8
FN6185.2
September 21, 2006
Application Information
The ISL59115 is a single-supply rail-to-rail triple (one s-video
channel and one composite channel) video amplifier with
internal sync tip clamps, a typical -3dB bandwidth of 9MHz
and slew rate of about 40V/µs. 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
ISL59115 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 ISL59115 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 ISL59115’s sync clamp. The Y and
CVBS inputs’ 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 input through the diode, forcing
current into the coupling capacitor until the voltage at the
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 C channel is slaved to the Y channel
and clamped to a 500mV level.
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 ISL59115, 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. 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 9MHz.
Output Coupling
The ISL59115 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 ISL59115’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 ISL59115 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.
FIGURE 21. PACKAGE POWER DISSIPATION vs AMBIENT
TEMPERATURE
FIGURE 22. PACKAGE POWER DISSIPATION vs AMBIENT
TEMPERATURE
Typical Performance Curves (Continued)
JEDEC JESD51-3 AND SEMI G42-88
(SINGLE LAYER) TEST BOARD
0.8
0.7
0.5
0.3
0.2
0.1
0
0 255075100 150
AMBIENT TEMPERATURE (°C)
POWER DISSIPATION (W)
12585
0.6
0.4
J
A
=
1
9
4
°
C
/
W
µ
TQ
FN
1
0
515mW
JEDEC JESD51-7 HIGH EFFECTIVE
THERMAL CONDUCTIVITY TEST BOARD -
QFN EXPOSED DIEPAD SOLDERED TO
PCB PER JESD51-5
3
2.5
2
1.5
1
0.5
0
0 255075100 150
AMBIENT TEMPERATURE (°C)
POWER DISSIPATION (W)
775mW
J
A
=
1
2
9
°
C
/
W
µ
T
Q
F
N
1
0
12585
ISL59115
