ISL59112_07 Datasheet, PDF (5/8 Pages) Intersil Corporation – 40MHz Rail-to-Rail Video Buffer

ISL59112
V_DD
SYNC CLAMP
V_DD
V_DD
+
-
C_IN
IN
IN
R_IN
75Ω
100nF
V_DC
+
-
EN
GND
EN = GND: SHUTDOWN I_DD~0
EN = V_DD: ACTIVE I_DD~2.0mA
SAG
NETWORK
OUT
R₆
R₇
SAG
R₅
AC COUPLING
CAPACITOR
C₅
47µF
C₄
22µF
R₄
75Ω
75Ω
FIGURE 11. BLOCK DIAGRAM
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 described in both the performance
characteristics section and the features section, 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 11.
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 11 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 and 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.
5
FN6142.4
March 15, 2007