2PSec/DIV

VSW

VCOM

+

-

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:

LM6588

www.ti.com

SNOSA77D – MAY 2003 – REVISED MARCH 2013

Figure 29. VCOM Driver Test Circuit

Figure 29 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 30

and Figure 31. Figure 30 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 31

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 VCOM Driver applications.

Figure 30. VSW and VCOM Waveforms from VCOM

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