ISL59112 Datasheet, PDF (4/6 Pages) Intersil Corporation – 40MHz Rail-to-Rail Video Buffer

ISL59112
Application Information
The ISL59112 is a single supply rail-to-rail output buffer
achieving a -3dB bandwidth of around 40MHz and slew rate
of about 85V/µs while demanding only 2mA of supply
current. This part is ideally suited for applications with
specific micropower consumption and high bandwidth
demands. As the performance characteristics above and the
features described below, the ISL59112 is designed to be
very attractive for portable composite video applications.
The ISL59112 features a sync clamp and SAG network at
the output facilitating reduction of typically large AC coupling
capacitors. See Figure 3.
Internal Sync Clamp
The typical embedded video DAC operates from a ground
referenced single supply. This becomes an issue because
the lower level of the sync pulse output may be at a 0V
reference level to some positive level. The problem is
presenting a 0V input to most single supply driven amplifiers
will saturate the output stage of the amplifier resulting in a
clipped sync tip and degrading the video image. A larger
positive reference may offset the input above its positive
range.
The ISL59112 features an internal sync clamp and an offset
function to level shift the entire video signal to the best level
before it reaches the input of the amplifier stage. These
features are also helpful to avoid saturation of the output
stage of the amplifier by setting the signal closer to the best
voltage range.
The simplified block diagram of the ISL59112 in Figure 3 is
divided into four sections. The first, Section A is the Sync
Clamp. The AC coupled video sync signal is pulled negative
by a current source at the input of the comparator amplifier.
When the sync tip goes below the comparator threshold the
output comparator is driven negative, The PMOS device
turns on clamping sync tip to near ground level. AC Output
Coupling and the SAG Network
Composite video signals carry viable information at
frequencies as low as 30Hz up to 5MHz. When a video
system output is AC coupled it is critical that the filter
represented by the output coupling capacitor and the
surrounding resistance network provide a band pass
function with a low pass band low enough to exclude very
low frequencies down to DC, and with a high pass band pass
sufficiently high to include frequencies at the higher end of
the video spectrum.
SAG AC COUPLING
NETWORK CAPACITOR
C₅
R_OUT
R₆
R₇
C₄
R_L
R₅
R₄
FIGURE 4. SAG NETWORK AND AC COUPLING CAPACITORS
Typically this is accomplished with 220µF coupling capacitor,
a large and somewhat costly solution providing a low
frequency pole around 5Hz. If the size of this capacitor is
even slightly reduced we have found that the accompanying
phase shift in the 50Hz to 100Hz frequency range results in
field tilt resulting in a degraded video image.
The internal SAG network of the ISL59112 replaces the
220µF AC coupling capacitor with a network of two smaller
capacitors as shown in Figure 4. Additionally, the network is
designed to place a zero in the ~30Hz range, providing a
small amount of peaking to compensate the phase response
associated with field tilt.
DC Output Coupling
The ISL59112 internal sync clamp makes it possible to DC
couple the output to a video load, eliminating the need for
any AC coupling capacitor saving board space and
additional expense for capacitors. Additionally, this solution
completely eliminates the issue of field tilt in the lower
frequency. The trade off is greater demand of supply current.
Typical load current for AC coupled is around 3mA
compared to typical 6mA used when DC coupling.
ENABLE
+
-
R_OUT
TELEVISION
OR VCR
FIGURE 5. DC COUPLE
4
FN6142.1
August 15, 2005