LMH6640MFSLASHNOPB datasheet(13/22 Pages) TI1

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10:

RF2

300:

RF1

3 k:

68n

47n

HIGH

CURRENT

DRIVER

VOUT

IOUT

18:

LMH6640

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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 VCOM 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 VCOM 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 VCOM application.

Figure 33, shows a typical simplified VCOM application with the LMH6640 buffering the VCOM 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 VCOM 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 VCOM load.

The modeled TFT VCOM 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 VCOM 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 20

Ω-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 54

Ω as

shown in Figure 33.

Figure 33. LMH6640 in a VCOM Buffer Application with Simulated TFT Load

RS is sometimes used in the panel to provide additional isolation from the load while RF2 provides a more direct

feedback from the VCOM. RF1, RF2, and RS 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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