+

-

VSW

RL1

10:

-6.5V

VCOM

+6.5V

IOUT

C1

10nF

C2

10nF

C3

10nF

C4

10nF

RL3

10:

RS

10:

RL2

10:

VCOM LOAD

0V

5V

RF2

300:

RF1

3k:

+

-

APPROX

VDD/2

CLOAD

50 - 200nF

VCOM LOAD

RESR

20 - 100:

VCOM DRIVER

VCOM

LM6584

SNOSA44C – MAY 2003 – REVISED MARCH 2013

www.ti.com

Figure 29. VCOM Driver with Simplified Load

A VCOM 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 LM6584 slew rate of 14V/

μs:

IOUT = 14V/μs x 50nF = 700mA

(1)

700mA exceeds the maximum current specification for the LM6584 and almost all other op amps, confirming that

a VCOM driver’s speed is limited by its peak output current. In order to minimize VCOM transients, the op amp used

as a VCOM Driver must supply large values of output current.

Figure 30. VCOM Driver Test Circuit

Figure 30 is a common test circuit used for measuring VCOM driver response time. The RC network of RL1 to RL3

and C1 to C4 models the distributed RC load of a VCOM line. This RC network is a gross simplification of what the

actual impedance 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

VSW, generating large current pulses in the RC network. Scope photos from this circuit are shown in Figure 31

and Figure 32. Figure 31 shows the test circuit generates positive and negative voltage spikes with an amplitude

of ±3.2V at the VCOM node, and both transients settle-out in approximately 2μs. As mentioned before, the speed

at which these transients settle-out is a function of the op amp’s peak output current. The IOUT trace in Figure 32

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 VCOM Driver applications.

16

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