Freescale - LED driver eliminates audible noise and visual artifacts for LCDs

The MC34845 LED driver is ideal for backlighting applications in notebook displays ranging in size from seven inches to more than 17 inches, as well as other applications that use multiple low-power LEDs, such as lighting products.

According to DisplaySearch, the penetration rate of LED backlights in large-size (10 inches and greater) thin film transistor (TFT) LCDs is expected to grow from approximately 7 percent in the fourth quarter of 2008 to 23 percent in the fourth quarter of 2009. By 2012, more than half of all large-size TFT LCDs – and 100 percent of notebook PCs – will have LED backlights.

Factors driving the fast adoption of LED backlighting technology include energy-efficient operation leading to extended battery life, and the ability to design slimmer displays for thinner, lighter laptops. LED backlighting also offers an environmentally friendly solution compared to CCFL displays, which contain mercury.

“The outstanding performance, accuracy and flexibility of the MC34845 driver helps manufacturers avoid a host of issues associated with LED drivers operating at lower frequencies,” said Michael Jennings, market segment manager for display products in Freescale’s Analog, Mixed-Signal and Power Division. “Advanced technologies such as the MC34845 are helping fuel the rapid adoption of LED backlights in notebook PC markets worldwide.”

The high-speed LED driver supports up to 100 kHz PWM, with pulses down to 200ns. This performance is suitable for 1000:1 dimming range at 25 kHz with excellent linearity across the range, supporting imperceptible dimming steps. The driver is also very accurate, supporting ±2% current matching maximum across the operating range of -40 degrees C to +85 degrees C, which helps improve display uniformity. The outputs are driven using a single direct drive PWM input.

The MC34845 also features a boost converter to produce the required voltage to power LEDs. Two versions are available for boost switching frequencies of either 600 Khz (MC34845) or 1.2 MHz (MC34845A). The input range supports 2-cell and 3-cell lithium-ion batteries or a fixed-input supply. High-voltage output capability enables the device to drive up to 96 white LEDs, more than competing 6-channel designs. A dynamic headroom control automatically sets the output voltage depending on the voltage requirements of the LEDs.

The MC34845 supports low-power shutdown with 1uA max supply current. In addition, an auto shutdown mode can enable the device to enter shutdown after a period of inactivity on the PWM input.