AN2687

LCD drive signals

Doc ID 14296 Rev 1

9/22

2.1.2

Contrast calculation

The performance of an LCD driving system is defined by the contrast.

Contrast (D) = Vrms(On) / Vrms(Off)

For the quadruplex signal as described on the previous page:

Vrms(On) =

=

Vrms(On) =

Vrms

(Off) =

For comparison, a hardware LCD drive uses 1/3 bias voltage. With 1/3 bias control, the

contrast value (D) is 1.73. Therefore, 1/3 bias gives only a small contrast advantage. This

advantage is reduced to zero when using software contrast control.

1

T

---

ft

0

T











1

T

---

V

DD

V

DD



2

dt

V

DD

2

----------





2

dt

2T

8

-------

T



+

T

8

---

2T

8

-------



+

0

T

8

---















1

T

---

V

DD



2

T

8

---

V

DD



2

T

8

---

V

DD



2

4

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

6T



8

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



+



+









0.661V

DD

=

1

T

---

0

dt

0

dt

V

DD

2

----------





2

dt

2T

8

-------

T



+

T

8

---

2T

8

-------



+

0

T

8

---















0.433V

DD

=