Driver circuits are available in a variety of topologies. These include series (switch-mode) drivers, and parallel (non-inductor) drivers. How LEDs are driven depends on the level of efficiency desired, the relation of the battery voltage driving the IC to the voltage range, current consumption when the LEDs are unlit, the ability to accurately regulate LED current, size and cost constraints, and whether or not dimming is needed.
LED power management hinges on the ability to efficiently drive these devices at the right voltage and current levels. Power management is also influenced by the manner of driving them, either in series, in parallel, or by using multiple parallel strings of series-driven LEDs.
Being diodes, LEDs require a minimum forward voltage drive to operate (that is glow) to allow for current flow. That driving voltage cannot exceed the LED’s limit. Otherwise, LED current will increase exponentially, providing an even greater light output, heating up the LED in the process and shortening its lifetime dramatically. Thus, it is not surprising that most LEDs are driven with constant-current sources.
Generally, it is not a good idea to drive a number of LEDs in parallel. Doing so can lead to non-uniform current sharing even when the LEDs are all rated for the same forward-voltage drop. This can be alleviated by using a separate driver for each LED string, an expensive proposition, or ideally a single driver than can handle multiple strings, with each string using a ballast resistor. The resistor’s value can be calculated by taking into account the variations in forward-voltage drops for each diode and how closely is the need to match the currents for each string.
Some manufacturers, like Philips Lumileds, offer matched and cross-connected serial/parallel LED string arrays like the Luxeon products. This simplifies a designerís task of having to match voltage and current values in LED lighting designs.
There are many commercially available LED driver ICs on the market. There are also microcontrollers designed for intelligent LED driving applications. Popular switch-mode LED drivers make use of buck, boost, buck-boost, and single-ended primary inductor converter (SEPIC) circuits.
Driver topologies can range from complex switching regulators to simple linear regulator with feedback loops. The former types are more costly, provide higher efficiency level and are ideal for high-light-output applications like street light illumination. The latter types are less expensive, don’t provide as much efficiency, and are ideal for backlighting, architectural and interior-lighting applications.
PULSE-WIDTH MODULATED CONTROL
Many commercially available LED drivers employ pulse-width modulation (PWM) for LED brightness control. While this may suffice for many LED circuit designs, pulsing LEDs in high current and high voltage applications (several amperes and tens of volts) can be problematic and not practical, requiring the need for specialty driver equipment made for this purpose.
Some LED IC driver manufacturers claim that using an external electrolytic capacitor that’s needed to smoothen out the rectified ac to dc power can lead to shorter LED lifetimes. They point out that the usual 40,000-hour lifetimes given for LEDs can be shortened considerably due to the external capacitor burning up due to the roughly 40% heat caused by the LED’s heat dissipation. Some, like China’s DALI Power, claim that their LED drivers eliminate such a capacitor by having the smoothing function performed electronically within the chip itself.
Fairchild Semiconductor claims that their FL7701 LED lamp driver has improved reliability since no electrolyticcapacitor is needed for the input, output, or the VDD supply. This driver is meant for space-saving designs, saving up to 10% pc-board space in a tiny SOP-8 package (Fig. 1).
The smart non-isolated buck driver uses a digital technique that allows it to automatically detect the ac input voltage condition, allowing it to create a special internal reference signal that results in high power-factor correction (PFC). It can also work from a dc source.
Space-saving LED drivers are important for hand-held electronics like mobile phones, digital cameras, toys, hand-held computers, etc., where space is at a premium. A space-saving LED driver is the power-efficient STLA02 from STMicroelectronics (Fig. 2). Aimed at backlighting LEDs in high-tech handheld electronic devices, it is housed in a 6-lead DFN package. It performs high-side current sensing which allows the backlight negative low-side terminal to be connected directly to ground with no need for low-side sensing. It can drive up to six LEDs from a 2.5-V battery.
STMicroelectronics also recently introduced the STLED25 driver that provides five current sources that can be connected directly to the LED high-side terminals, allowing the low side to be connected to ground. This eliminates the need to connect each LED channel back to the control IC, enabling a compact, robust and reliable design. The STLED25 operates from 2.3 V to 5.5 V and is aimed at battery-powered devices.
Dimming Challenges
A big challenge for designers is to allow LEDs to perform the dimming function in place of place legacy triacs and SCRs used for dimming fluorescent and incandescent lamps. Although dimming is easy with a constant-current dc source, it is not so simple using PWM power sources. A rule of thumb is that any PWM source over 100 Hz is sufficient for dimming purposes. However, when one factors in PFC combined with slow rates and short duty cycles, unwanted harmonics may be applied to the LED.
