LED Power Management: Techniques Proliferate

Jan 4, 2012 2:14 PM
Allan, Roger

There are many power management methods that can be used for LED lighting. You can drive them via a number of dedicated driver ICs available on the market, or even use microcontroller units (MCUs) for more intelligent driving.

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.

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