CAPACITIVE LOAD AND STABILITY
Figure 5 shows the OPA743 in a dual supply buffered
reference configuration for the DAC7644.
The OPA743 series op amps can drive up to 1000pF pure
capacitive load. Increasing the gain enhances the amplifier’s
ability to drive greater capacitive loads (see the typical
performance curve “Small Signal Overshoot vs Capacitive
Load”).
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.
One method of improving capacitive load drive in the unity-
gain configuration is to insert a 10Ω to 20Ω resistor inside
the feedback loop, as shown in Figure 4. This reduces
ringing with large capacitive loads while maintaining DC
accuracy.
T
ypical 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 10kΩ, 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
OPA4743’s ability to operate on 12V supplies, to drive
heavy resistive loads (as low as 1kΩ), 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.
RS
20Ω
OPA743
VOUT
VIN
CL
RL
FIGURE 4. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
During conversion, the DAC’s 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.
APPLICATION CIRCUITS
The OPA743 series op amps are optimized for driving
medium-speed sampling data converters. The OPA743 op
amps buffer the converter’s input capacitance and resulting
charge injection while providing signal gain.
NC 48
NC 47
DAC7644
NC 46
NC 45
+V
V
OUTA Sense 44
OUTA 43
REFL AB Sense 42
REFL AB 41
REFH AB 40
REFH AB Sense 39
OUTB Sense 38
OUTB 37
V–
VOUT
V
V
–2.5V
Negative
Reference
Ref
1/2
V
OPA2743
500pF
V
V+
Ref
V
1/2
OPA2743
500pF
V
Positive
Reference
+2.5V
V
VOUT
–V
FIGURE 5. OPA743 as Dual Supply Configuration-Buffered References for the DAC7644.
OPA743
10
SBOS201
TI [ TEXAS INSTRUMENTS ]
