One chip drives the entire headlight assembly. How does this controller do this?

Because the driver that meets the power and functional requirements of each light string in the light cluster must accept a relatively wide range of battery voltages and convert them to a variety of light string voltages using a buck-boost topology - it must be small and versatile to easily fit into the very limited space of the light cluster and produce very low EMI, thereby minimizing the R&D effort and eliminating the need for expensive EMI metal shielding enclosures. And it should also be highly efficient.

The Power by Linear™ LT8391A 2MHz buck-boost controller is unique in meeting all of the above requirements, driving the entire headlight cluster, yet using only a single controller.

The LT8391A is the first of its kind 2MHz buck-boost controller for LED current regulation. The LT8391A’s very high 2MHz switching frequency allows the use of a single small inductor and achieves a small total solution size for high power LED applications. Unlike monolithic converters that have the power switch built into the IC package, controllers such as the LT8391A can drive an external power switch at much higher peak currents, such as 10A. Such peak currents would burn out the small IC packages of typical integrated converters. In contrast, controllers using external 3mm × 3mm synchronous MOSFETs can deliver much higher power. These MOSFETs can be placed in a small space along with hot loop capacitors for very low EMI. The unique peak switch current sense amplifier architecture places the sense resistor next to the power inductor (outside the critical input and output hot loops), which also reduces EMI. Optional spread spectrum frequency modulation (SSFM) further reduces the controller’s EMI.

The 2MHz LT8391A 16V, 1.5A (24W) buck-boost LED driver in Figure 1 has up to 93% efficiency with an EMI filter and gate resistor, as shown in Figure 2. When optional EMI components are removed, efficiency can be improved by 1%–2%. Using small 3mm × 3mm MOSFETs and a single high power inductor, the converter’s temperature rise is low even at 24W of power dissipation. At 12V input, all components rise by less than 25°C above room temperature. At 6V input, the hottest component rises by less than 50°C on a standard 4-layer PCB with no heat sink or cooling airflow. The converter can run continuously at full 24W load in the face of input transients as low as 4.3V, or reduce load current with analog or PWM dimming when the input drops for an extended period of time. 8A–10A sense resistors make it possible to deliver this high power at low VIN.

Figure 1: The LT8391A 2MHz 16V, 1.5A automotive buck-boost LED driver meets CISPR 25 Class 5 EMI specifications.

Figure 2: Efficiency of the LED driver solution shown in Figure 1. Measurements made using the 16V, 1.5A demonstration circuit DC2575A LED driver (with and without optional EMI components)

The LT8391A features state-of-the-art PWM dimming features and open-circuit LED fault protection. This synchronous buck-boost controller regulates current through a string of LEDs whose voltage may or may not be within the input voltage range (e.g., 9V–16V automotive battery or 18V–32V truck battery). It operates from cold-crank inputs as low as 4.0V and withstands input transients up to 60V. The LT8391A provides up to 2000:1 PWM dimming ratios at 120Hz, and it can use its internal PWM dimming signal generator (without an externally supplied PWM clock) to achieve accurate dimming ratios up to 128:1.

The 2MHz LT8391A LED driver shown in Figure 1 is designed for automotive headlamps. It uses AEC-Q100 components and meets CISPR 25 Class 5 radiated EMI standards. Spread spectrum frequency modulation (SSFM) reduces EMI and also operates flicker-free while performing PWM dimming, as shown in Figure 7. The small size of the LT8391A is highlighted by its small inductor and exceptionally small input and output EMI filters. For a 2MHz converter, no large LC filters are required, instead only small ferrite beads are used to reduce high frequency EMI.

Automotive EMI requirements are not easily met with high power converters. High power switches and inductors placed on a large PCB area and close to large capacitors can create undesirable hot loops, especially when a large sense resistor is included. The unique LT8391A buck-boost architecture removes the sense resistor from the buck and boost switch pair hot loops, resulting in low EMI.

