LM8207MTSLASHNOPB datasheet(14/32 Pages) TI1

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OUT_VREF = 1.295 x ¨

§ R

1 + R2

OUT_VREF = 1.295 x ¨

§ R

1 + R2

+ R1 x IIN_V

REF

LM8207

OUT_VREF

IN_VREF

1.295V

VREF

IIN_V

REF

LM8207

SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013

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PRINCIPLE OPERATION of the LM8207

The LM8207 combines three basic functions used in TFT displays:

•

Voltage Reference

–

To improve picture quality and to reduce brightness variations, a highly stable reference voltage is

available. It has a low drift over the operation temperature range. This output voltage (OUT_VREF) is used

as the reference voltage to define the gamma correction values.

•

Gamma Buffers

–

The gamma correction curve can be defined easily using an external chain of precision resistors. To

ensure load independent gamma correction levels, 18 gamma buffers, each having a low output

resistance, can be used to drive the TFT display column drivers.

•

VCOM Buffer

–

The VCOM buffer supplies a common voltage, which is applied to the back plate of all the pixels. Writing

color information to all the pixels will cause high current variations at the VCOM level so this VCOM buffer is

designed for driving large output currents.

These three functions are discussed in detail in the following sections.

VOLTAGE REFERENCE

The internal Voltage Reference of the LM8207 can be used to improve picture stability. This accurate reference

is highly stable over the operation temperature range. The output voltage (OUT_VREF) of the Voltage Reference

can be set using two external resistors. In the next two sections, the possibilities for setting the output voltage of

the Voltage Reference and the operating range of the Voltage Reference are discussed.

SETTING THE OUTPUT VOLTAGE OF THE VOLTAGE REFERENCE

The output voltage of the Voltage Reference Amplifier (OUT_VREF) can be set using the internal reference in

combination with the internal amplifier and two external resistors. In Figure 33 a typical application circuit for VREF

is given.

Figure 33. Typical Application Circuit for VREF

To calculate the output voltage of the Voltage Reference Amplifier (OUT_VREF) use the following equation:

(1)

As can be seen in the 16V Electrical Characteristics table, IIN_VREF has a typical value of −10 nA. Using resistor

values for R1 = 9 kΩ and R2 = 1 kΩ this results in a gain of 10 and OUT_VREF = 12.95 V an error will be

introduced of

−10 nA*9 kΩ = −90 μV. This error can be neglected. The simplified formula for calculating the

OUT_VREF is:

(2)

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