Dec 2005 Simplify High-Resolution Video Designs with Fixed-Gain Triple Multiplexers

DESIGN FEATURES
Simplify High-Resolution
Video Designs with Fixed-Gain
by Jon Munson
Triple Multiplexers
Introduction
V+
The LT6555 and LT6556 triple video
multiplexers offer up to 750MHz
performance in compact packages,
requiring no external gain-setting
resistors to establish a gain of two
or unity. A single integrated circuit,
in a choice of either 24-lead SSOP or
24-contact QFN (4mm × 4mm), performs fast switching between a pair of
three-channel video sources, such as
RGB or component HDTV.
The LT6555 provides a built-in
gain of two that is ideal for driving
back-terminated cables in playback or
signal routing equipment. The LT6556
provides a unity-gain function, in the
same footprints, that is ideal as an
input selector in high-performance
video displays and projectors.
The three video channels exhibit
excellent isolation between themselves
(50dB typical at 100MHz) and the inactive inputs (70dB typical at 100MHz)
for the highest quality video transmission. Excellent channel-to-channel
gain-matching preserves high fidelity
color balance.
The increasing popularity of the
UXGA professional graphics format
(1600 × 1200), which generates a
whopping 200-megapixel-per-second
flow, has put exceptional demands on
the frequency response of video amplifiers. For instance, pulse-amplitude
RINA
GINA
BINA
LT6555
75Ω
75Ω
75Ω
75Ω AGND
100kHz
Figure 2. Wide frequency response
of circuit in Figure 1
20
1GHz
75Ω
×2
RINB
GINB
BINB
75Ω
ROUT
GOUT
75Ω
75Ω
×2
75Ω
75Ω
BOUT
SELECT A/B
75Ω
ENABLE
DGND
V
–
6555 TA01a
Figure 1. The LT6555 in an RGB cable driving multiplexer circuit
waveforms like those of RGB baseband
video, generally require reproduction
of high-frequency content to at least
the 5th harmonic of the fundamental
frequency component, which is 2.5
times the video pixel rate, accounting
for the 2-pixels-per-fundamentalcycle relationship. This means that
UXGA requires a flat frequency response to beyond 0.5GHz! The wide
bandwidth performance of the LT6555
and LT6556 makes them ideally suited
to such high performance video applications.
response and crosstalk anomalies
can plague the circuit development
process. The LT6555 and LT6556
conveniently solve these problems by
providing internal factory-matched
resistors and an efficient 3-channel,
2-input group, flow-through layout
arrangement.
Figure 1 shows the typical RGB
cable driver application of an LT6555,
and its excellent frequency and time
response plots are shown in Figures
2 and 3 (as implemented on demo
1.8
Easy Solution for MultiChannel Video Applications
Baseband video generated at these
higher rates is processed in either native red, green and blue (RGB) domain
or encoded into component luma plus
blue and red chroma channels (YPbPr);
three channels of information in either
case. With frequency response requirements extending to beyond 500MHz,
amplifier layouts that require external
resistors for gain setting tend to be
real-estate inefficient, and frequency
1.6
1.4
1.2
OUTPUT (V)
0dB
3dB/DIV
75Ω
×2
1.0
0.8
0.6
0.4
0.2
VIN = 0V TO 700mV
VS = ±5V
RL = 150Ω
TA = 25°C
0
–0.2
–0.4
0
2
4
6
8 10 12 14 16 18 20
TIME (ns)
Figure 3. Fast pulse response
of circuit in Figure 1
Linear Technology Magazine • December 2005
DESIGN FEATURES
V+
V+
BIAS
V
EN
1k
40k VREF
TO OTHER
OUTPUT
STAGES
40k
+
V
46k
770Ω
–
OUT
VREF
SEL
INA
V+
V+
INB 100Ω
100Ω
V–
360Ω
360Ω
360Ω
360Ω AGND
V–
DGND
VREF
VREF
SELECT
TO OTHER
INPUT STAGES
V–
V–
Figure 4. Simplified internal circuit functionality of the LT6555 and LT6556
circuit 892A-A). Frequency markers in
Figure 2 show the small-signal –0.5dB
response beyond 500MHz and –3dB
response above 600MHz. The LT6556,
when used to drive high impedances,
provides bandwidth to 750MHz,
though the LT6556 demo circuit 892AB uses 75Ω back termination (rather
than 1kΩ), resulting in performance
similar to the LT6555.
Taking a Look
at the Internal Details
The LT6555 and LT6556 integrate
three independent sections of circuitry
that form classic current-feedback
amplifier (CFA) gain blocks, but
with switchable input sections, all
implemented on a very high-speed
fabrication process. The diagram in
Figure 4 shows the equivalent internal circuitry (one LT6555 section
shown).
