ENGINEERING FEATURE

LEDs and Drivers for Lighting Designs

Energy savings, economy and other imperatives are propelling manufacturers to seek energy, space and cost efficiencies. LED lighting solutions can perfectly satisfy all these needs. As a global provider of products, services and solutions to industrial and commercial users of electronic components, Arrow works with customers and the solution providers of LED alternatives to incandescent, halogen and fluorescent lighting.

Our multi-skilled team specialises in providing a wide range of solutions from components to complete solutions utilising high brightness and ‘lighting class’ LEDs. This feature looks at a few of the LED products that are enabling this shift to solid state designs and the driver technologies available to engineers implementing LED-based lighting solutions.

LED light sources can be 85% efficient compared to the 85% of energy wasted on heat production by traditional incandescent light bulbs. Coupled with LEDs’ low lifetime cost of ownership, this explains their growing popularity for a myriad of applications. Legislation to force replacement of inefficient lighting will be another major source of market growth in future years.

Unlike traditional lighting, however, LEDs do not simply ‘plug in’ to ordinary domestic lighting sockets or incandescent lighting arrays. At minimum, they need power conversion, regulation and other circuit conditioning, so a concurrent increase in the number and functionality of driver ICs is also under way.

Emerging applications for LED lighting are outdoor, portable, personal and indoor directional lighting, emergency vehicle and architectural lighting.

High Brightness LED Products
Avago Technologies is one of Arrow’s suppliers looking to satisfy the requirements for high-brightness LEDs. Its range of 0.5W PL CC-4 packaged devices offer bright output at a 120° viewing angle in a wide colour range for decorative, mood and automotive lighting applications. This enables good light mixing, even when viewed from a close distance, and their small size enables a higher packing density, significantly increasing the possible image resolution for signage applications.

Figure 1: Avago’s PL CC-4 LED Range

The package enhances heat dissipation, enabling the LED to operate at ambient temperatures up to +110°C when driven at up to 150mA. The ASMT-QxBx LEDs are highly reliable with a long lifetime, owing to their enhanced silicone resin material and very low thermal resistance (only 60K/W (junction to pin). In addition to general illumination, channel, counter lighting and panel backlighting, qualification to AEC-Q101 equips them for internal and external automotive applications such as dome, map and reading lamps, exterior signalling or, with AlInGaP colours, side repeater applications.

Two low profile series from Everlight are the EHP-A07 1W and the new EHP-B03 5W LEDs. Both series offer binning capacity for warm and cool white applications. The EHP-A07 has a 120° viewing angle and luminous flux output greater than 28lm at 350mA. Its optical efficiency is 36lm/W and junction to lead thermal resistance is 17K/W. It offers standard industry colours.

Figure 2: Everlight’s EHP-803 5W Diamond Insulated LEDs

The EHP-B03 series offers standard or high power operation at 700mA or 1000mA. Its light output is 250lm at 700mA with 50lm/W optical efficiency. Its design allows effective heat dissipation with lower thermal resistance, typically 11K/W. It is reflow solderable for easy handling and offers high colour uniformity with narrow warm white and cool white binning in a convenient package.

Figure 3: Everlight ‘Star’ Design with Heat Spreader Layer

The XLamp series of power LEDs, XR-E, XR-C, XP-E and XP-C from Cree^® are bright, reliable and environmentally friendly power LEDs. Cree believes that its XPE and XPC are the world's smallest package high-power LEDs. The XLamp XR-E LEDs are ‘lighting class’, delivering high white brightness with good white-point stability. XLamp XR-C LEDs give flexibility and performance to create a new generation of LED lighting products. Typical efficiency is up to 90lm/W in cool white. The Correlated Colour Temperature (CCT) is from 2,600K to 10,000K and currents up to 1000mA are possible. Lifetimes up to 50,000 hours are achievable.

Figure 4: Cree's XPE Smallest Packaged LED

General Lighting and Projection Possible
Osram has produced a range of LEDs suitable for lighting and projection applications. OSTAR^® products are compact light sources using OSRAM’s proprietary ThinFilm/ThinGaN^® chip technology. The arrays consist of multiple power LED chips, on a low thermal resistance carrier, with screw holes for mounting, connectors and features for precision mounting of secondary optics.

They use precisely aligned optics and high-luminance surface-emitting chips. Several variations are available with lenses; a variety of mechanical, colour and optical variations provide maximum flexibility. For projection, these comprise four chip square or six chip rectangular arrangements in red, green, blue and RGB with or without a hemispherical lens, from 2,700K to 6,500K white.

Driver Technologies, Buck, Boost and All
We turn now to the technologies necessary to drive these high performance LEDs, some of which are application specific.

Take, for example, Infineon’s TLE 4242 G and TLE 4241 GM integrated and adjustable constant current sources suitable for automotive applications; these can supply constant current up to 500mA to white or colour LEDs independently from supply voltage or LED forward voltage. This provides appropriate operating conditions for constant brightness and extends LED lifetime. Adjustable output current and PWM input, for flexible use, regulated, constant colour brightness and open-load detection are key features in addition to protection from short circuit, overheating, reverse polarity transients and input voltages up to 45V.

Linear Technology offers inductor-based, step-down switching LED drivers that provide high power and current (350mA to 10A) yet are tiny and efficient. Key features include PWM dimming, a wide input voltage range, high-side sensing and a high switching frequency. The LT3475, for instance, delivers constant colour with a 3000:1 dimming range and accurate, adjustable LED current control from 50mA to 1.5A from a 4V to 36V operating voltage range. High-side current sense allows grounded cathode LED operation. Switching frequency ranges from 200kHz to 2MHz.

