Mastering the Art of Illumination
Nazzareno (Reno) Rossetti and Yin Wu from Maxim Integrated discuss how a versatile LED controller can streamline the design of automotive exterior lighting.
LEDs are increasingly popular in car exterior lighting because of their excellent lighting properties, efficiency, and design flexibility. They can be used in various shapes and incorporate diverse features.
The different LED setups needed for automotive lighting, along with the changing car battery voltage, require numerous specialized integrated circuits for different vehicle lighting functions. A flexible controller IC, however, can handle multiple architectures and make automotive exterior lighting design much simpler.
Powering LEDs
LEDs are used in many car functions, from single lamps to chains and grids. They light up high beams, low beams, fog lights, daytime running lights (DRLs), position lights, and turn signals.
Car batteries usually provide an input voltage of 12V, but this can rise to 16V when fully charged. Cars with start-stop technology experience significant voltage drops when the engine starts, so the power can go well below the typical 12V, sometimes to 6V or less.
The switching regulators that power the LEDs must meet specific criteria. They need to work across the entire voltage range supplied by the battery and withstand 60V voltage spikes. The current amplitude must be precise to maintain the LED’s color.
Dimming is achieved by slicing the LED current using pulse-width modulation (PWM), which reduces brightness without changing color. The PWM dimming frequency must be above 100Hz to avoid flickering detectable by the human eye.
To limit radio frequency interference, a high and stable PWM switching frequency above the AM frequency band is necessary. Spread-spectrum modulation is required to meet EMI standards. High efficiency is also important to reduce heat and boost system reliability.
Basic Headlight Design
A basic headlight setup with a series of LEDs employs a boost converter. A boost controller IC has three feedback loops: one for tight output current control (current loop), one for overvoltage protection (OVP loop), and one for overcurrent protection (OCP loop) within the string of 12 diodes, generating 42V (3.5V per LED).
Along with current and voltage control, the IC should offer dimming, spread spectrum, and similar features. High-side current sensing via resistors (Rx) protects the LED system from short circuits to ground or battery input.
Flexible Architecture
An ideal LED controller should support various configurations to implement different features. Besides the boost configuration, consider the buck-boost configuration as well.
A buck-boost mode configuration is necessary when dealing with a short diode string, such as two or three LEDs (7 or 10.5V), compared to a battery voltage that can range below 6V (cold crank) up to 16V. For input-to-output isolation concerns, a single-ended primary inductance converter (SEPIC) or a continuous output current converter (CHUK) might be suitable.
A single controller supporting multiple architectures offers clear benefits, such as cost savings and ease of reuse. It can drive LEDs in boost, high-side buck, SEPIC mode, or buck-boost mode configurations. This device can serve as a single-channel high-brightness LED (HB LED) controller for various automotive front-lighting applications like high beam, low beam, DRLs, turn signals, and fog lights.
