Electronic Design

Behind The Bright Lights, LED Drivers Evolve To Meet New Requirements

In applications ranging from small flashlights to stage lighting systems, high-brightness LEDs can generate usable levels of light with improved efficiency, longer lifetimes, and smaller dimensions than conventional light sources. Special effects such as dimming, sequencing, and blinking can also be achieved. Accurate current control is essential for an effective LED lighting solution, and a wide variety of LED driver ICs is available to help engineers optimize their designs.

LEDs for Lighting

Power LEDs for lighting applications can produce sufficient luminous flux that, when driven at their maximum rated current, and by combining the devices in a suitably sized array, allows use in applications such as flashlights, room lighting, exterior lighting, and electronic signage. The range of devices available can sustain drive currents up to well over 1 A in some white high-power LEDs.

Since the drive current effectively determines the light output of the LED, all LEDs in the array must be driven at constant current to ensure acceptable uniform lighting in the end product. To ensure uniform current, the LEDs are usually connected in series. The forward voltage drop across the LEDs also must be considered, though. Each LED in the string contributes an individual forward voltage drop, or VF. This is usually around 3.4 V nominal, but can vary from a minimum of around 2.5 to 4 V or higher.

However, the voltage applied to the string must be at least equivalent to the sum of the forward voltages of all LEDs in the string. LED manufacturers use a binning system to identify devices with closely related color and luminous intensity for a given current. VF must be allowed to vary for the current to be controlled. By specifying a bin when ordering parts, design engineers can ensure a uniform light.

Battery-Voltage Applications

To achieve sufficient voltage to drive the necessary number of LEDs in series, a boost converter may be used to step up the supply in battery-powered applications such as flashlights or portable devices with LED-backlit displays. At the other end of the scale, in applications such as electronic billboards or traffic signage where large numbers of LEDs are required, the driver topology may have a high output voltage up to around 40 V.

Alternatively, one or more multichannel driver ICs may be used. In these devices, current matching between channels must be very close to prevent variation in brightness from one string to the next. The latest multi-output drivers, for example, achieve current matching well within the light-output tolerance of modern power LEDs.

Many designers looking to take advantage of the small size and high efficiency of LED illumination are concerned with portable battery-powered products that have low operating voltages. A dual-cell supply, for example, will have a voltage range of 1.8 to 2.5 V for nickel-cadmium (NiCd) and nickel-meal-hydride (NiMH) chemistries or up to 3 V for alkaline cells.

A driver such as the Zetex ZXSC310 is a constant-current boost converter that may be used to step up the low-voltage supply to drive a power LED with nominal VF of 3.4 V. By operating from an input voltage as low as 0.8 V, the driver can supply constant current to the LED as the battery voltage decays.

The ZXSC310 is useful in flashlight applications and to control LED backlights in small portable devices. A single external pin controls normal operation or 5-µA shutdown mode, or it can be connected to a pulse-width modulation (PWM) signal to control the dimming of the LED.

The National Semiconductor LM3410 is another example of a boost converter for use in low-voltage equipment. It can convert an input voltage from 2.7 to 5.5 V to an output from 3 to 24 V for display backlighting and other handheld applications.

Among non-battery-powered applications, boost-type drivers allow the replacement of low-voltage halogen lamps in interior lighting applications. By accepting an input voltage up to 18 V, a driver such as the Texas Instruments TPS61160/1 boost converter can step up the nominal 12-V dc supply for a standard low-voltage halogen lamp fitting to an output high enough to drive six or 10 white LEDs.

Figure 1 shows how the TPS61161, which has an integrated 40-V/0.7-A N-channel MOSFET switch, can drive 10 LEDs. The dimming control pin provided can be used as a one-wire digital interface or as a PWM input, which allows designers to implement versatile control modes. Buck Converters as LED Drivers

Most drivers for general-purpose illumination applications are designed to operate from generally higher dc voltages, usually converted from mains voltage and ranging from around 5 V up to 30 V or more. The National LM3406, for example, is a self-contained buck regulator with an input range of 6 to 32 V and capable of supplying constant forward current up to 1.5 A. Figure 2illustrates a typical application circuit, with an external resistor to set the LED current and a dedicated input pin for PWM dimming.

The Zetex ZXLD1350 is a similar buck regulator operating from a 7- to 30-V input and capable of supplying up to 350-mA LED current. An external pin is provided for current adjustment, which can accept a PWM dimming signal or a simple dc voltage to adjust the output above or below the value determined in the normal way using an external sense resistor.

Multichannel and Multidriver Designs

Using a buck regulator topology, the number of LEDs that can be driven is determined according to the maximum output voltage, which cannot exceed the input voltage applied. Large LED arrays for applications such as signage or stage lighting, for example, may require several drivers each controlling a string of LEDs.

Alternatively, a multichannel driver may be used. The eight-channel TI TLC5917 driver delivers up to 120 mA per channel. It also provides eight regulated current ports that can operate at up to 17 V. Output current accuracy better than ±3% between channels effectively ensures uniform light output from LEDs in different strings.

Moreover, accuracy better than ±6% between ICs enables engineers to drive even larger arrays of LEDs by using several TLC5917 drivers. By also building in open-load, shorted-load, and overtemperature protection, this driver can indicate device failures in large arrays of LEDs as well.

As power LEDs penetrate an increasing variety of lighting applications, many further driver variants can be expected to emerge to permit higher drive currents, easier control, and greater freedom for designers to use the optimum number of LEDs for their target application.

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