Feedback resistors are provided
on-chip to set the closed-loop gain
to either unity or two, depending
on the part. The nominal feedback
resistances are chosen to optimize
flat frequency response. The LT6555
is intended to drive back-terminated
50Ω or 75Ω cables (for effective loading of 100Ω to 150Ω respectively),
while the LT6556 is designed to drive
ADCs or other high impedance loads
(characterized with 1kΩ as a reference
loading condition).
Linear Technology Magazine • December 2005
Common to all three CFAs in each
part is a bias control section with a
power-down command input. The
input select logic steers bias current
to the appropriate input circuitry,
enabling the input function of the
selected signal. The shutdown function
includes an internal on-chip pull-up
resistance to provide a default disable command, which when invoked,
reduces typical power consumption to
less than 125µA for an entire threechannel part. During shutdown mode
the amplifier outputs become high
impedance, though in the case of the
LT6555, the feedback resistor string
to AGND is still present. The parts
come into full-power operation when
the enable input voltage is brought
within 1.3V above the DGND pin.
The typical on-state supply current of
about 9mA per amplifier provides for
ample cable-drive capacity (>40mA)
and ultra-fast 2.2V per nanosecond
slew rate performance.
Expanding MUX
Input Selection
The power-down feature of the LT6555
and LT6556 may be used to control
multiple ICs in a configuration that
provides additional input selections.
Figure 5 shows a simple 4-input RGB
selecting cable driver using two LT6555
devices with the enable pins driven by
complementary logic signals. The
shared-output connections between
the devices need to be kept as short
as possible to minimize printed-circuit
parasitics that might affect frequency
response. This circuit would be ideal
in an A/V control-unit for driving the
component-video output, for example.
The same basic expansion concept applied to an LT6556 pair would be ideal
at the input section of a four-source
HD video display.
Operating with the
Right Power Supplies
The LT6555 and LT6556 require a
total power supply of at least 4.5V, but
depending on the input and output
swings required, may need more to
avoid clipping the signal. The LT6556,
having unity gain, makes the analysis
simple—the maximum output swing
is (V+ – V- – 2.6)VP–P and governed only
by the output saturation voltages. This
means a total supply of 5V is adequate
for standard video (1VP–P). For the
LT6555, extra allowance is required
for load-driving, so the output swing
is (V+ – V- – 3.8)V. This means a total
supply of about 6V is required for the
output to swing 2VP–P, as when driving
cables. For best dynamic range along
with reasonable power consumption,
a good choice of supplies would be
±3V for the LT6556 and +5V/–3V for
the LT6555.
21
DESIGN FEATURES
RED 1
GREEN 1
BLUE 1
75Ω
V+
LT6555 #1
IN1A
IN1B
5V
OUT1
×2
75Ω
75Ω
RED 2
GREEN 2
BLUE 2
IN2A
OUT2
×2
IN2B
75Ω
75Ω
IN3A
75Ω
OUT3
×2
IN3B
AGND
DGND
SEL
V
RED 3
GREEN 3
BLUE 3
75Ω
VREF
EN
75Ω
–
75Ω
–3V
5V
75Ω
V
LT6555 #2
IN1A
75Ω
OUT1
×2
IN1B
75Ω
+
75Ω
ROUT
GOUT
BOUT
75Ω
75Ω
RED 4
GREEN 4
BLUE 4
IN2A
OUT2
×2
IN2B
75Ω
75Ω
IN3A
75Ω
OUT3
×2
IN3B
AGND
DGND
SEL
VREF
EN
SEL0
V–
NC75Z14
SEL1
SEL1 SEL0 OUTPUT
0
0
1
0
1
2
1
0
3
1
1
4
–3V
Figure 5. A 4-to-1 video multiplixer using the shutdown feature for expansion
Since many systems today lack a
negative supply rail, a small LTC19833 solution can be used to generate a
simple –3V rail for local use, as shown
in Figure 6. The LTC1983-3 solution
is more cost effective and performs
at high frequencies better than ACcoupling and resistor network biasing
techniques that might otherwise be
employed. For example, Figure 7
shows the typical AC-coupling networks used when operating from a
single supply. With six input networks
and three large output capacitors required, the AC-coupled method uses
more board space and adds parasitics
to the signal path that can degrade
frequency response.
continued on page 12
OFF ON
VOUT
VIN
LTC1983-3
(SOT23-6)
SHDN
VOUT = –3V
IOUT = UP TO 100mA
COUT
10µF
GND
C–
C+
7V TO 12V
INPUT
22µF*
IN
80.6Ω
CFLY
1µF
Figure 6. Generating a local –3V
supply with four tiny components
22
6.8k
2.2k
AGND
LT6555
OR
LT6556
OUT
75Ω
220µF**
+
VIN
3V TO 5.5V
CIN
10µF
* AVX 12066D226MAT
** SANYO 6TPB220ML
75Ω
NOTE: ONLY ONE INPUT AND ONE OUTPUT SHOWN
Figure 7. AC-coupling techniques for single-supply operation
Linear Technology Magazine • December 2005
DESIGN FEATURES
IGBT Drive
Most camera flashes are capable of
redeye reduction and light-feedback
flashing. These features quench, or
stop, the flash before the capacitor
drains completely. This added level
of control requires a high current,
high voltage Insulated Gate Bipolar
Transistor (IGBT). An IGBT has the
advantage of a BJT’s high voltage
and high current capabilities but
does not need base current since it
has a MOSFET gate as the input. The
tradeoff for these two advantages is
speed. Since a flash is on the order
of milliseconds, speed is not an issue
in this application and an IGBT fits
perfectly for the role.