Figures 3 and 4 show the measured EMI of the 24W LED driver shown in Figure 1. Despite the 2MHz operating frequency and 24W power of the controller, this buck-boost LED driver still meets the CISPR 25 Class 5 radiated and conducted EMI specifications. Class 5 specifications are the most stringent requirements and goals for most automotive EMI testing. Converters that do not meet Class 5 EMI specifications are either designed outside the automotive circuit or must be enclosed in a large metal EMI shield. Although the shield is bulky and does not cause assembly problems, the cost of adding a shield is expensive.

Figure 3: LT8391A demonstration circuit DC2575A meets CISPR 25 Class 5 automotive radiated EMI specification requirements

Figure 4: LT8391A demonstration circuit DC2575A meets CISPR 25 Class 5 automotive conducted EMI specifications

LED headlight clusters can be innovative and artistic. High and low beams can be "wrapped" with stylish and distinctive daytime running lights (DRL). Because the DRLs are only needed when the high and low beams are off, a single LED driver can be used to power the high and low beam LEDs or the DRLs. This only works if the LED driver has a flexible input-to-output ratio and can step up and down the input-to-output voltage. Buck-boost designs meet this requirement.

The multi-beam LT8391A buck-boost LED driver in Figure 5 is capable of driving LED string voltages from 3V to 34V. This enables it to drive a low-beam string and create a high-beam by adding LEDs to the low-beam string. The same driver can then be switched to drive a higher voltage, but lower current DRL.

Figure 5: LT8391A multi-beam LED headlight cluster solution for low beam, high beam, and DRL lamps

Switching from a low-beam-only LED to a low/high-beam combination string generates no spikes in the output voltage or LED current, as shown in Figure 6a. The LT8391A smoothly transitions between the boost, 4-switch buck-boost, and buck operating regions. Changing from a low-to-high-beam string without LED spikes can be a challenge for the converter, but this multi-beam LED circuit does it with ease. Switching from high-and-low-beam combination mode to low-beam-only mode is also very clean, without any harmful LED spikes, as shown in Figure 6b.

Figure 6: Waveforms showing smooth switching between high beam + low beam, low beam, and DRL LED strings for the LT8391A multi-beam LED lamp application circuit shown in Figure 5

The same is true when switching to or from the DRL string. Figure 6c shows how to turn off the low beam and smoothly connect the DRL to the output capacitor. Even when the LED current changes from 1A (high and low beam) to 700mA (DRL with 8 LEDs), there are no problems. Additional trim LEDs or signal LEDs can be added, and the DRL can be flashed as a signal light. Figure 6d shows how to PWM dim the DRL using the internally set PWM generator and then smoothly switch to low beam when darkness falls.

The automotive environment requires a robust solution that can function properly in the presence of LED short and open circuit conditions. LED short and open circuit conditions are safely handled with the multi-beam LED lamp solution shown in Figure 6 and reported through the converter's fault flags.

The LT8391A is available in a 4mm × 5mm 28-lead QFN package for small form factor applications and a 28-lead TSSOP FE package for automotive designs. Both packages feature a thermally enhanced GND pad to help dissipate the power dissipated by the internal INTVCC LDO due to the higher voltage.

The internal LDO INTVCC regulator of these converters can drive four synchronous MOSFETs at 2MHz with approximately 15nC gate charge. Figure 7 shows the small size of the LT8391A FE 2MHz 16V, 1.5A demonstration circuit (DC2575A, based on the design shown in Figure 1). For this high power general purpose application, only a single 5mm × 5mm inductor is required.

Figure 7: Compact solution: 2MHz demonstration circuit DC2575A using LT8391A drives 16V LED at 1.5A

Figure 8: PWM dimming with internal and external PWM options; 1% and 0.05% respectively

The LT8391A 2MHz, 60V buck-boost LED driver controller powers LED strings in automotive headlights. Features of the device include its low EMI four-switch architecture and spread spectrum frequency modulation to meet CISPR 25 Class 5 EMI specifications. The unique high switching frequency allows it to operate above the AM band, requiring very little EMI filtering. Its small size and versatility enable it to be used in headlight cluster LED strings with a variety of voltages and currents.