LM6588 datasheet(12/16 Pages) NSC

12 / 16 page

TFT Display Application (Continued)

8KHz to 160KHz, which is much lower than the gain band-

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

COM load adds very

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

allows the V

COM Driver to remain stable. This was verified by

measuring the small-signal bandwidth of the LM6588 with

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

and 20

Ω, the LM6588 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.

COM Driver’s large-signal response time is determined by

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 LM6588

slew rate of 14V/µs:

OUT = 14V/µs x 50nF

= 700mA

700mA exceeds the maximum current specification for the

LM6588 and almost all other op amps, confirming that a

COM driver’s speed is limited by its peak output current. In

order to minimize V

COM transients, the op amp used as a

COM Driver must supply large values of output current.

Figure 5 is a common test circuit used for measuring V

COM

driver response time. The RC network of R

L1 to RL3 and C1

to C

4 models the distributed RC load of a VCOM line. This RC

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

SW, generating large

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

spikes with an amplitude of ±3.2V at the V

COM node, and

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 LM6588 can sink and source

peak currents of −200mA and 200mA. This ability to supply

large values of output current makes the LM6588 extremely

well suited for V

COM Driver applications.

20073429

FIGURE 4. V

COM Driver with Simplified Load

20073430

FIGURE 5. V

COM Driver Test Circuit

20073431

FIGURE 6. V

SW and VCOM Waveforms from VCOM

20073432

FIGURE 7. V

SW and IOUT Waveforms from VCOM Test

Circuit

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