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Wei-Chung Cheng, Member, IEEE, Massoud Pedram, Fellow, IEEE - page 4 / 8

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Normalized CCFL Luminance

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Normalized Transmissivity

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Power Consumption (W)

Normalized TFT-LCD Power

Fig. 3: (a) Luminance/Power characterization of CCFL (b) Transmissivity/Power characterization of TFT-LCD panel.

III. TFT-LCD GRAYSCALE CONTROL AND POWER MODELING

In a TFT-LCD display, each sub-pixel has an individual liquid crystal cell, a thin-film-transistor (TFT) and a capacitor. The electrical field of the capacitor controls the orientation of the liquid crystals within the cell, which indeed determines the transmissivity. The capacitor is charged and discharged by its own TFT. The gate electrode of the TFT controls the timing of charging/discharging when the pixel is scanned for refreshing its content. The source electrode of the TFT controls the amount of charge that determines the transmissivity of the liquid crystal cell. The gate electrodes and source electrodes of all TFTs are driven by a set of gate drivers and source drivers, respectively. A single gate driver drives all gate electrodes of the pixels on the same row. The gate electrodes are enabled at the same time the row is scanned. A single source driver drives all source electrodes of the pixels on the same column. The source driver supplies the desired voltage level (called grayscale voltage) according to the pixel value. In other words, ideally, the transmissivity t(v(x)) is a linear function of the grayscale voltage v(x), which is a linear function of the pixel value x. If there are 256 grayscales, then the source driver must be able to supply 256 different grayscale voltage levels. For the source driver to provide a wide range of grayscales, a number of reference voltages are required. The source driver mixes different reference voltages to obtain the desired grayscale voltages. Typically, these different reference voltages are fixed and designed as a voltage divider. For example in [10], an LCD reference driver [11] is used with a 10-way voltage divider. Assume that the transmissivity of the TFT-LCD is linear and the resistors of the voltage divider are identical.

If k identical b e t w e e n V k a n d

resistors r1…rk ground, then the

are connected in output voltage from

series rk is

i V

i k

Vk

.

(6)

1

1 b

t

bt

b

*

  • 0

    gl

gu

1

=

0

gl

t=cx+d gu 1

x

x

(a)

(b)

Fig. 4: The transmissivity function (a) and luminance function (b) when using a programmable LCD reference driver.

A. Programmable LCD Reference Driver

Our approach to CBCS is to control the mapping of v(x) in order to control the transmissivity function t(x). We propose using a programmable LCD reference driver (PLRD) described as follows.

The PLRD is implemented by adding an extra logic to the original voltage divider expressed by (6). The logic contains a number of p-channel and n-channel switches and multiplexers. The PLRD takes two input arguments gl a n d g u , a n d t h e n c o n n e c t s r g u , r g u + 1 r k t o V k a n d r 0 , r 1 r g l t o g r o u n d . I n t h i s w a y , t h e o u t p u t v o l t a g e s e e n f r becomes o m r k

' Vi , gl , gu

V k i g l , l gu g 0,

Vk ,

guik

gligu

0igl

.

(7)

Clearly, the PLRD performs a linear transformation ( l i m i t e d b y 0 a n d V k ) o n t h e o r i g i n a l r e f e r e n c e v o l t a g e s and therefore, provides the CBCS policy a mechanism for adjusting the TFT-LCD transmissivity function as shown in Fig. 4a. The luminance function is shown in Fig. 4b. ,

The similar concept of PLRD has been implemented in TFT-LCD controllers such as [12] to control contrast or gamma-correction. The PLRD represents a class of linear transformations on the backlight-scaled image. It covers both brightness scaling (adjusting gu and gl simultaneously) and contrast scaling (adjusting gu-gl). On the other hand, non-linear transformations are not desired in backlight scaling because they cannot preserve the uniformity of contrast.

B. TFT-LCD Power Characterization The TFT power can be modeled by a quadratic function

of pixel value x[0,255] [13]:

PTFT (x)=c0+c1x+c2x2 (Watt).

(8)

We performed the current and power measurements on [10]. The measurement data are shown in Fig. 3b. Plugging into (8), the coefficients are found as:

c0=2.703E-3, c1=2.821E-4, c2=2.807E-5.

(9)

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