standard. The red line represents the CISPR 25 Class 5 limit, the most stringent emission specification in the automotive industry. With few additional components to form the input filter (shown in Figure 4), the LT8638S can meet the stringent peak and average limits of the CISPR 25 Category 5 specification.
Figure 4. LT8638S circuit with very low EMI.
Figure 5. Radiated and conducted EMI for the circuit shown in Figure 4 (12 V input converted to 3.3 V output at 10 A).
Higher current monolithic power solution using the LT8648S
Complex ADAS requires the use of multiple SoCs, as well as multiple cameras and sensors. For example, a contactless ADAS might employ multiple very power-hungry chips, as well as as many as 11 cameras. The LT8648S has higher output current capability than the LT8638S and can be installed on the intermediate supply rails required by these complex ADASs. The LT8648S is a monolithic 42 V, 15 A buck regulator with output current and power levels similar to power controller solutions using external MOSFETs. Its current capability can be further extended by using multiple LT8648S in parallel.
Figure 6 shows a schematic of a 3.3 V/25 A, 2 MHz circuit using two LT8648S devices in parallel. The two LT8648S regulators have a common input and output. The EN/UV and SS pins are connected to ensure that both regulators start up simultaneously at the same slew rate. The LT8648S uses peak current mode control so that the error amplifier output VC voltage is related to the load current. By connecting the VC and FB pins, the two parallel LT8648S achieve good current balance without the use of external circuitry. The CLKOUT pin of the U1 LT8648S is connected to the SYNC/MODE pin of the U2 LT8648S. Once connected, the two LT8648S regulators are synchronized and support 180° phase shift.
Figure 6. 2 MHz 3.3 V/25 A application using two LT8648S in parallel.
Figure 7 shows the efficiency and temperature rise of the circuit of Figure 6. The temperatures of U1 and U2 are nearly the same, indicating good current balance for this parallel application. High switching frequency and external compensation support fast transient response. Figure 8 shows the load transient response of the circuit shown in Figure 6.
Figure 7. Efficiency and temperature rise for the circuit shown in Figure 6.
Figure 8. 10 A to 20 A load transient response of the circuit shown in Figure 6.
This article describes two high current 42 V monolithic Silent Switcher regulators, the LT8638S and LT8648S. They feature high efficiency and low emissions, alleviating heat dissipation and EMI issues in harsh automotive application environments. The LT8638S and LT8648S integrate MOSFETs to provide a small footprint solution for the high current intermediate power supplies required by rapidly evolving automotive ADAS.