DN396 - High Resolution Video Solutions Using Single 5V Power

High Resolution Video Solutions Using Single 5V Power
Design Note 396
Jon Munson
Introduction
Video cable driver amplifier 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®1983-3.
Such additional circuitry is unnecessary in typical 1VP-P
video connections, such as HD component video, if the
cable driver amplifiers simply offer near rail-to-rail output
capability when powered from 5V.
Standard definition and SVGA (800 × 600 pixel) low voltage
devices have been available from Linear Technology for
some time (see Design Note 327), but a number of recent
device developments have made it possible to produce
high resolution video devices that operate on a single 5V
power supply. Some parts that fit this mold include the
LT6556, a UXGA-resolution (1600 × 1200 pixel) RGB 2:1
input-port buffered multiplexer (MUX); the LT6557 and
LT6558 UXGA fixed gain triple amplifiers that include
on-chip biasing to minimize external part count; and the
LT6559 triple amplifier that provides flexible, cost effective
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 1VP-P nominal (Y-channel,
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Corporation. All other trademarks are the property of their respective owners.
5V
1
R
22mF
76.8W
5V
5.1k
5.1k
22mF
EN
IN2 A
A/B
4.7nF
V+
VREF
22mF
SELECT
5.1k
5V
5.1k
B
R/PR
OUT1
IN3 A
76.8W
10mF
5.1k
7
3
DGND
220nF
G
76.8W
PC
RGB
V+
5V
6
2
IN1 A
5.1k
V–
AGND1
8
G/Y
OUT2
LT6556
5V
22mF
PR
76.8W
IN1 B
V
AGND 2
V+
IN2 B
V+
4.7nF
5.1k
Y
76.8W
B/PB
4.7nF
–
5.1k
5V
22mF
V+
OUT3
5.1k
TO VIDEO
ADCS
5.1k
5V
220nF
ANGD 3
22mF
5.1k
PB
IN3 B
76.8W
5.1k
V–
DN396 F01
Figure 1. LT6556 Provides Input Video Selector and ADC Driver for Multimedia Display System
09/06/396
1.5VP-P worst-case when AC-coupled) and the required
gain is unity for digitizing by an analog-to-digital converter
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 settling time. The part is available in either SSOP-24
or QFN-24 packaging and includes layout-friendly flow
through pinouts. For the AC-coupling shown, the outputs
swing approximately ±0.7V about the mid-supply level,
within an available range of 2.6VP-P. Though not explicitly
shown in Figure 1, coupling to the ADC inputs usually
involves series resistances to reduce capacitive loading
of the amplifiers to preserve the smoothest frequency
response and optimal settling.
High Resolution Single-Supply Cable Driver
The LT6557 is a triple video amplifier specifically 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 gain-setting resistors to establish
a nominal gain of two and incorporates a single-resistorprogrammable input biasing 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
EN
GND
LT6557
V+
5V
412Ω
22µF
IN R
IN R
GND R 500Ω
+
–
OUT R
500Ω
V+ R
75Ω
220µF
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 current-feedback op amp (CFA) with individual
channel enables, the LT6559 offers great flexibility 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 3VP-P of available output swing on 5V due to its
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 filters 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.
75Ω
5V
5V
22µF
EN
IN G
IN G
GND G 500Ω
+
–
OUT G
75Ω
220µF
22µF
VIDEO IN
500Ω
V+ G
75Ω
5V
3.3k
5V
+
–
1/3 LT6559 75Ω
220µF
VIDEO OUT
301Ω
22µF
IN B
3.3k
IN B
GND B 500Ω
+
–
OUT B
75Ω
220µF
DN396 F03
301Ω
500Ω
V+ B
75Ω
5V
22µF
DN396 F02
Figure 2. LT6557 Provides Low Part Count
UXGA-Resolution Cable Driver on 5V Supply
Figure 3. HD Video Cable Driver Using Economical
LT6559 (Depicting One Channel of Three)
Data Sheet Download
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For applications help,
call (408) 432-1900, Ext. 2525
Linear Technology Corporation
dn396f LT/TP 0906 409K • PRINTED IN THE USA
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