Design of automotive power clusters

Cluster displays are becoming increasingly large to accommodate more and more data available to drivers and to provide this information in real time. Cluster displays are shifting from traditional mechanical-based solutions to LCD-based designs, giving drivers more options to customize them and personalize their driving experience.

This trend presents some challenges for design engineers. While the motor drive is electromechanical and doesn't exhibit obvious interference from issues like EMC, the problem on an LCD display can be perceived as visible ripples (discontinuous lines), which can distract the driver. In fact, the motor drive can be shielded better than an LCD display; the LCD display must be more open, rather than hidden behind the front panel.

Secondly, modern cluster displays need to operate under both cold and warm start conditions. During these phases of vehicle operation, they can be particularly susceptible to large transients in system inputs and outputs. Such conditions can also cause display interference that may not be visible on the motor drive, such as blurred graphics or distortion.

Finally, as cluster displays become larger, the PCB grows to support the display, as it forms the backbone of the overall unit structure. The PCB significantly increases the cost of the overall solution and can become the most expensive item in the bill of materials. Therefore, cluster PCBs are typically designed as two-layer designs to maintain low cost. Two-layer designs are more susceptible to noise because only a few copper layers minimize noise passing through the power planes. This presents a considerable challenge for engineers who must design such systems.

Figure 1: Cluster power reference design.

    The TI Designs reference design, a CISPR 25 Category 5 rated 14W multi-output power supply reference design for automotive cluster units, can help overcome these problems or guide engineers to adopt best practices (Figure 1). This reference design is a two-layer board power supply optimized for conducted electromagnetic interference (EMI) in automotive dashboard units. The power supply has two cascaded buck regulators. The first-stage buck regulator uses an LM26003 to generate 5V at 2A output. The second stage uses an LM26420 to generate two outputs: 2.8V at 1.5A and 1.8V at 2A (total maximum output power of 14W).

    The LM26003 is a wide V-IN asynchronous buck regulator, and the LM26420 is a 5V input dual-channel 2A high-frequency synchronous buck regulator. Both meet AEC-Q100 Class 1 automotive standards.

This reference design has an input voltage range of 6.5V to 38V, making it suitable for 12V automotive battery systems and covering most cold and hot start conditions. If specifications require voltages below 6.5V, a pre-boost or buck-boost configuration is necessary. The two-layer board design includes an additional input filter stage to improve conducted EMC performance and minimize any challenges to the display, such as flickering lines, or information not being visible or displayed correctly. The board has been tested to automotive EMC standard CISPR 25, and its conducted emissions meet CISPR 25 Class 5 requirements. See Figure 2.

Figure 2: Conducted EMC in the 150kHz to 30MHz range using a clustered power reference design.

This design meets the stringent requirements of large LCD displays, manages thermal constraints efficiently (due to the limited number of PCB layers for heat dissipation), and keeps costs to a minimum for noise suppression with as few components as possible.

    The PMP9458 reference design is a 2-layer board power supply optimized for conducted EMI in automotive dashboard units. The power supply features two cascaded buck regulators: the first stage uses an LM26003 to produce a 5V, 2A output, and the second stage uses an LM26420 to produce two outputs: 2.8V, 1.5A and 1.8V, 2A (maximum total output power 14W). The LM26003 is a wide input voltage asynchronous buck regulator, and the LM26420 is a 5V input dual 2A high-frequency synchronous buck regulator. Both are available in automotive-grade versions, compliant with AEC-Q100 Class 1 standards. The design has an input voltage range of 6.5V to 38V, making it suitable for automotive 12V battery systems. The 2-layer board design optimizes PCB layout and includes an additional input filter stage to improve conducted EMI performance.

feature

    • EMI-optimized 2-layer PCB design

    • Complies with CISPR 25 Category 5 conducted emissions standards

    • Wide input voltage range of 6.5V to 38V

    Both the LM26003 and LM26420 are available in versions compliant with the AEC-Q100 standard.

    • Power-on/off can be configured via jumper pins

    This circuit board has been tested, including design documents and test reports.

 The LM26420 regulator is a monolithic, high-efficiency, dual-path PWM step-down DC/DC converter. This device can drive two 2-A loads via an internal 75-mΩ PMOS top switch and an internal 50-mΩ NMOS bottom switch, achieving optimal power density using state-of-the-art BICMOS technology. World-class control circuitry allows for on-time as low as 30 ns, enabling exceptionally high-frequency switching across the entire 3V to 5.5V input operating range, with a minimum output voltage of 0.8V.

Despite its high operating frequency, efficiency up to 93% is easily achieved. It features ultra-low standby current, including external shutdown. The LM26420 utilizes current-mode control and internal compensation to provide high-performance regulation under a wide range of operating conditions.

The LM26003 is a switching regulator designed to meet the high efficiency requirements of applications with standby modes. This device features a low-current sleep mode to maintain efficiency under light load conditions and current-mode control for precise regulation over a wide input voltage range. In shutdown mode, the quiescent current typically drops to 10.8 µA, while in sleep mode it is below 40 µA. A forced PWM mode can also be used to disable sleep mode.

The LM26003 device provides up to 3A of continuous load current and a fixed current limit via an internal N-channel switch. It features a wide input voltage range of 4.0 V to 38 V and can operate with input voltages as low as 3 V during line transients.

The operating frequency can be adjusted from 150 kHz to 500 kHz via a single resistor and can be synchronized with an external clock.

Other features include power-good, adjustable soft-start, enable pin, input undervoltage protection, and an internal bootstrap diode for reducing component count.