INPUT VOLTAGE Most LCD backlight drivers run off a 12-V-dc input, although applications may range from 5 to 48 V dc. CCFL control loops may be open or closed. Open-loop designs require regulated input supplies, because strike voltage and output current vary with input voltages. Closed-loop CCFL drivers provide constant strike voltage and current over a range of input voltages. As a result, they’re more desirable in applications that don’t have a regulated input. Typically, battery-powered applications fall into this category.
For LED drivers, VCC must be greater than the minimum that’s required to power the LED string and sense resistor. The dc-dc boost stage must be closed loop to provide a relatively stable VCC with no- or full-load conditions.
While obvious major differences exist between CCFL and LED backlighting of LCDs, the driver designer must observe certain similarities and basic principles. These include allowing for the backlight’s ambient temperature, paying particular attention to low temperatures for CCFLs, and high temperatures for LEDs.
The key challenge for CCFL backlights concerns CCFL packaging and driver layout due to high voltages, while the key challenge for LED backlights revolves around the load packaging due to thermal management. Either technology is best served by PWM as opposed to analog dimming. Though this article focuses on the major considerations, other factors, of course, must be considered in optimizing the driver design for application-specific demands, cost control, manufacturability, and reliability.
Highlights
Thermal Considerations Ambient temperature is a key consideration for backlight-driver designs. CCFL starting or strike voltage is inversely proportional to temperature. The time a CCFL needs to reach specified brightness is also inversely proportional to temperature. LED backlights, on the other hand, are less sensitive to low temperatures.
Dimming Analog dimming of CCFL backlights, wherein the driver output current is modulated to change lamp brightness, provides coarse dimming to about 30% of full brightness—not enough dynamic range for most application requirements. PWM dimming brings significantly better dimming control for both CCFLs and LEDs. Here, the CCFL or LED is pulsed on and off at a fixed frequency, and the duty cycle is modulated to provide variable brightness.
Input Voltage Most LCD backlight drivers run off a 12V-dc input, although applications may range from 5 to 48 V dc. CCFL control loops may be either open loop, requiring regulated input supplies, or closed loop, which provides constant strike voltage and current over a range of input voltages. For LED drivers, VCC must be greater than the minimum that's required to power the LED string and sense resistor.
We are a professional CCFL (Cold Cathode Lamp) factory, we can proivde all kinds of LCD replacement or repair Backlight tube. Out-side diameter from 1.80-4.0mm, length from 10-600mm.
sissy -July 10, 2009
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