BUF04701

9

SBOS214B

www.ti.com

RAIL-TO-RAIL OUTPUT

A class AB output stage with common-source transistors is

used to achieve rail-to-rail output. This output stage is ca-

pable of driving 1k

Ω loads connected to any point between

V+ and V–. For light resistive loads (> 100k

Ω), the output

voltage can swing to 100mV from the supply rail. With 2k

Ω

resistive loads, the output is specified to swing to within

200mV of the supply rails while maintaining high open-loop

gain (see the typical characteristic curve

Output Voltage

Swing vs Output Current).

CAPACITIVE LOAD AND STABILITY

The BUF04701 can drive up to 1000pF pure capacitive load.

One method of improving capacitive load drive is to insert a

10 to 20

Ω resistor in series with the output, as shown in

Figure 5. This reduces ringing with large capacitive loads

while maintaining DC accuracy.

FIGURE 6. BUF04701 as LCD Display Buffer.

1/4

BUF04701

V

IN

V

OUT

20

Ω

1/4

BUF04701

V

DC

R

S

20

Ω

C

L

100nF

GMA1

GMA2

GMA3

GMA4

GMA5

1/4

BUF04701

R

S

20

Ω

C

L

100nF

GMA6

GMA7

GMA8

GMA9

GMA10

LCD Source Driver

1/4

BUF04701

R

S

20

Ω

C

L

100nF

1/4

BUF04701

R

S

20

Ω

C

L

100nF

FIGURE 5. Improving Capacitive Load Drive.

APPLICATION CIRCUITS

REFERENCE BUFFER FOR LCD SOURCE DRIVERS

In modern high-resolution TFT-LCD displays, gamma correc-

tion 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 Digital-to-

Analog Converters (DACs) to convert the 6-bit data into analog

voltages applied to the LCD. These DACs typically require

external voltage references for proper operation. Normally

these external reference voltages are generated using a simple

resistive 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 of a

DAC is only around 10k

Ω to 16kΩ, 6 to 8 strings 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 ability of the BUF04701 to operate on 12V supplies, to

drive heavy resistive loads (as low as 2k

Ω), and to swing to

within 200mV of the supply rails, makes it very well suited as

a buffer for the reference voltage inputs of LCD source drivers.

During conversion of the DAC, internal switches create

current glitches on the output of the reference buffer. The

capacitor CL (typically 100nF) functions as a charge reservoir

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

resistor RS isolates the outputs of the BUF04701 from the

heavy capacitive load and helps to improve settling time.

4-POLE LOW-PASS SALLEN-KEY FILTER

The high open-loop gain and wide bandwidth of the BUF04701

make it optimal for active filtering applications. Figure 7 shows

the BUF04701 in a 4-pole Butterworth low-pass active filter

configuration of 20kHz bandwidth.

FIGURE 7. BUF04701 Configured as a 4-Pole Sallen-Key Butterworth Low-Pass Filter.

1/4

BUF04701

1/4

BUF04701

V

IN

V

OUT

16.8k

Ω

6.8nF

1.68k

Ω

19.4k

Ω

680pF

330pF

2.2nF

2.18k

Ω