{"id":1511,"date":"2014-04-30T10:28:51","date_gmt":"2014-04-30T10:28:51","guid":{"rendered":"http:\/\/bitmap2lcd.com\/blog\/?p=1511"},"modified":"2014-07-30T14:27:06","modified_gmt":"2014-07-30T14:27:06","slug":"about-led-back-light-driving-methods","status":"publish","type":"post","link":"https:\/\/bitmap2lcd.com\/blog\/about-led-back-light-driving-methods\/","title":{"rendered":"About LED Back light driving methods"},"content":{"rendered":"

About LED Back light driving methods<\/h3>\n

( A Hantronix Application Note<\/a> )<\/p>\n

\u00a0LED Back Light Driving Methods<\/p>\n

I. Introduction:<\/strong>
\nLED back lights on LCD modules are generally driven with a DC voltage through a
\ncurrent limiting resistor. This simple approach is perfectly acceptable for most
\napplications. When the primary consideration is an extra bright display, the lowest
\npossible power consumption, or a back light that can be controlled over a very wide
\nbrightness range another method is needed. The purpose of the paper is to describe this
\nmethod.<\/p>\n

II. Description:<\/strong>
\nBy using a pulse width modulation scheme several advantages can be realized over the
\nsimple DC voltage method. The main advantage is in efficiency. The LED’s are pulsed
\nwith a high current for a short period of time. For example consider the HDM16216L-7.
\nThe nominal LED driving current for this display is 120 mA which produces a typical
\nbrightness of 50 NIT. If, instead of a DC or constant current, we apply5 times the current,
\n600ma, for 1\/5 of the time, the average current is the same, 120ma. See Figure #1. The
\naverage brightness of the LED would also be the same if measured electronically. The
\ndifference is in the perceived<\/span> brightness<\/p>\n

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The human eye has a certain amount of persistence. If exposed to a bright light the eye
\nwill “remember” the light for a short period of time. This allows us to view a motion
\npicture or TV screen as a steady image when in fact it is flickering at 24 to 30 times a
\nsecond. When the LED is flashed on brightly for a short time and then turned off the eye
\n“remembers” the light at the high brightness level. The result is that the perceived
\nbrightness of the back light is closer to the high pulsed brightness than to the lower DC
\nbrightness.<\/p>\n

This effect can be used to advantage in several ways. If the brightest possible back light is needed the display can be pulsed at a 1:4 on\/off ratio with5 times the typical current. The pulse repetition frequency should be greater than 100Hz so the flickering is not perceptible to the eye but not greater than about 1kHz.<\/p>\n

This technique can also be used to give a “normal” looking brightness level to the display
\nbut at a lower average current to save power. The average power can be cut by a factor
\nof at least 50% to produce a given perceived brightness level. This can be a big advantage in battery operated equipment.<\/p>\n

The third use of this method is to facilitate a wide range brightness control for the LED
\nback light. By varying the on\/off ratio a very wide range of brightness can be achieved
\nwhile maintaining a very even appearing back light. See Figure #2. One can also very the
\nbrightness by simply varying the DC current to the LED’s but at low current the individual
\nLED emitters start to become visible resulting in an uneven looking back light. To implement this technique the peak current should be set at the specified typical current for the display and the on\/off ration of the pulses varied from near 100% on to near 0% on.<\/p>\n

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