8KHz to160KHz, which is much lower than the gain band-

width of most op amps. As a result, the V

little phase lag when op amp loop gain is unity, and this

allows the V

measuring the small-signal bandwidth of the LM6584 with

the RC load of Figure 4. When driving an RC load of 50nF

and 20

Ω, the LM6584 has a unity gain frequency of 6.12MHz

with 41.5˚C of phase margin. If the load capacitor is in-

creased to 200nF and the resistance remains 20

Ω, the unity

gain frequency is virtually unchanged: 6.05MHz with 42.9˚C

of phase margin.

AV

the op amp’s maximum output current, not by its slew rate.

This is easily shown by calculating how much output current

is required to slew a 50nF load capacitance at the LM6584

slew rate of 14V/µs:

I

= 700mA

700mA exceeds the maximum current specification for the

LM6584 and almost all other op amps, confirming that a

V

order to minimize V

V

Figure 5 is a common test circuit used for measuring V

COM

driver response time. The RC network of R

to C

network is a gross simplification of what the actual imped-

ance is on a TFT panel. However, it does provide a useful

test for measuring the op amp’s transient response when

driving a large capacitive load. A low impedance MOSFET

driver applies a 5V square wave to V

current pulses in the RC network. Scope photos from this

circuit are shown in Figure 6 and Figure 7. Figure 6 shows

the test circuit generates positive and negative voltage

both transients settle-out in approximately 2µs. As men-

tioned before, the speed at which these transients settle-out

is a function of the op amp’s peak output current. The I

OUT

trace in Figure 7 shows that the LM6584 can sink and source

peak currents of −310mA and 320mA. This ability to supply

large values of output current makes the LM6584 extremely

well suited for V

GAMMA BUFFER

Illumination in a TFT display, also referred to as grayscale, is

set by a series of discrete voltage levels that are applied to

each LCD pixel. These voltage levels are generated by

resistive DAC networks that reside inside each of the column

driver ICs. For example, a column driver with 64 Grayscale

20059229

FIGURE 4. V

20059230

FIGURE 5. V

20059231

FIGURE 6. V

20059232

FIGURE 7. V

Circuit

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