Datasheet

R
S
10:
+
-
R
F2
300:
R
F1
3 k:
68n
C1
47n
C2
47n
C3
47n
C4
HIGH
CURRENT
DRIVER
V
OUT
I
OUT
R
1
18:
R
2
18:
R
3
18:
LMH6640
www.ti.com SNOSAA0B –FEB 2004–REVISED MARCH 2013
APPLICATION INFORMATION
Application Notes
With its high output current and speed, one of the major applications for the LMH6640 is the V
COM
driver in a TFT
panel. This application is a specially taxing one because of the demands it places on the operational amplifier’s
output to drive a large amount of bi-directional current into a heavy capacitive load while operating under unity
gain condition, which is a difficult challenge due to loop stability reasons. For a more detailed explanation of what
a TFT panel is and what its amplifier requirements are, please see the Application Notes section of the LM6584
found on the web at: http://www.ti.com/lit/pdf/snosb08
Because of the complexity of the TFT V
COM
waveform and the wide variation in characteristics between different
TFT panels, it is difficult to decipher the results of circuit testing in an actual panel. The ability to make simplifying
assumptions about the load in order to test the amplifier on the bench allows testing using standard equipment
and provides familiar results which could be interpreted using standard loop analysis techniques. This is what
has been done in this application note with regard to the LMH6640’s performance when subjected to the
conditions found in a TFT V
COM
application.
Figure 33, shows a typical simplified V
COM
application with the LMH6640 buffering the V
COM
potential (which is
usually around ½ of panel supply voltage) and looking into the simplified model of the load. The load represents
the cumulative effect of all stray capacitances between the V
COM
node and both row and column lines.
Associated with the capacitances shown, is the distributed resistance of the lines to each individual transistor
switch. The other end of this R-C ladder is driven by the column driver in an actual panel and here is driven with
a low impedance MOSFET driver (labeled “High Current Driver”) for the purposes of this bench test to simulate
the effect that the column driver exerts on the V
COM
load.
The modeled TFT V
COM
load, shown in Figure 33, is based on the following simplifying assumptions in order to
allow for easy bench testing and yet allow good matching results obtained in the actual application:
• The sum of all the capacitors and resistors in the R-C ladder is the total V
COM
capacitance and resistance
respectively. This total varies from panel to panel; capacitance could range from 50 nF-200 nF and the
resistance could be anywhere from 20Ω-100Ω.
• The number of ladder sections has been reduced to a number (4 sections in this case) which can easily be
put together in the lab and which behaves reasonably close to the actual load.
In this example, the LMH6640 was tested under the simulated conditions of total 209 nF capacitance and 54Ω as
shown in Figure 33.
Figure 33. LMH6640 in a V
COM
Buffer Application with Simulated TFT Load
R
S
is sometimes used in the panel to provide additional isolation from the load while R
F2
provides a more direct
feedback from the V
COM
. R
F1
, R
F2
, and R
S
are trimmed in the actual circuit with settling time and stability trade-
offs considered and evaluated. When tested under simulated load conditions of Figure 33, here are the resultant
voltage and current waveforms at the LMH6640 output:
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