■ LED system switching technology utilizes various topologies


Designing power conversion regulators is a challenging and complex task, especially in diverse systems involving numerous regulators used in automotive power supply systems. This article will help you select the right switching topology for your automotive LED driver by examining the benefits, tradeoffs, and suitable applications for each topology.

LEDs differ from traditional filament bulbs or gas lamps. Utilizing specific semiconductor bonding principles, LEDs can produce light of a wide range of colors, extending beyond the entire visible spectrum to infrared (IR) and ultraviolet (UV). In automotive applications, LEDs can enhance safety in all driving conditions, day and night. Their improved efficiency extends the battery life of electric vehicles, and the use of multiple LEDs in a single system prevents single-element failures.

Additionally, LEDs are versatile and can be driven in a variety of ways. Because the output from LEDs is well-controlled light, LED loads differ from traditional loads in terms of power systems. LEDs generate light using only a precisely regulated current through semiconductor junctions, and their terminal voltages are independent of system ground (or the chassis in automotive systems). Therefore, LED systems can utilize a variety of topologies using switching technology.

■ Selecting the right switching topology for a vehicle LED system requires careful consideration of the overall system level.

Selecting the right switching topology for an automotive system requires consideration of many factors at the system level. These include minimum input voltage, maximum string voltage, chassis return capability, output short-circuit capability, maximum input current, output/LED current, and PWM dimming.

○ Step-down (buck) converter

A step-down (buck) LED driver regulates the LED string current using a voltage higher than the total LED string voltage. The buck LED driver can be safely shorted to system ground.

It features chassis returns (one wire is used for power) and is easily configurable for matrix or animation applications.

The circuit examples in Figures 1 and 2 show a system diagram in which the controller modulates the upper switch to control current.


Step-down LED drivers feature constant frequency operation, high efficiency through excellent switching control and low resistance switches, high accuracy across the analog dimming range, excellent EMI, and well-designed spread spectrum frequency modulation.
○ Step-up (boost) converter

A step-up (boost) LED driver regulates the LED string current using a voltage lower than the total LED string voltage. This topology is useful when multiple LEDs must be powered in a single string. Typical 12V automotive systems operate from 6V to 18V, so the LED driver must operate at a minimum of 6V and provide a high step-up ratio. The circuit examples in Figures 3 and 4 show a system diagram in which the controller modulates the high-side switch to control current.


○ Boost-buck using a boost converter

Some step-up (or boost) LED drivers can be configured to return the LED cathode to the power supply. This configuration is called a boost-buck. The total output voltage is VIN (VBATTERY), which is added to the total LED string voltage. The advantage of this topology is that it can drive LED strings that are higher, lower, or equal to the supply voltage. This topology is limited only by the converter. The lower limit is limited by the minimum power supply voltage of the controller IC, and the upper limit is limited by the maximum output voltage of the controller IC.


○ Buck mode using boost converter

Some step-up (boost) LED drivers can also be configured to step down the supply voltage (rather than being referenced to ground like a standard buck). This is called a buck-mode configuration. This configuration is subject to the same limitations as the buck. That is, the total LED string voltage must be lower than the input power.
○ Buck-boost converter

The buck-boost LED driver regulates LED current using a power supply that is either higher or lower than the total LED string voltage. This converter modulates the high-side switch connected to the input voltage in step-down mode, and the low-side switch on the output side in step-up mode. This topology offers greater flexibility at the expense of complexity. The VIN and VOUT ranges are limited only by the controller IC. Suitable for use in matrix applications.
■ Select the appropriate switching topology and configuration according to application needs

Automotive LED lighting systems can be driven in a variety of ways using switching regulators. Lighting designers can select the appropriate switching topology and configuration to meet the needs of different applications, optimizing complexity, efficiency, EMI, and safety.

※ About the author

Joshua Caldwell spent ten years at Linear Technology (now Analog Devices) as a design engineering leader, where he oversaw the definition, design, and development of monolithic buck, boost, and controller LED drivers. He holds a bachelor's degree in electrical engineering from the University of Colorado. His hobbies include cycling and drawing.