LIGHTING LIGHTING

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LEDs are becoming verypopular in automotive exte-rior lighting, thanks to their

superior lighting characteristics,efficiency and flexibility in designimplementation. They can be usedin a variety of light shapes andwith different features.

The required multiplicity ofLED configurations in automotiveexterior lighting, combined withthe variability of the car battery

voltage, forces the use of a largenumber of integrated circuits,each tailored to a specific vehiclelighting function. A flexible con-troller IC can support many archi-tectures and simplify automotiveexterior lighting design.

Powering LEDsLEDs have many automotive ap-plications and are used in diversearrangements from a single LED

IN CONTROLIN CONTROL

Nazzareno (Reno)Rossetti and YinWu explain howa flexible LEDcontroller cansimplifyautomotive exteriorlighting design

lamp to LED strings and matrices.Functions such as high beam, lowbeam, fog lights, daytime runninglights (DRLs), and position andturn signal lights can all be imple-mented with LEDs.

Powered from a car battery, theinput voltage is typically 12V butcan be as high as 16V on a fullycharged battery. Vehicles employ-ing start-stop technology experi-ence large voltage dips when the

engine starts, so the lower limitfor the power source can be wellbelow the typical 12V, often 6V oreven lower.

The switching regulators power-ing the LEDs must meet specificrequirements. They must operateover the entire voltage range pro-vided by the battery and be able tosurvive 60V dump voltage tran-sients. The current amplitude mustbe very accurate, since it controlsthe LED colour.

For light dimming, time-slicingthe LED current by using pulse-width modulation or PWM re-duces the light’s brightnesswithout affecting its colour. ThePWM dimming frequency mustbe above 100Hz to be undetectedas a flicker by the human eye.

A high and well-controlledPWM switching frequency abovethe AM frequency band is re-quired to reduce radio frequencyinterference. Spread-spectrummodulation is also necessary tomeet EMI standards. Finally, highefficiency helps reduce heat gen-eration and improves system relia-bility.

Basic headlightA basic headlight architecture thatcan accommodate a series ofLEDs uses a boost converter. Inthe boost controller IC of Fig. 1,one of the three feedback loops(current loop) ensures tight con-trol of the output current. Theother two feedback loops performovervoltage protection (OVPloop) and overcurrent protection(OCP loop) for the string of 12diodes, which develops 42Vacross the string (3.5V per LED).

In addition to current and volt-age control, the IC must beequipped with all the features pre-

viously described – dimming,spread spectrum and so on. High-side current sensing via the resis-tors, Rx, is required to protect theLED system in case of shortsfrom the output to the ground orbattery input.

Flexible architectureIdeally, an LED controller shouldhave a flexible architecture thatsupports multiple configurationsthat can implement different fea-tures. As well as the boost config-uration, the buck-boostconfiguration should also be con-sidered.

A buck-boost mode configura-tion is necessary if the diodestring is short, for example two orthree LEDs (7 or 10.5V), against abattery voltage that can vary fromless than 6V (cold crank) up to16V. If the concern is input-to-output isolation, then a single-ended primary inductanceconverter (sepic) for discontinu-ous output current or a chuck(continuous output current) con-verter may be the right device.

A single controller that supportsmany architectures, such as theone in Fig. 2, has clear advantagesof economies of scale and ease ofreuse. It can drive LEDs, allowingboost, high-side buck, sepic modeor buck-boost mode configura-tions. The device can be a single-channel high-brightness LED (HBLED) controller for automotivefront-lighting applications such ashigh beam, low beam, daylightrunning lights, turn-signal indica-tors and fog lights.

Low EMI and noiseA 200kHz to 2.2MHz programma-ble switching frequency can allowthe device to operate well outside

LIGHTING EMC

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the AM radio frequency band,avoiding interference with the au-tomotive radio signal.

Built-in spread-spectrum modu-lation also improves electromag-netic compatibility performance.

Spread-spectrum ditheringadded to the oscillator can allevi-ate EMI problems in the LEDcontroller.

The boost converter oscillator(RT pin in Fig. 2) can be synchro-nised to the positive-going edge ofthe PWM dimming pulse(PWMDIM).

This means that the NDRV pulsegoes high at the same time as thepositive-going pulse onPWMDIM.

Synchronising the RT oscillatorto the PWMDIM pulse guaranteesthat the switching-frequencyvariation over a period of aPWMDIM pulse is the same fromone PWMDIM pulse to the next.This prevents flickering during

Fig. 2: Boost LED system

Fig. 1: Typical boost LED control system

PWM dimming when spreadspectrum is added to the RToscillator.

ConclusionAn LED controller can support ahigh number of architectures forautomotive exterior lighting and

greatly simplifies the design. Theflexible design options use boost,high-side buck, sepic mode orbuck-boost mode configurations,providing clear advantages ineconomies of scale and ease ofreuse.

In addition, a high switchingfrequency allows operation abovethe AM radio frequency bandwhile built-in spread-spectrummodulation can reduce electro-magnetic interference.

Nazzareno(Reno) Rossettiis an analogueand powermanagementprofessional

and Yin Wu asemiconductorbusinessprofessional,both at MaximIntegrated

In 1998, the Volkswagen GolfMark IV had 17 ECUs, twoCan networks and just 434

Can signals. In 2010, the GolfMark VI had 49 ECUs, five Cannetworks and 6516 Can signals.Today, typical vehicle electricalsystems have more than 60 pri-mary networked (Can or Flexray)ECUs and as many as 20,000 Cansignals.

This growth in complexity overthe past 20 years has been driven

Making animpact

Anthony Martin and Alastair Ruddleconsider how to ensure resilienceamong increasingly complex vehiclesystems within their intendedelectromagnetic environment.

by the consumer’s desire for com-fort and convenience, and the in-dustry’s drive for market sharethrough value, efficiency andsafety. However, this will quicklybe surpassed by the rapid integra-tion of new technologies intofuture vehicles.

AutonomyIt is widely acknowledged that au-tonomous vehicles offer the appli-cation that artificial intelligence

and machine learning have beenwaiting for, and that this introduc-tion will be sooner than we think.There have been significantstrides in the development ofbasic algorithms used in machinelearning and an increase in theamount of quality data available.

Infra-red sensors, lidar systems,360˚ vision systems, wireless con-nectivity and more combine toprovide machine learningalgorithms with a wealth of richinformation.

Wireless technologies and theirassociated benefits are now anindispensable part of modernsociety. With demand increasingand implementation costsreducing, they are becomingavailable across most vehicleson the market.