Like a MOSFET, the gate acts like
a capacitor. The IGBT driver’s job is
to charge and discharge the gate. The
IGBT driver does not need to be fast,
and actually a fast driver can potentially destroy the device. The IGBT
turns on when the IGBTIN pin is above
1.5V and turns off when the IGBTIN
pin is below 0.3V. When the input is
high, the driver draws a small amount
of current to hold the gate high with a
PNP. When the input is low, the driver
has zero quiescent current. During
transitions the driver is capable of
delivering 150mA of current.
The speed of the driver needs to be
carefully controlled or the IGBT may
be destroyed. The IGBT driver does not
need to pull up the gate fast because
of the inherently slow nature of the
IGBT. A rise time of 2µs is sufficient
to charge the gate of the IGBT and
create a trigger pulse. With slower
rise times, the trigger circuitry does
not have a fast enough edge to create
the required 4kV pulse. The fall time
of the IGBT drive is critical to the safe
operation of the IGBT. The IGBT gate
is a network of resistors and capacitors. When the gate terminal is pulled
low too quickly, the capacitance closest to the terminal goes low but the
capacitance further from the terminal
remains high, causing a small portion
of the IGBT device to handle the full
100A of current which quickly destroys
the device. The pull down circuitry
therefore needs to be slower than the
internal RC time constant in the gate
of the IGBT. To slow down the driver,
a 20Ω series resistor is integrated into
the LT3485.
Which Part to Use
The LT3484 and LT3485 families of
photoflash capacitor chargers suit
about any photoflash need. The basic
photoflash functionality in each part
is identical and both parts are capable
of operating from 2AA cells. The integrated IGBT drive and voltage output
monitor differentiate the LT3485 from
the LT3484, along with its higher current capabilities. The LT3484 is the
smallest solution available if quenching the bulb is not needed. When
using an IGBT to trigger the flash, the
LT3485 offers valuable board space
savings over the LT3484 by eliminating
several external components. Table 1
shows the major functional differences
between these seven parts.
Once the decision is made on the
integrated IGBT driver, choosing a
current option is a matter of balancing the inherent trade-off between
input current and charge time. For
a given photoflash capacitor size, the
device which results in the highest
input current offers the fastest charge
time. The limit on how much current
the photoflash charger can draw is
usually set by the battery technology used, and how much load they
LT6555/56, continued from page 22
Demonstration
Circuits Available
The LT6555 and LT6556 have Demo
Boards available that make evaluation
of these parts a simple plug-and-play
operation. To evaluate the LT6555 ask
for DC858A (SSOP-24 package) or
DC892A-A (QFN package). To evalu12
ate the LT6556 ask for DC892A-B (in
QFN package). All three of these demo
circuits have high-quality 75Ω BNC
connections for best performance
and illustrate high-frequency layout
practices that are important to obtaining the best performance from these
super-fast amplifiers.
can handle. The LT3485-3 offers the
fastest charge times of the chargers
discussed here.
The following equation predicts the
charge times (T) in seconds for the
seven parts:
T=
(
COUT • VOUT(FINAL)2 – VOUT(INIT)2
τ • VIN
)
where COUT is the value of the photoflash capacitor in Farads, VOUT(FINAL)
is the target output voltage, VOUT(INIT)
is the initial output voltage, VIN is the
battery voltage to which the flyback
transformer is connected, and τ is
the charge time coefficient listed in
Table 1.
The charge time coefficients for
each part are different depending on
the transformer due to differences in
efficiency and average input current.
The charge time coefficients are given
for Kijima Musen and TDK transformers, with part numbers and typical
specifications for these transformers
listed in Table 2.
Conclusion
The LT3484 and LT3485 provide
simple, efficient capacitor charging
solutions for digital still cameras
and integrated digital cameras in cell
phones. The high level of integration
reduces the amount of external components while also producing tightly
controlled output voltage and average
input current distributions. The three
current limits in the LT3484 family
and the four current limits in the
LT3485 family allow for flexibility in the
trade-off between input current and
charge time. The LT3485 saves even
more space for some applications by
integrating an IGBT driver and voltage
output monitor.
For further information on any
of the devices mentioned in
this issue of Linear Technology,
visit www.linear.com, use the
reader service card or call the LTC
literature service number:
1-800-4-LINEAR
Linear Technology Magazine • December 2005
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