8
FN6277.0
September 21, 2006
Application Information
The ISL59116 is a single-supply rail-to-rail triple (two in,
three out) video amplifier with internal sync tip clamps, a
typical -3dB bandwidth of 9MHz and slew rate of about
40V/µs. The Y and C channels are internally mixed to create
a third CVBS (composite) video output. This part is ideally
suited for applications requiring high composite and S-video
performance with very low power consumption. As the
performance characteristics and features illustrate, the
ISL59116 is optimized for portable video applications.
Internal Sync Clamp
Embedded video DACs typically use ground as their most
negative supply. This places the sync tip voltage at a
minimum of 0V. Presenting a 0V input to most single supply
amplifiers will saturate the output stage of the amplifier
resulting in a clipped sync tip and degraded video image.
The ISL59116 features an internal sync clamp and offset
function that level shifts the entire video signal to the optimum
level before it reaches the amplifiers’ input stage. These
features also help avoid saturation of the output stage of the
amplifier by setting the signal closer to the best voltage range.
The simplified block diagram on the front page shows the
basic operation of the ISL59116’s sync clamp. The Y input’s
AC-coupled video sync signal is pulled negative by a current
source at the input. When the sync tip goes below the
comparator threshold, the comparator output goes high,
pulling up on the Y input through the diode, forcing current
into the coupling capacitor until the voltage at the Y input is
again 0V, and the comparator turns off. This forces the sync
tip clamp to always be 0V, setting the offset for the entire
video signal.
The Sallen Key Low Pass Filter
The Sallen Key is a classic low pass configuration. This
provides a very stable low pass function, and in the case of
the ISL59116, a three-pole roll-off at 9MHz. The three-pole
function is accomplished with an RC low pass network placed
in series with and before the Sallen Key. The first pole is
formed by an RC network, with poles two and three generated
with a Sallen Key, creating a nice three-pole roll-off at 9MHz.
Output Coupling
The ISL59116 can be AC or DC coupled to its output. When
AC coupling, a 220µF coupling capacitor is recommended to
ensure that low frequencies are passed, preventing video
“tilt” or “droop” across a line.
The ISL59116’s internal sync clamp makes it possible to DC
couple the output to a video load, eliminating the need for
any AC coupling capacitors, saving board space, cost, and
eliminating any “tilt” or offset shift in the output signal. The
trade off is larger supply current draw, since the DC
component of the signal is now dissipated in the load
resistor. Typical load current for AC coupled signals is 5mA
compared to 10mA for DC coupling.
Output Drive Capability
The ISL59116 does not have internal short circuit protection
circuitry. If the output is shorted indefinitely, the power
dissipation could easily overheat the die or the current could
eventually compromise metal integrity. Maximum reliability is
maintained if the output current never exceeds ±40mA. This
limit is set by the design of the internal metal interconnect.
Note that for transient short circuits, the part is robust.
Short circuit protection can be provided externally with a
back match resistor in series with the output placed close as
possible to the output pin. In video applications this would be
a 75 resistor and will provide adequate short circuit
protection to the device. Care should still be taken not to
stress the device with a short at the output.
Power Dissipation
With the high output drive capability of the ISL59116, it is
possible to exceed the +125°C absolute maximum junction
temperature under certain load current conditions.
Therefore, it is important to calculate the maximum junction
temperature for an application to determine if load conditions
or package types need to be modified to assure operation of
the amplifier in a safe operating area.
The maximum power dissipation allowed in a package is
determined according to:
Where:
T
JMAX
= Maximum junction temperature
T
AMAX
= Maximum ambient temperature
JA
= Thermal resistance of the package
The maximum power dissipation actually produced by an IC
is the total quiescent supply current times the total power
supply voltage, plus the power in the IC due to the load, or:
for sourcing:
for sinking:
Where:
V
S
= Supply voltage
I
SMAX
= Maximum quiescent supply current
V
OUT
= Maximum output voltage of the application
R
LOAD
= Load resistance tied to ground
I
LOAD
= Load current
PD
MAX
T
JMAX
T
AMAX
–
JA
---------------------------------------------
=
PD
MAX
V
S
I
SMAX
V
S
V
OUT
–+
V
OUT
R
L
----------------
=
PD
MAX
V
S
I
SMAX
V
OUT
V
S
–+ I
LOAD
=
ISL59116
