LED Drivers for backlighting applications are required to drive one or more strings of LEDs. LED-backlit LCD displays have now replaced CCFL-based backlighting panels and dominate televisions, automotive instrument clusters, cell-phones, PDA, driver information systems, notebooks, DVD players and monitors. A variety of LED drivers are offered for backlighting to match appropriate requirements of input voltage range, number of LEDs per string, number of strings and LED current. For instance, automotive drivers require specific voltage and temperature ratings. The major challenge in the design lies in making high picture quality at lowest cost and smallest PCB area. Recent LED drivers meet high performance requirements with many advanced features like fault protection, error detection/diagnostics, auto power savings/shutdown, PWM dimming, etc. Let’s walk through the reference designs of some LED drivers designed for such LCD-backlighting applications.
- Reference Design for LCD TV LED Backlighting: This reference design implements a LED driver with high output voltage and high efficiency required for LCD TV backlighting applications. This circuit drives a string of white LEDs in series with a forward voltage up to 230V for use in LCD TV Backlighting. For dimming, a PWM dimming circuit is implemented using MOSFET switches that alternate the LED current from a fixed level of 110mA in dim signal high state to zero in dim signal low state. This changes the intensity of the LED load at a dimming rate set between 500Hz and 5KHz. A Discontinuous Flyback converter topology is used for fast transient response while dimming. A high transfer ratio from Vin to Vout is required that is implemented with a transformer with a turns ratio to improve efficiency and transient performance. More on this Reference Design
- Boost Backlight LED Driver for a TFT Display: This is a reference design for an LED backlight driver for a TFT display. The design is implemented using a MAX16809 16-Channel LED Driver that controls a boost power supply. A boost power supply is used because its output voltage is always greater than the input power-supply voltage. The input voltage is 8V to 18V, and the LEDs are grouped into three parallel strings of six to eight LEDs (34V) in series with each string drawing 150mA. The MAX16809 has a Switch-Mode Boost and SEPIC Controller. The boost power supply operates at a frequency of 200kHz, which is fast enough for small power components and yet slow enough to prevent overheating the switching MOSFET. More on this Reference Design
- 12-inch (30.48cm) TFT Display LED Driver for Automotive Applications: This reference design explains an LED Driver for a 12-inch (30.48cm) TFT display. The design is well-suited for automotive applications which are subject to input and output voltage variations. It tends to achieve a high dimming ratio of 4000:1. This design can handle an input voltage range of 16V to 36V with 50V transients with the help of a single-ended primary-inductor converter (SEPIC, essentially a boost converter followed by a buck-boost converter) power supply which allows the input voltage to be greater, or lesser, than the output power supply. A MAX16809 16-channel LED driver drives eight parallel strings of 8 WLEDs (white LEDs) in series. Each string draws 70mA and the maximum LED voltage is 31 volts. The switching frequency of a SEPIC power supply is 200kHz, which is fast enough for small power components and yet slow enough to prevent overheating of the switching MOSFET. More on this Reference Design