LMH6640MF National Semiconductor, LMH6640MF Datasheet - Page 11

• Manufacturer Part Number: LMH6640MF
• Description: IC,Operational Amplifier,SINGLE,BIPOLAR,TSOP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LMH6640MF

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Typical Performance Characteristics
1 kΩ for A
Application Notes
With its high output current and speed, one of the major
applications for the LMH6640 is the V
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 Applica-
tion Notes section of the LM6584 found on the web at:
http://www.national.com/ds.cgi/LM/LM6584.pdf
Because of the complexity of the TFT V
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 famil-
iar 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
application.
Figure 1, shows a typical simplified V
the LMH6640 buffering the V
around
simplified model of the load. The load represents the cumu-
lative effect of all stray capacitances between the V
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
load.
The modeled TFT V
the following simplifying assumptions in order to allow for
easy bench testing and yet allow good matching results
obtained in the actual application:
1
⁄
2
V
of panel supply voltage) and looking into the
= −1. R
COM
L
tied to V
load, shown in Figure 1, is based on
COM
+
/2. Unless otherwise specified. (Continued)
potential (which is usually
COM
COM
COM
application with
driver in a TFT
waveform and
Differential Gain/Phase vs. IRE
COM
COM
COM
At T
11
J
In this example, the LMH6640 was tested under the simu-
lated conditions of total 209 nF capacitance and 54Ω as
shown in Figure 1.
R
isolation from the load while R
feedback from the V
the actual circuit with settling time and stability trade-offs
considered and evaluated. When tested under simulated
load conditions of Figure 1, here are the resultant voltage
and current waveforms at the LMH6640 output:
• The sum of all the capacitors and resistors in the R-C
• The number of ladder sections has been reduced to a
= 25˚C, V
FIGURE 1. LMH6640 in a V
S
ladder is the total V
spectively. This total varies from panel to panel; capaci-
tance could range from 50 nF-200 nF and the resistance
could be anywhere from 20Ω-100Ω.
number (4 sections in this case) which can easily be put
together in the lab and which behaves reasonably close
to the actual load.
is sometimes used in the panel to provide additional
+
= 16 V, V
Simulated TFT Load
20086225
COM
−
COM
= 0V, R
. R
F1
capacitance and resistance re-
COM
, R
F
F2
F2
= 330Ω for A
Buffer Application with
, and R
provides a more direct
S
are trimmed in
V
= +2, R
www.national.com
20086235
F
=