The right LED driver for every application
In choosing a suitable LED driver, it is usually necessary to ensure that the driver is direct voltage operated. If alternating voltage is connected to the application input, it needs to be converted into direct voltage. There are three ways of doing this: complete power supply units (PSUs), discrete PSUs, or a complete LED driver solution. PSUs are recommended for lighting applications in relatively low volumes, as they keep down development costs. Discrete PSUs essentially comprise a transformer and an AC/DC converter, such as an ICE driver from Infineon or a VIPer driver from STMicroelectronics. For large volumes, discrete PSUs are not only more cost-effective than bought-in solutions, they can also be tailored closely to the requirements of the application. One complete solution capable of working directly on the rectified mains voltage is the ICL8001G from Infineon. Its minimal external component count [figure 1] makes it a space-saving alternative, ideal for cost-sensitive designs subject to tight space constraints.
Figure 1: Infineon ICL8001G external components. For full resolution, click here.
Once the DC voltage supply to the LED driver has been safeguarded, the required module can be selected as either driver-specific (that is, based on the mode of operation of the module) or application-specific. Driver-specific modules are subdivided into linear and DC/DC drivers. Applications involving special LED drivers include array, backlight, flashlight and automotive.
Low-cost linear drivers
Linear modules cover an input voltage range up to 40V and output currents up to approximately 2.5A with an external transistor, with the output current being adjustable by an external resistor. The example presented here is the low-cost BCR402U from Infineon [figure 2]:
Figure 2: BCR402U from Infineon
Like all drivers in the BCR family, it features a very low minimum input voltage of 1.4V and a maximum input voltage of 40V. If the input voltage is higher than 40V, LEDs can be operated above the module [figure 3].
Figure 3: Using the BCR402U with input voltage above 40V
Owing to its negative temperature coefficient, the current falls as the temperature rises. This prevents overheating of the module. The negative temperature coefficient also guarantees uniform current distribution where multiple drivers are configured in parallel. Despite the small SC-74 package, a power loss of max. 750mW is possible. The Infineon driver is also AEC-Q(101) qualified, meaning it conforms to the high quality demands of the automotive industry.
The choice of the required DC/DC driver depends on the voltage scenario: If the input voltage is greater than the output voltage, a buck driver must be used. If the voltage ratio is reversed, however, a boost driver is required. Where the voltage fluctuates at the input, meaning it is greater than or less than the output voltage, buck/boost drivers are used.
One possible model for buck applications is the ZXLD1362ET5TA from Diodes/Zetex [figure 4]. It features a broad input voltage range from 6V to 60V and up to 1A output current, and thanks to its maximum switching frequency of 500kHz it enables small passive external components to be used.
Figure 4: ZXLD1362 from Diodes/Zetex
The built-in MosFET makes an external switch unnecessary, saving space on the circuit board and cutting cost. The driver itself is also a cost-effective product, and is available in the space-saving SOT 23-5 package. The ZXLD family incorporates a large number of products which are all pin- and package-compatible, enabling the application to be adapted to changing circumstances – such as when a lower output current is needed – with no cost-intensive redesign.
LED drivers for special applications: LED arrays
Because every application entails different demands, manufacturers have developed special drivers for some areas. For array applications, such as video walls, LED drivers with a SPI interface are used. These feature multiple outputs, enabling different LED strings to be connected in parallel to one driver for example. In the case of large backlights in particular, it is important to ensure that all parallel configured LED strings are powered with the same current rating, so as to avoid differing brightnesses. One option for such a scenario is the STP08DP05 from STMicroelectronics, which features eight constant current outputs. With input voltages from 3V to 5.5V, data can be received via SPI at max. 30MHz. The output voltage of the LEDs is specified as max. 20V, and the output current as 5mA to 100mA. The STP08DP05 also enables LED dimming. The driver additionally features two protective functions: "open detection" and "short detection", which protect the module and the application against destruction.
Backlight and flashlight
Mobile phone displays, satellite navigation units and device controls of all kinds are classed as typical backlight applications. Flashlight applications include torches and other kinds of flashing lights. For all these applications, the question arises as to whether the driver can be battery-powered. If it can, the LED driver needs to have the lowest possible no-load current and be capable of working with a low input voltage of around 1.0V.
The automotive sector offers a broad range of applications for LEDs, both for interior and exterior lighting. The key factors in these applications are to ensure protection of the input against overvoltage by means of an external FET and to detect whether the output voltage is impermissibly high, such as due to a faulty LED. One product suitable for such applications is the ISL78100 from Intersil, with which three to eight high-power LEDs can be operated. The input voltage range of the module is specified as 5V to 16V. The module additionally permits both boost and buck configuration [figure 5].
Figure 5: Typical configuration of the ISL78100 in buck mode, including overcurrent and output voltage protection
Fluctuations in battery voltage may cause the input voltage to fall below the output voltage, meaning that the driver would have to alternate between buck and boost mode. This can be prevented by referring the output voltage to the input voltage instead of to ground. To do this, the shunt resistor is connected to the input voltage in order to measure the current. The shunt ensures that the output voltage is always greater than the input voltage, so the driver operates continuously in boost mode. The module additionally features an input for a temperature sensor. If the sensor measures temperatures above a preset limit, the driver reduces the output current of the LEDs, thereby substantially extending their service lives.
Rutronik has devised a Selection Tree which developers can use as a quick and easy means of finding the right driver for their application. The tool provides all the necessary details, including links to manufacturers’ data sheets and to the Rutronik Webg@te eCommerce platform, as well as an overview of component technical specifications. It also enables users to check availability and place orders at the click of a mouse. And lighting experts are also available via Webg@te to answer specific questions directly.[figure 7].
Figure 7: Useful in selecting drivers: the Rutronik Selection Tree. For full resolution, click here.
www.rutronik.com/leddriver LED Driver Selection Tree
www.rutronik.com/deltapsuselectiontree PSU Selection Tree
About the author: Axel Stang is Product Sales Manager Power Semiconductors & LED Drivers at Rutronik Elektronische Bauelemente GmbH.