High-brightness LED driver puts 150 mA in four strings

Sept. 3, 2009
Cost-effective solution simplifies lighting designs, offers high performance

The MAX16814 is a highly integrated four-string HB (high-brightness) LED driver capable of delivering up to 150mA per string. This device contains the features necessary to implement a high-performance backlight driver for LCD TVs, monitors, industrial and automotive displays and other SSL (solid-state lighting) applications. Through this integration, the driver gives designers a cost-effective solution that greatly simplifies their lighting designs.

Among the product’s features is a switching DC-DC controller to generate the voltage required to drive multiple strings of LEDs. It also features four linear current sinks with internal power MOSFETs to provide programmable current up to 150mA per string. An adaptive output-voltage control scheme optimizes efficiency and minimizes power dissipation in the LED current-sink paths. This architecture eliminates the need for multiple external components, thus saving space and easing design in space-constrained applications.

HB LEDs are rapidly replacing CCFLs as the backlighting technology of choice in LCD TVs, desktop monitors, and automotive displays. The main drivers for this conversion are the lower energy consumption, smaller form factor, and higher reliability of HB LEDs compared to CCFLs; additionally, HB LEDs do not contain hazardous chemicals such as mercury.

Most of these applications use multiple LED strings and, of course, need to keep cost as low as possible. Multistring drivers present an excellent solution to these requirements, as they reduce cost and improve integration at the same time. However, their use presents a number of design challenges related to efficiency, power dissipation, dimmability, fault protection, and EMI (electromagnetic interference).

Multistring drivers are composed of a switching converter section and multiple linear current sinks. In order to optimize efficiency, the output voltage of the switching section must be set to the supply voltage required by each string of LEDs. Output-voltage optimization is critical for high-temperature applications (e.g., automobiles) that must minimize power dissipation to keep the circuit temperature at an acceptable level.

Whereas traditional drivers require a complex external circuit to set the output voltage, the MAX16814 performs this function internally. The device's adaptive output-voltage control scheme automatically detects the voltage required by the HB LEDs, thus ensuring the lowest possible power dissipation. This architecture improves integration and lowers solution cost substantially.

Applications that are exposed to changing ambient-light conditions require a wide dimming range. These applications must be able to accommodate lighting conditions that vary from intense sunlight to complete darkness to ensure optimal visibility throughout the day. This capability is even more important when the application is life critical, as is the case in automotive, industrial, and medical systems. In most multistring drivers, the PWM dimming duty cycle cannot go below a certain minimum value due to the need to set the switching section's output voltage correctly. Consequently, these drivers are typically limited to a maximum dimming range on the order of 100:1. The MAX16814 employs a unique, patent-pending technique to deliver an extremely wide PWM dimming range of 5000:1 with a 200Hz dimming frequency. It thus allows optimizing display performance in all lighting environments.

The MAX16814 features open and short fault protection. In the event that an LED shorts or becomes an open circuit, the driver detects the fault and automatically turns off the affected string, while keeping all other strings active. This protection scheme minimizes the effect of the failure and prevents further damage to the system. In both cases, an output pin signals the fault condition to the rest of the system. Additionally, the driver features thermal protection to prevent damage to the circuit from over-temperature conditions. Combined with the device's high input-voltage capability and wide operating-temperature range, these fault protections virtually guarantee reliable operation in harsh electrical and thermal conditions.

The MAX16814 allows users to program the switching frequency from 200kHz to 2MHz, thus enabling operation above the AM radio band and avoiding interference. In addition, its oscillator-synchronization feature minimizes EMI noise when multiple drivers are used in the same module. The device thereby reduces the number, size, and cost of EMI filter components required in the application.

For enhanced flexibility, the switching regulator can be configured as a boost or SEPIC converter; the voltage across the driven LEDs can therefore be either higher or lower than the supplied input voltage. Additional features include cycle-by-cycle current limiting, output overvoltage protection, and overtemperature protection. The MAX16814 is fully specified over the -40 degrees Celsius to +125 degrees Celsius automotive temperature range, and its wide input-voltage range (4.75V to 40V) allows it to withstand 40V load dumps. The device is available in thermally enhanced, 4mm x 4mm, 32-pin TQFN and 20-pin TSSOP packages. Prices start at $1.39 (1000-up, FOB USA). For more information, please visit: www.maxim-ic.com/Backlight-Driver.


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