New trend to improve the performance of LCD TV LED backlight
Many traditional CCFL backlight systems cannot add phase delay to CCFL lamps. Based on this, many LED backlight designers continue this practice without adding any phase delay to the LED backlight string, except for advanced liquid crystals with reduced noise and many high-end direct-lit backlight architectures. Designers of these traditional CCFL backlight systems often do not have control signals that can be used for multi-channel pulse width modulation phase delay. In addition, limited time and resources limit the addition of new control interfaces and microcontrollers for TV systems. The latest generation of LED backlight IC controllers can solve this problem by generating a user-programmable phase delay for the LED backlight current sink pulse width modulated dimming signal. Figure 7 shows that the LCD TV designed for CCFL backlight has a simple pulse width modulation dimming signal in the range of 120Hz to 480Hz, and uses a programmable phase delay. In this case, an edge-lit backlight application can use 6 LED current sink channels, thereby realizing the upgrade from the existing CCFL LCD TV to the LED backlight. These 6 channels have a phase delay of 60 degrees, which can be used to evenly distribute the channel on / off time and reduce the peak current and average current through the LED string AC-DC or DC-DC boost power supply. This improves power supply efficiency, and by reducing peak inductor current, low-cost inductors with lower saturation current specifications can be selected for the system. in conclusion With the continuous development of LED TV backlight systems, the new LCD TV LED backlight control IC has brought better solutions for designers. When selecting the LED backlight control IC, the following system characteristics should be considered: LED VF specifications and tolerances The total value of LED string voltage required according to the amount of LED VF Can the system use ICs with integrated current sinks or require controllers with external MOSFETs? Calculate the power consumption caused by the voltage mismatch of parallel LEDs and the maximum LED current required Once the highest level of system parameters is reduced, IC can be selected. The designer must solve the problem of the power consumption of the LED string in the worst case. The LED power supply design should have a positive feedback signal from the control IC current sink to reduce the excessive supply voltage of the LED string, thereby maximizing the overall system power conversion efficiency. When arranging the printed circuit board of the backlight power supply, please consider the thermal properties of the circuit board and the placed components, disperse the power consumption as much as possible, and maximize the distribution of the heat and heat source of the circuit board. In addition, designers should take advantage of advanced new features in the latest generation of LED backlight control ICs, such as pulse width modulation dimming phase delay control, power supply current detection feedback, and the configuration of multiple LED strings and / or channels for any number of LEDs based on the established string Function. ICs with better voltage ratings are now available on the market. They can work directly from 12V to 24V power supplies and can be turned off at 50V. For those designs that require higher voltage and LED current, the controller that drives the external MOSFET can be selected and can be upgraded to meet the requirements of today's advanced LCD TV backlight applications.
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