Datasheet

+
-
APPROX
V
DD
/2
C
LOAD
50 - 200nF
V
COM
LOAD
R
ESR
20 - 100:
V
COM
DRIVER
V
COM
V
COM
COLUMN
LINE
R R
R
R
R
C C
C
C
C
LM6588
SNOSA77D –MAY 2003–REVISED MARCH 2013 www.ti.com
Figure 27. Model of Impedance between V
COM
and Column Lines
A V
COM
driver is essentially a voltage regulator that can source and sink current into a large capacitive load. To
simplify the analysis of this driver, the distributed RC network of Figure 27 has been reduced to a single RC load
in Figure 28. This load places a large capacitance on the V
COM
driver output, resulting in an additional pole in the
op amp’s feedback loop. However, the op amp remains stable because C
LOAD
and R
ESR
create a zero that
cancels the effect of this pole. The range of C
LOAD
is 50nF to 200nF and R
ESR
is 20Ω to 100Ω, so this zero will
have a frequency in the range of 8KHz to 160KHz, which is much lower than the gain bandwidth 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 28. When driving an RC load of 50nF and 20Ω, the LM6588 has a unity gain frequency of
6.12MHz with 41.5°C of phase margin. If the load capacitor is increased to 200nF and the resistance remains
20Ω, the unity gain frequency is virtually unchanged: 6.05MHz with 42.9°C of phase margin.
Figure 28. V
COM
Driver with Simplified Load
A V
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:
I
OUT
= 14V/μs x 50nF = 700mA (1)
700mA exceeds the maximum current specification for the LM6588 and almost all other op amps, confirming that
a V
COM
driver’s speed is limited by its peak output current. In order to minimize V
COM
transients, the op amp used
as a V
COM
Driver must supply large values of output current.
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