12
FN7489.6
May 4, 2006
The V
REF
Pin
Applying a voltage to the V
REF
pin simply places that
voltage on what would usually be the ground side of the gain
resistor of the amplifier, resulting in a DC-level shift of the
output signal. Applying 100mV to the Vref pin would apply a
-100mV DC level shift to the outgoing signal. The charge
pump provides sufficient bottom room to accommodate the
shifted signal. V
REF
may be connected to ground for back
porch at ground.
Note: The V
REF
input is the common point of the 3 amps
minus input resistors. Any common resistance on V
REF
input will share the voltage induced on it with all the other
amps, so using a resistor source to get offset will cause
cross talk and gain change for the offset for all amps and
amp +input gain change. Offset on the V
REF
pin must be low
impedance to prevent gain error and cross talk. A transistor
emitter follower should work like an NPN MMBT3904 with
the emitter connected to the V
REF
pin and 1k pull down to V-
with 1µF cap bypass to ground and the collector to V+ and
base to V offset source. If better tempco is needed then a
diode may be used in series with the pot to ground. A 499Ω
resistor may be added in series with the collector to prevent
damage when testing.
See the Block Diagram on page 8.
The V
EE
Pin
The V
EE
pin is the output pin for the charge pump. A
voltmeter applied to this pin will display the output of the
charge pump. This pin does not affect the functionality of the
part. One may use this pin as an additional voltage source.
Keep in mind that the output of this pin is generated by the
internal charge pump and a fully regulated supply that must
be properly bypassed. We recommend a 0.1µF ceramic
capacitor placed as close to the pin and connected to the
ground plane of the board.
Input, Output, and Supply Voltage Range
The ISL59830 is designed to operate with a single supply
voltage range of from 0V to 3.3V. The need for a split supply
has been eliminated with the incorporation of a charge pump
capable of generating a bottom rail as much as 1.6V below
ground, for a 4.9V range on a single 3.3V supply. This
performance is ideal for NTSC video with its negative-going
sync pulses.
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and the same
frequency and phase response as DC levels are changed at
the output. This is especially difficult when driving a standard
video load of 150Ω because of the change in output current
with changing DC levels. Special circuitry has been
incorporated into the ISL59830 for the reduction of output
impedance variation with the current output. This results in
outstanding differential gain and differential phase
specifications of 0.06% and 0.1°, while driving 150Ω at a
gain of +2. Driving higher impedance loads would result in
similar or better differential gain and differential phase
performance.
NTSC
The ISL59830, generating a negative rail internally, is ideally
suited for NTSC video with its accompanying negative-going
sync signals; easily handled by the ISL59830 without the
need of an additional supply as the ISL59830 generates a
negative rail with an internal charge pump referenced at
negative 1/2 the positive supply.
YPbPr
YPbPr signals originating from a DVD player requiring three
channels of very tightly-controlled amplifier gain accuracy
present no difficulty for the ISL59830. Specifically, this
standard encodes sync on the Y channel and it is a negative-
going signal; easily handled by the ISL59830 without the
need of an additional supply as the ISL59830 generates a
negative rail placed at negative 1/2 the positive supply.
Additionally, the Pb and Pr are bipolar analog signals and
the video signals are negative-going; and again easily
handled by the ISL59830.
Driving Capacitive Loads and Cables
The ISL59830, internally-compensated to drive 75Ω cables,
will drive 10pF loads in parallel with 1kΩ with less than 5dB
of peaking. If less peaking is required, a small series resistor,
usually between 5Ω to 50Ω, can be placed in series with the
output. This will reduce peaking at the expense of a slight
closed loop gain reduction. When used as a cable driver,
double termination is always recommended for reflection-
free performance. For those applications, a back-termination
series resistor at the amplifier's output will isolate the
amplifier from the cable and allow extensive capacitive drive.
However, other applications may have high capacitive loads
without a back-termination resistor. Again, a small series
resistor at the output can help to reduce peaking. The
ISL59830 is a triple amplifier designed to drive three
channels; simply deal with each channel separately as
described in this section.
DC-Restore
When the ISL59830 is AC-coupled it becomes necessary to
restore the DC reference for the signal. This is accomplished
with a DC-restore system applied between the capacitive
"AC" coupling and the input of the device. Refer to
Application Circuit for reference DC-restore solution.
Amplifier Disable
The ISL59830 can be disabled and its output placed in a
high impedance state. The turn-off time is around 25ns and
the turn-on time is around 200ns. When disabled, the
amplifier's supply current is reduced to 80mA typically,
reducing power consumption. The amplifier's power-down
can be controlled by standard TTL or CMOS signal levels at
ISL59830