National Semiconductor’s broad portfolio includes energy efficient buck regulators for driving LEDs in various applications. Integrated switching MOSFETs, online design tools, extensive protection features and fast dimming capability are among the features making design easier without sacrificing functionality. The LED drivers feature low feedback voltages and very high efficiency to support energy efficient lighting.

The LM3404 and LM3404HV are constant-current buck regulators capable of up to 1A output current; the LM3404HV can support up to 15 1W or 3W LEDs in series. The LM3405/05A are 1A constant-current buck regulators designed to provide simple, high efficiency solutions for driving high power LEDs. They feature a low 205mV feedback voltage to reduce heat dissipation and can support up to five 1W or 3W LEDs in series. Figure 5 shows the M3402/02HV evaluation board with a female 6-pin SIP connector and two standard 94mm turret connectors for easy connection to the LED array.

Figure 5: LM3402/02HV Evaluation Board

NXP Semiconductor offers a range of high-efficiency AC/DC LED driver solutions; the SSL152x, SSL16xx and SSL1750 families are offline Switched Mode Power Supply (SMPS) controllers for efficiently driving high brightness LEDs. They all offer a full set of protection features and a driver solution with a minimum of additional components. The SSL152x family, for example, suits indoor lighting applications below 15W; it operates directly from the rectified universal mains and is ideal for retrofitting LED lamps and for LED driver solutions in cabinets, kitchen and other lighting applications in the home. It is fully compatible with wall-mounted dimmers based on transistors and thyristors (TRIAC). For solid state lighting applications from 15W to 24W, the SSL1623PH has a special heat spreader beneath the IC package. For applications above 25W, NXP offers the SSL1750 with fly-back control and power-factor correction integrated into one IC.

ON Semiconductor offers solutions for AC mains or battery powered LED applications, addressing LCD backlighting, flashlights and a wide DC-input range of applications including automotive, solar and commercial/landscape lighting, lighting ballasts and power-factor correction solutions. They feature a very wide temperature range of -40°C to +125°C and a wide choice of packages, suitable for automotive and commercial environments.

The Recom International RCD-24 series of step-down constant current sources integrates a high level of functionality, including both analogue and PWM/digital dimming control. These are complete modules rather than chips, making for easy installation in situ. Recom plans introductions of 1.5A versions, 48V input and modules incorporating resistive dimming via potentiometer for smooth, analogue dimming.

The RCD-24 dimming controls are fully independent and can combine to offer maximum design flexibility. A remote on/off function helps energy saving applications and the series offers a compact, fully featured and efficient solution for driving high-power white LEDs at 300mA, 350mA, 500mA, 600mA and 700mA.

Figure 6: Recom’s RCD-24 Series

The RCD-24 series allows designers to use LEDs from a variety of manufacturers in their lighting designs without needing additional external components. Their compact form factor allows the drivers to fit inside the LED lamp fixtures or to be mounted separately for easy installation and exchange.

Reference Designs and Devices
Zetex Semiconductors has a dedicated chipset and reference design for MR16-compatible LED lamps that fits the complete driver electronics into the neck of the lamp. It reduces the component count by up to 50% compared to some existing solutions and significantly decreases the total cost of lamp manufacture.

As lighting manufacturers are increasingly looking for LED technology to provide much longer lifetime and greater energy efficiency than halogen technology, Zetex created the MR16 reference design 'top-down', to provide efficiency and ease of manufacture. The chipset consists of two devices, the ZXLD1350 monolithic LED driver and the ZXSBMR16PT8 full bridge, to handle power rectification and current control.

Figure 7: Zetex Controller and MR16 Lamp

In addition, the design includes a bill of materials for the required passive devices and a circuit board layout. The driver can drive up to three 1W LEDs and includes facilities for pure DC operation. As for any switching regulator, circuit board layout can reduce radiated EMI; the design keeps critical track lengths to a minimum and maximises ground areas around such tracks.

STMicroelectronics’ range of reference designs for offline LED driver applications includes all common architectures, including constant current, dimmable current, fly-back and step-down current source. For electrically isolated applications such as architectural and general lighting, STMicroelectronics recommends combining its VIPer family of integrated offline switching regulators and its TSM family of voltage and current controllers.

The VIPer12A and VIPer22A devices combine a dedicated current-mode PWM controller and high-voltage power MOSFET in a single chip DIP-8 or SO-8 package. They feature internal 60kHz oscillator, overcurrent and overvoltage protection and automatic burst mode for low loads. These are the basis of a set of complete reference designs implementing isolated fly-back architecture, supplied as application notes and evaluation boards. These designs can deliver a constant current of 350mA and an output voltage from 3V to 27V for driving up to eight LEDs. A variant with integrated PWM dimming is available.

Figure 8: STMicroelectronics Reference Design Schematic

Accuracy and Speed for Billboards
A family of linear LED drivers from Texas Instruments, the TLC59xxx, offers current regulation accuracy up to 1% channel to channel and 3% chip to chip. Serial data-input devices, they can run at up to 30MHz. Image display speed can be improved by the devices quick turn-on and turn-off times. The drivers also feature a small amount of voltage headroom over the LEDs' VF to bias the internal linear element. The TLC59xxx family provides flexibility for designers of high-quality signage applications such as video walls and billboards.

Arrow stocks a wide range of devices from a number of the world’s leading suppliers, only a few of which are featured here. Many of the devices can be found in the lighting catalogue, available online.

For further information, including a copy of the Lighting Guide, please click here