MC34845 LED driver quietens LED backlights
Freescale Semiconductor
MC34845 LED driver
Freescale Semiconductor has introduced the MC34845 LED driver with high-speed pulse width modulation (PWM) capability.
This six-channel device is designed to allow LED backlights to operate at frequencies above the audible range, thereby eliminating noise issues and reducing visual artefacts.
In addition, the driver's accuracy and flexibility offer display uniformity and a wide dimming range.
The MC34845 LED driver is ideal for backlighting applications in notebook displays ranging in size from 7-17in (178-432mm), 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 (10in and greater) thin-film transistor (TFT) LCDs is expected to grow from approximately seven per cent in the fourth quarter of 2008 to 23 per cent in the fourth quarter of 2009.
By 2012, more than half of all large-size TFT LCDs - and 100 per cent 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 with CCFL displays, which contain mercury.
The high-speed LED driver supports up to 100kHz PWM, with pulses down to 200ns.
This performance is suitable for 1000:1 dimming range at 25kHz with excellent linearity across the range, supporting imperceptible dimming steps.
The driver is also accurate, supporting +/- two per cent current matching maximum across the operating range of -40C to +85C, which helps to 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 600Khz (MC34845) or 1.2MHz (MC34845A).
The input range supports two-cell and three-cell lithium-ion batteries or a fixed-input supply.
High-voltage output capability enables the device to drive up to 96 white LEDs.
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.
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