Display Technology Figure 3: 90% efficient 28 Watt LED backlighting circuit
voltage to as low as 6V upon ignition, after which it typically returns to a nominal 12V. Linear Technology’s LT3760 has been developed and can survive and regulate a fixed output voltage throughout both of these conditions. Its input voltage range of 6V to 40V makes it suitable for the automotive environment. Even, when VIN is greater than VOUT, which could occur
during a 40V transient, it can regulate the required LED current.
Most automotive LCD backlighting applications require between 20W and 35W of LED power. The LT3760 can boost the automotive bus voltage (6V to 18V/nominal 12V) to as high as 44V to drive up to eight parallel strings, each containing ten 80mA LEDs in series. Figure 3 shows a schematic of the LT3760
driving eight parallel strings, each string comprised of ten 80mA LEDs, for a total of 28W. The LT3760 utilizes an adaptive feedback loop design which adjusts the output voltage so that it is slightly higher than the highest voltage LED string. This minimises power lost through the ballast circuitry and helps to optimise the efficiency. The circuit offers efficiency of greater than 90% which is important because it eliminates any requirement for heat sinking, enabling a very compact low profile footprint. Equally important for driving arrays of LEDs is to provide accurate current matching to insure that the backlighting brightness remains uniform across the entirety of the panel. The device uses a fixed frequency, constant current boost controller topology. Using a single 60V external N-Channel MOSFET, it is capable of driving eight strings of up to ten 100mA LEDs connected in series. Its switching frequency is programmable and synchronisable between 100kHz and 1MHz enabling it to deliver optimum efficiency while minimising the size of the external components. Its design also enables it to run four strings of 200mA LEDs or even two strings of 400mA LEDs. Each string of LEDs can use the same number of LEDs or can be run asymmetrically with a different number of LEDs per string. It also offers direct PWM with dimming ratios as high as 3,000:1 and analogue dimming via the control pin which offers ratios as high as 25:1. In applications which require dimming ratios of up to
30,000:1 these two dimming functions can be combined to reach the required ratio. It appears that as automotive display technology is perfected, most manufactures prefer to use higher LED currents (50mA to 100mA) to make the display more readable in bright ambient lighting and a higher dimming ratio to compensate for varying ambient conditions. Integrated protection features include open and short circuit protection and a FAULT diagnostic pin. The continual acceleration of LED backlighting applications in automotive displays is driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new LED driver ICs. As a result, these LED drivers need to be able to provide constant current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations, operate with high efficiency, offer very wide dimming ratios and have a variety of protection features to enhance system reliability. Of course, these LED driver circuits must also offer a very compact, low-profile and thermally efficient solution footprint. But while automotive lighting systems will continue to demand higher performance LED drivers, there are plenty of designers who will have the IC solutions to meet that demand.
Linear Technology |
www.linear.com
Jeff Gruetter is Sr. Product Marketing Engineer, Power Products, Linear Technology
COMPONENTS IN ELECTRONICS
Distributor Directory ’11
Now in its 20th year, the Distributor Directory has established itself as the essential handbook for electronics buyers and specifiers. No other resource provides such comprehensive and up-to-date information and listings on the manufacturers and their products offered by over 500 distributors within the UK.
/
www.cieonline.co.uk
Order today and you can save up to 25% off the cover price of £65 (incl P&P). Be quick, this offer is only available for a limited period only! Save 25% when you order 5 copies or more copies. Save 15% when you order 1-4 copies
Call our credit card hotline on 08450 138426 quoting reference DD11PA Or go to
www.cieonline.co.uk to print the order form
Components in Electronics October 2011 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52