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.