Optoelectronics
Automotive LED driver power conversion topology guide
By Joshua Caldwell, design director, Analog Devices I
n many systems - including the myriad of regulators deployed in automotive power delivery systems - the design of power conversion regulators is often a difficult and complex task. This article aims to simplify the selection process by explaining the benefits, trade-offs, and applications for different switching topologies used for LED drivers. LEDs are unlike traditional electrical light producing filament or gas components. Utilising specific semiconductor junctions, LED manufacturers can produce specific colours of light spanning the entire visible range - as well as IR and UV. In automotive applications, LEDs can increase the safety in both daylight and nighttime driving scenarios. Increased efficiency can extend battery life in electric vehicles, and multiple LEDs in a single system can eliminate single-component failures. Due to their versatility, LEDs offer the capability of being driven in many different ways. Since the output from LEDs is well-controlled light, LED loads are unlike traditional loads to a power system. LEDs only rely upon accurately regulated current, through the semiconductor junction, to produce light, where the relative voltages at the terminals to the system ground (or chassis in an automotive system) are unrelated. As a result, LED systems can take advantage of the different topologies offered by switching technologies.
How to select the correct switching topology for automotive LED systems
The choice of a particular switching topology in an automotive system is related to the complete system design; considerations should be taken into account for minimum input voltage, maximum string voltage, chassis return capability, shorted output capability, maximum input current, output/LED current, and PWM dimming.
54 September 2023 Components in Electronics
www.cieonline.co.uk.uk
Step-down (buck) converters Step-down (or buck) LED drivers regulate the current in an LED string from a voltage that is higher than the total LED string voltage. Buck LED drivers can be safely shorted to the system ground, making them both intrinsically safe. They can have the capability of chassis return (one wire for
Figure 1. Buck converter.
Benefits to Buck LED Drivers
Grounded string–chassis return
Matrix switches can shunt entire string
Higher bandwidth (>1/5 of f SW)
Best EMI performance Smallest inductorsizing
Table 1. Advantages and trade-offs of using buck converters as LED drivers
power), and they can easily be adapted to matrix or animation applications. Figure 1 and an example schematic in Figure 2 show basic system diagrams with the controller modulating the high-side switch for current control.
Several critical features to look for in
step-down LED drivers are fixed frequency operation, high efficiency through excellent switching control and low resistance switches, high accuracy throughout the analogue dimming range, and, for excellent EMI, a properly designed spread spectrum frequency modulation.
Figure 2. Buck converter example: LT3932.
Trade-Offs to Step-Down LED Drivers
Input voltage must be higher than LED voltage
Pre-boost regulator required in most automotive systems
Applications High beam/low beam Turn signals/animation Matrix headlamps Short-safe systems
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