OPA2743EA Datasheet, PDF(10/17 Page) Texas Instruments – 12V, 7MHz, CMOS, Rail-to-Rail I/O OPERATIONAL AMPLIFIERS

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OPA2743EA Datasheet, PDF (10/17 Pages) Texas Instruments – 12V, 7MHz, CMOS, Rail-to-Rail I/O OPERATIONAL AMPLIFIERS

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CAPACITIVE LOAD AND STABILITY

The OPA743 series op amps can drive up to 1000pF pure

capacitive load. Increasing the gain enhances the amplifierâs

ability to drive greater capacitive loads (see the typical

performance curve âSmall Signal Overshoot vs Capacitive

Loadâ).

One method of improving capacitive load drive in the unity-

gain configuration is to insert a 10â¦ to 20â¦ resistor inside

the feedback loop, as shown in Figure 4. This reduces

ringing with large capacitive loads while maintaining DC

accuracy.

20â¦

OPA743

VOUT

VIN

FIGURE 4. Series Resistor in Unity-Gain Buffer Configura-

tion Improves Capacitive Load Drive.

APPLICATION CIRCUITS

The OPA743 series op amps are optimized for driving

medium-speed sampling data converters. The OPA743 op

amps buffer the converterâs input capacitance and resulting

charge injection while providing signal gain.

Figure 5 shows the OPA743 in a dual supply buffered

reference configuration for the DAC7644.

REFERENCE BUFFER FOR LCD SOURCE DRIVERS

In modern high resolution TFT LCD displays, gamma cor-

rection must be performed to correct for nonlinearities in the

glass transmission characteristics of the LCD panel. The

typical LCD source driver for 64 Bits of Grayscale uses

internal DAC to convert the 6-Bit data into analog voltages

applied to the LCD. These DAC typically require external

voltage references for proper operation. Normally these ex-

ternal reference voltages are generated using a simple resis-

tive ladder, like the one shown in Figure 6.

Typical laptop or desktop LCD panels require 6 to 8 of the

source driver circuits in parallel to drive all columns of the

panel. Although the resistive load of one internal string DAC

is only around 10kâ¦, 6 to 8 in parallel represent a very

substantial load. The power supply used for the LCD source

drivers for laptops is typically in the order of 10V. To

maximize the dynamic range of the DAC, rail-to-rail output

performance is required for the upper and lower buffer. The

OPA4743âs ability to operate on 12V supplies, to drive

heavy resistive loads (as low as 1kâ¦), and to swing to within

325mV of the supply rails, makes it very well suited as a

buffer for the reference voltage inputs of LCD source drivers.

During conversion, the DACâs internal switches create cur-

rent glitches on the output of the reference buffer. The

capacitor CL (typically 100nF) functions as a charge reser-

voir that provides/absorbs most of the glitch energy. The

series resistor RS isolates the outputs of the OPA4743 from

the heavy capacitive load and helps to improve settling time.

NC 48

DAC7644

NC 47

NC 46

NC 45

VOUTA Sense 44

VOUTA 43

VREFL AB Sense 42

VREFL AB 41

VREFH AB 40

VREFH AB Sense 39

VOUTB Sense 38

VOUTB 37

VOUT

500pF

VOUT

500pF

1/2

OPA2743

1/2

OPA2743

Vâ

â2.5V

Ref

Negative

Reference

V+

+2.5V

Positive

Ref Reference

âV

FIGURE 5. OPA743 as Dual Supply Configuration-Buffered References for the DAC7644.

OPA743

SBOS201

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