Both National Semiconductor (now Texas Instruments) with its LM3448 and Texas Instruments (TI) with its TPS92070 provide phase-dimmable LED lighting drivers (Fig. 3). The former is an adaptive constant off-line ac/dc constant-current LED regulator with a 600-V MOSFET and is optimized for 2-W to 8-W dimmable applications. The latter is a flyback AC/DC constant-current LED driver controller optimized for 6-W to 8-W applications.
Austria Microsystems makes available the space-saving AS12130 LED driver aimed at dot-matrix displays that features 8-bit dimming control for each of the 132 LEDs it drives and requires no external resistor. It features the industry’s highest efficiency and the smallest size package, taking up just 5 mm2 of pc-board space. Key to the driver’s small size is Austria Microsystems’ 12-by-11 cross-plexed driving technique.
BUCK REGULATOR LeD DRIVERS
International Rectifier has introduced a new line of high-efficiency and low-cost buck regulator LED drivers for non-isolated lamps that is compatible with electronic dimming, supporting 0 to 100% current-controlled dimming, rather than an integrated triac-dimming circuit that other drivers include (Fig. 4). The IRS2980 is the first in LEDrivIR family of high-voltage LED drivers. It combines a high-voltage output range up to 600 V maximum and uses hysteretic average current-mode control for precise regulation. Alternative methods for current regulation often use a buck regulator with a low-side MOSFET switch, which regulates the current in the driver MOSFET rather than the actual LED.
MCUs Have a Role
Modern microcontroller units (MCUs) are powerful devices that can offer power-management functions for LED lighting applications including embedded applications such as LED backlighting. Many of these applications can be handled by 8-bit and 16-bit MCUs, especially where low cost and small size are important issues. 32-bit MCUs may be required for some applications that require advanced communications protocols. However, even some 8-bit MCUs can be used to handle communications standards like the Digital Addressable Lighting Interface (DALI). Some LED drivers make use of digital signal processors (DSPs) as well as dedicated intelligent analog drivers. And some MCUs use independent analog drivers. Nearly all MCU driver manufacturers offer development kits to assist in the design implementation process
One of the most elegant MCUs for LED driver control is the MSP 430 MCU from TI, which gives designers efficient, flexible and scalable intelligent management and control. TI’s Piccolo MCUs for LED driving allow designers to dig deeper into the hardware and software used for digital power control. The company’s C2000 dc-dc LED lighting developer’s kits show the proper power topologies needed for driving single and multiple shared power stages.
Microchip Technology offers the MCP1650/51/52/53, a 750-kHz gated oscillator boost controller ideal for space-limited designs and packaged in an 8- or 10-pin MSOP package. Developed for high-power, portable applications like LED driving, it can deliver 5 W of power to the load while consuming only 120 µA of quiescent current at no load. It operates over a wide input voltage range (2.0 V to 5.5 V).
8-bit microcontrollers
There is some concern about driving LEDs using switched-mode power supplies since they might not meet international EMC certification. That’s why Freescale Semiconductor created a dual-string lighting controller based on its SM08MP16 8-bit microcontroller, the HBLED1 (Fig. 5). The embedded closed-loop control algorithm that is implemented in the microcontroller ensures the optimal flow of current though a variable number of high-brightness LEDs, maximizing their lifetimes and avoiding undesirable visual blemishes.
The switched-mode power supply used is a discrete buck-boost topology. It is designed to power from 1 to 18 LED strings (from 0 V to approximately -65 V, continuous) and can supply up to 500 mA of current, running at a frequency of 350 kHz.
Other suppliers offering 8-bit microcontrollers include NEC Electronics. The µPD78F8025 all-flash MCU includes integrated constant-current 4-chanmel high-current drivers with 8 kbytes of flash memory. It can be used in buck-boost topologies. The chip was designed for general home-appliance and office lighting applications.
NEC’s offering comes with several interfaces including I2C and universal asynchronous receiver-transmitter (UART) ports, as well as an onboard analog-to-digital converter (ADC) that can be used for light and temperature sensing, and fined-grained control of a power supply. NEC claims that its design minimizes pc-board space.
SINGLE-CHIP SOLUTION
Cypress’s PowerPSoC family of integrated embedded power controllers offers the industry’s first single-chip solution for both controlling and driving high-power LEDs. The PowerPSoC family power controllers integrate four constant-current regulators and four 32-V MOSFETs with Cypress’s PSoC programmable system-on-chip, which includes a microcontroller, programmable analog and digital blocks, and memory (Fig. 6).
This high level of integration provides users with a single-chip solution for high-quality LED-based lighting products, and extends into other embedded applications, such as white goods and industrial control. The Power PSoC controllers enable users to design end systems with fewer components, faster design cycles, lower power consumption and higher reliability.
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