Software contrast control

AN1048

16/21

4

Software contrast control

The software contrast control is under pending patent from STMicroelectronics. The use of

this technique with a non-STMicroelectronics' Microcontroller has to be agreed by

STMicroelectronics.

The LCD contrast is controlled entirely by software without the use of any external

components. LCD contrast can be adjusted to the optimal value depending on the operating

voltage of the LCD used. The LCD contrast is controlled by varying the timing of dead phase

as shown in the LCD timing diagram.

Deadtime can be used to decrease as well as to increase the Vrms of the LCD. Deadtime is

the voltage compensation time to regulate rms voltage up and down. Dead time can be

implemented either after each control period or at the end of the frame. To avoid flickering,

the duration of the dead time must be adjusted depending on the quality of LCD and the

frequency of the frame.

In the example in Figure 9, the Rms value of the LCD decreases if the duration of dead time

is increased and Rms value increases if the duration of dead time decreases. In Figure 10,

this works the opposite way.

4.1

Contrast calculations

Let the frame period = T + xT

where T - Active Time, xT- Dead Time

x- Proportion of dead time, Vx - Voltage during the dead time

Vrms ON

()

1

TxT

+

-----------------

ft

()

2

t

d

0

TXT

+

()

∫

=

Vrms ON

()

1

TxT

+

-----------------

Vcc

()

2

tVcc

–

()

2

tVcc

() 2

⁄

()

2

tVx

2

() t8

⋅

d

0

xT

8

-------

∫

+

d

2T

8

-------

T

∫

+

d

T

8

---

2T

8

-------

∫

+

d

0

T

8

---

∫

⎝⎠

⎜⎟

⎜⎟

⎜⎟

⎛⎞

=

Vrms ON

()

1

TxT

+

-----------------

Vcc

()

2 T

8

---

Vcc

()

2 T

8

---

Vcc

()

2

4

------------------- 2T

8

-------

Vx

2 xT

8

-------

⋅

⎝⎠

⎛⎞

8

⋅

+

⋅

+

⋅

+

⋅

⎝⎠

⎜⎟

⎛⎞

=