White LED Back Lighting Solutions
The LCD panel is a major portion of the digital photo frame, using white LEDs for back lighting. Regulators and drivers (LM2733 & LM27313) are applied to the white LED BLU solution for major models and key power areas are bias (LM2622) and video buffering (LMH6683). Audio and other solutions can be applied as shown on Figure 1.
The LED module in Figure 2 is composed of 24 total white LEDs, 3 in series and 8 sets in parallel (landscape). There are many types of LED modules. A 7" model uses 3 by 7 array (or 7 by 3 array). Since the required voltage and current depends on these arrays, the selection of LED drivers is important early in the design phase. Figure 2 is an example circuit for driving 8"LCD modules.
Figures 2 & 3 show two circuits of white LED back lighting for 8" digital photo frame LCDs. Note the differences between the two designs (#2 for landscape, #3 for portrait layouts). The most important fact in designing a circuit for driving LED back lighting is to keep constant current during LED forward bias. Since LEDs are a current driven device, the intensity of light depends on the quantity of current. Keeping constant current is necessary to maintain the intensity of light and improve the LED life-time. Which layout you use is a critical part of the design solution.
Figure 2. 8" LCD Module with circuit for 3 x 8 backlight LEDs
Figure 3. 8 by 3 LED Module driving Circuit for 8" LCD
#3 Steps to Design Process
Now that some basics are covered, actual design implementation can be discussed. The different array types and component selections all play a major role in circuit layout. It is important for the current to be controlled and to maintain the working range of the specific LEDs chosen. By following these simple steps, backlighting for digital frames can be designed successfully.
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