ISL59111 Datasheet, PDF (7/10 Pages) Intersil Corporation – 8MHz Rail-to-Rail Composite Video Driver

ISL59111
SYNC CLAMP
V_DD
V_DD
V_DD
+
-
C_{IN IN}
R₁
IN
R_IN ^100nF
SALLEN KEY LOW PASS FILTER
C₂
R₂
R₃
V_DC
+
-
C₁
C₃
EN
GND
EN=GND: SHUTDOWN I_DD~0
EN=V_DD: ACTIVE I_DD~1.5mA
SAG
AC COUPLING
NETWORK CAPACITOR
OUT
R₆
R_{7 SAG}
C₅
47µF
C₄
R_OUT
75Ω _RL
75Ω
22µF
R₅
R₄
FIGURE 22. BLOCK DIAGRAM
Application Information
The ISL59111 is a single supply rail-to-rail output amplifier
achieving a -3dB bandwidth of around 8MHz and slew rate
of about 40V/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 ISL59111 is designed to be
very attractive for portable composite video applications.
The ISL59111 features a sync clamp, low pass function, and
SAG network at the output facilitating reduction of typically
large AC coupling capacitors. See Figure 22.
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 ISL59111 features an internal sync clamp and 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
7
stage of the amplifier by setting the signal closer to the best
voltage range.
The simplified block diagram of the ISL59111 in Figure 22 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. The network
triggers on the sync tip of video signal.
The Sallen Key Low Pass Filter
The Sallen Key is a classic low pass configuration illustrated
in Figure 22. This provides a very stable low pass function,
and in the case of the ISL59111, a three-pole roll-off at
around 8MHz. The three-pole function is accomplished with
an RC low pass network placed in series with and before the
Sallen Key. One pole provided by the RC network and poles
two and three provided by the Sallen Key for a nice three-
pole roll-off at around 8MHz. If more aggressive, multiple-
pole roll-offs are needed, multiple ISL59111 can be placed in
series. There will, of course, be a loss of bandwidth as
additional devices are added.
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
FN6172.3
August 29, 2006