LM8261 datasheet(17/19 Pages) NSC

17 / 19 page

Application Notes: (Continued)

TFT applications:

Figure 8 below, shows a typical application where the

LM8261 is used as a buffer amplifier for the V

com signal em-

ployed in a TFT LCD flat panel:

Figure 9 shows the time domain response of the amplifier

when used as a V

com buffer/driver with VREF at ground. In

this application, the Op Amp loop will try and maintain its out-

put voltage based on the voltage on its non-inverting input

REF) despite the current injected into the TFT simulated

load. As long as this load current is within the range tolerable

by the LM8261 (45mA sourcing and 65mA sinking for +/−5V

supplies), the output will settle to its final value within less

than 2µs.

Output Short Circuit Current and Dissipation Issues:

The LM8261 output stage is designed for maximum output

current capability. Even though momentary output shorts to

ground and either supply can be tolerated at all operating

voltages, longer lasting short conditions can cause the junc-

tion temperature to rise beyond the absolute maximum rat-

ing of the device, especially at higher supply voltage condi-

tions. Below supply voltage of 6V, output short circuit

condition can be tolerated indefinitely.

With the Op Amp tied to a load, the device power dissipation

consists of the quiescent power due to the supply current

flow into the device, in addition to power dissipation due to

the load current. The load portion of the power itself could in-

clude an average value (due to a DC load current) and an AC

component. DC load current would flow if there is an output

voltage offset, or the output AC average current is non-zero,

or if the Op Amp operates in a single supply application

where the output is maintained somewhere in the range of

linear operation. Therefore:

total =PQ +PDC +PAC

Q =IS ·VS

Op Amp Quiescent

Power Dissipation

DC =IO ·(Vr -Vo)

DC Load Power

AC = See Table 1 below

AC Load Power

where:

s: Supply Current

s: Total Supply Voltage (V

+ -V−)

o: Average load current

o: Average Output Voltage

r:V

+ for sourcing and V− for sinking current

Table 1 below shows the maximum AC component of the

load power dissipated by the Op Amp for standard Sinusoi-

dal, Triangular, and Square Waveforms:

TABLE 1. Normalized AC Power Dissipated in the

Output Stage for Standard Waveforms

AC (W.Ω/V

Sinusoidal

Triangular

Square

50.7 x 10

−3

46.9 x 10

−3

62.5 x 10

−3

The table entries are normalized to V

2/R

L. To figure out the

AC load current component of power dissipation, simply mul-

tiply the table entry corresponding to the output waveform by

the factor V

2/R

L. For example, with ±15V supplies, a 600Ω

load, and triangular waveform power dissipation in the out-

put stage is calculated as:

AC= (46.9 x 10

−3) · [302/600]= 70.4mW

DS101084-59

FIGURE 7. Output Sinking Characteristics with Load

Lines

DS101084-61

FIGURE 8. V

com driver application schematic

DS101084-65

FIGURE 9. V

com driver performance scope photo

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