Single-chip LED-backlight drivers

11 June 2008 Opto-Electronics

STMicroelectronics has introduced three monolithic step-up converters for LED backlights and lighting, providing 30 mA or 85 mA current sources for six rows of up to 10 white LEDs. The LED7706, LED7707 and mobile-optimised PM6600 include on-chip MOSFETs to reduce part count, and operate up to 1 MHz to minimise filter components.

The 30 mA 7706 and 85 mA 7707, with an output voltage of 36 V, enable a single-chip solution for LCD panels as large as 17 inches. An external synchronisation pin allows connection of extra devices for larger screen sizes. The 7707's 85 mA output also satisfies standalone LED lighting applications.

The third device introduced, the PM6600, joins ST's PM66xx family, which provides a complete notebook and Ultra-Mobile PC (UMPC) power-management solution. Six 36 V constant-current channels of 30 mA serve applications up to large notebook size. Like the other two devices, the PM600 features a synchronisation pin to operate larger displays. In addition, it implements extensive fault-management to control backlight operation if the display sustains damage to the flexible cable or to groups of LEDs. Detection of intermittent faults, shorted LEDs, single or multiple open rows, or combinations of defects, allow fault-tolerant operation in the field and assist with maintenance and repair. The PM6600 accepts input voltages from 4,5 to 28 V.

All three devices include power-saving modes to optimise efficiency under light-load conditions, as well as PWM dimming down to 1% duty cycle. Each device is stable with a ceramic output capacitor to reduce bill-of-materials costs. There is also built-in protection against thermal overload, output over-voltage and MOSFET over-current. A soft-start function is also featured, to maximise the lifetime of the backlight.

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