LINEAR TECHNOLOGY LT6556 Technical data

High Resolution Video Solutions Using Single 5V Power

Design Note 396

Jon Munson

Introduction

Video cable driver amplifi er output stages traditionally
require a supply voltage of at least 6V in order to provide
the required output swing. This requirement is usually
met with 5V supplies by adding a boost regulator or a

®

small local negative rail, say via the popular LT
Such additional circuitry is unnecessary in typical 1V

198 3-3.

P-P

video connections, such as HD component video, if the
cable driver ampli fi ers simply of fer near rail-to-r ail output
capability when powered from 5V.

Standard defi nition and SVGA (800 × 600 pi xel) low voltage
devices have been available from Linear Technology for
so me tim e ( se e D esi gn No te 32 7) , b ut a nu mb er o f r ec en t
device developments have made it possible to produce
high resolution video devices that operate on a single 5V

5V

22mF

22mF

22mF

22mF

22mF

22mF

5.1k

IN1 A

5.1k
5V

5.1k

5V

5.1k

5.1k

5V

5.1k

5V

5.1k

5.1k
5V

5.1k

DGND

5.1k

IN2 A SELECT

V

REF

IN3 A

AGND1

5.1k

IN1 B

AGND 2

IN2 B

ANGD 3

5.1k

IN3 B

–

V

PC

RGB

R

1

76.8

W

6

G

2

76.8

W

7

B

3

76.8

W

8

P

R

76.8

W

Y

76.8

W

P

B

76.8

W

power supply. Some parts that fi t this mold include the
LT6556, a UXGA-resolution (1600 × 1200 pixel) RGB 2:1
input-port buffered multiplexer (MUX); the LT6557 and
LT6558 UXGA fi xed gain triple amplifi ers that include
on-chip biasing to minimize external part count; and the
LT6 5 59 tr ip le am pl ifi er that provides fl exible, cost effec tive
solutions in SXGA (1280 × 1024 pixel) products.

High Resolution Video Input-Port Multiplexer

High performance multimedia video display systems
usually include a multiple-input feature to select between
a VESA-compliant D-type PC connection and consumer
component video that uses RCA jacks. The incom­ing video signal is at most 1V

, LT, LTC and LTM are registered trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners.

5V

+

LT6556

OUT1

OUT2

OUT3

V

EN

A/B

+

V

–

V

+

V

–

V

+

V

+

V

DN396 F01

220nF

4.7nF

4.7nF

4.7nF

220nF

nominal (Y-channel,

P-P

10mF

R/P

R

TO VIDEO

G/Y

ADCS

B/P

B

09/06/396

Figure 1. LT6556 Provides Input Video Selector and ADC Driver for Multimedia Display System

1.5V

worst-case when AC-coupled) and the required

P-P

gain is unit y for digitizing by an analog-to -digital conver ter
set (ADC) or other signal routing. This input selection
function is readily implemented with the LT6556 on 5V
as shown in Figure 1, supporting all video resolutions
including UXGA by virtue of its 750MHz bandwidth and

6.5ns set tling time. The part is available in ei ther SSOP-24
or QFN-24 packaging and includes layout-friendly fl ow
through pinouts. For the AC-coupling shown, the outputs
swing approximately ±0.7V about the mid-supply level,
within an available range of 2.6V

. Though not explicitly

P-P

shown in Figure 1, coupling to the ADC inputs usually
involves series resistances to reduce capacitive loading
of the amplifi ers to preserve the smoothest frequency
response and optimal settling.

High Resolution Single-Supply Cable Driver

The LT6557 is a triple video amplifi er specifi cally engi­neered to provide UXGA level performance on a single
5V power supply. A quasi rail-to-rail output stage and an
almost slew-unlimited 400MHz large signal bandwidth
make this the part of choice for the most performance­critical applications.

The LT6557 has internal g ain-setting resis tors to establish
a nominal gain of two and incorporates a single-resistor­programmable input bi asing system to eliminate the usual
input divider resistors used in single supply applications.
As seen in Figure 2, the entire cable driver function is
largely reduced to the IC and blocking capacitors. The

BCV

+

5V

V

220µF

OUT R

V+ R

OUT G

V

OUT B

V

75Ω

5V

220µF

75Ω

+

5V

G

220µF

75Ω

+

5V

B

DN396 F02

+
–

500Ω

+
–

500Ω

+
–

500Ω

412Ω

75Ω

75Ω

75Ω

IN R

IN G

IN B

22µF

22µF

22µF

EN

GND

IN R

GND R

IN G

GND G

IN B

GND B

LT6557

500Ω

500Ω

500Ω

internal input biasing may be defeated for DC-coupled
applications, such as with direct digital-to-analog con­verter (DAC) output applications where a software con­trolled offset is introduced to set the signal dynamics. A
unity gain version, the LT6558, is also available.

Economical SXGA/HD Cable Driver

In cost sensitive applications like consumer video-play­back equipment, the LT6559 provides excellent bang-for­the-buck in a tiny QFN-16 (3mm × 3mm) package. As a
basic triple curren t-feedback op amp (CFA) with indi vidual
channel enables, the LT6559 offers great fl exibility in
forming various multiplexer, cable driver, and ADC driver
functions at low cost. Even though the LT6559 is not a
true rail-to-rail output device, there remains approxi­mately 3V

of available output swing on 5V due to its

P-P

high performance output stage design. Figure 3 shows
a typical AC-coupled application as an economical HD or
SXGA-grade triple cable driver (one channel shown for
brevity). As a general purpose CFA, the feedback resis­tor value (301Ω) optimizes the frequency response. This
circuit is ideal as an output buffer/driver for following
passive reconstruction fi lters such as for the increasingly
popular 1080p HD format (i.e. 60MHz lowpass).

Conclusion

As system designers continue to reduce the number of
supply voltages used within their products, pressure
to maximize analog performance on available 5V logic
supplies has led to the need for viable low voltage high
performance video solutions. For high resolution appli­cations, Linear Technology offers the LT6556 buffered
MUX, the LT6557/LT6558 AC-coupled amps, and the
economical LT6559 triple CFA—all well suited to operate
in the 5V environment.

5V

22µF

VIDEO IN

3.3k

3.3k

EN

+
–

301Ω

301Ω

22µF

5V

1/3 LT6559

DN396 F03

75Ω

220µF

VIDEO OUT

Figure 2. LT6557 Provides Low Part Count
UXGA-Resolution Cable Driver on 5V Supply

Data Sheet Download

http://www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

Figure 3. HD Video Cable Driver Using Economical
LT6559 (Depicting One Channel of Three)

For applications help,

call [phone], Ext. 2525

dn396f LT/TP 0906 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2006