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LED Backlight Drive Methods




Introduction:

LED backlights for LCD modules are generally driven with a DC voltage through a current limiting resistor. This approach is acceptable for most applications. This application note discusses issues such as extra bright display requirements, lowest possible power consumption rates and backlights controlled over a wide brightness range.


Description:

By using a pulse width modulation scheme, several advantages are realized over a simple DC voltage method. The main advantage is in efficiency. The LEDs are pulsed with high current for a short period of time. For example, consider the NHD-0216K1Z. The nominal LED driving current for this display is 120mA which produces a typical brightness of 50 NIT. If, instead of a DC or constant current, we apply 5 times the current (600mA), for 1/5 of the time, the average current is the same (120mA). See Fig.1 The brightness average of the LED would also be the same if measured by a meter. The only difference is the PERCEIVED brightness.


The human eye has a certain degree of persistence. If exposed to a bright light, the eye will remember the light for a short period of time. This allows us to view a motion picture or television screen as a steady image when in fact it is flickering at 24 - 30 times per second. When the LED is flashed "ON" for a short period of time and then turned off, the eye remembers the light at the high brightness level. The result is that the perceived brightness of the backlight is closer to the high pulsed brightness than to the lower DC brightness.


This effect provides several advantages as follows...


If the brightest possible backlight is required, the display can be pulsed at a 1:4 on/off ratio with 5x the typical current. The pulse frequency repetition should be greater than 100Hz but not greater then 1,000Hz, so the flickering is not perceived by the eye.


This technique can also be used to provide a normal looking brightness level to the display but at a lower average current to save power. The average power can be cut by a factor of at least 50% to produce a given perceived brightness level. This may provide a big advantage in battery operated products.


Another use of this method is to facilitate a wide range brightness control for the LED backlight. By adjusting the on/off ratio, a wide range of brightness can be achieved while maintaining a very even backlight appearance. See Fig.2. One can also vary the brightness by adjusting the DC current to the LEDs. But at low current the individual LED emitters start to become visable, resulting in an uneven looking backlight. To implement this technique, the peak current should be set at the specified typical current for the display and the on/off ratio of the pulses varied from near 100% on to near 0% on.






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