ISL59833

FN6334 Rev 1.00

Page 13 of 15

March 5, 2007

THE VREF PIN

Applying a voltage to the VREF 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.

VREF may be connected to ground for back porch at ground.

The ISL59833 buffers the VREF voltage before applying it to

the triple amplifiers, isolating the input from the amplifiers and

allowing it to be driven by moderate-impedance voltage

sources.

THE VEE PIN

The VEE 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 ISL59833 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

150

 because of the change in output current with changing

DC levels. Special circuitry has been incorporated into the

ISL59833 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 150

 at a gain of +2. Driving higher impedance loads

would result in similar or better differential gain and differential

phase performance.

NTSC

The ISL59833 (generating a negative rail internally) is ideally

suited for NTSC video with its accompanying negative-going

sync signals, which is easily handled by the ISL59833 without

the need for an additional supply as the ISL59833 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 ISL59833. Specifically, this

standard encodes sync on the Y channel and it is a negative-

going signal, which is easily handled by the ISL59833 without

the need for an additional supply as the ISL59833 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 ISL59833.

DRIVING CAPACITIVE LOADS AND CABLES

The ISL59833 (internally-compensated to drive 75

 cables)

will drive 10pF loads in parallel with 1k

 with less than 5dB of

peaking. If less peaking is required, a small series resistor,

usually between 5

 to 50, 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 ISL59833 is a triple amplifier

designed to drive three channels; simply deal with each

channel separately as described in this section.

DC-RESTORE

When the ISL59833 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 “ISL59833 + DC-

Restore Solution” on page 10.

AMPLIFIER DISABLE

The ISL59833 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 the EN pin. The applied

logic signal is relative to the GND pin. Letting the EN pin float

or applying a signal that is less than 0.8V above GND will

enable the amplifier. The amplifier will be disabled when the

remains active.

OUTPUT DRIVE CAPABILITY

The ISL59833 does not have internal short-circuit protection

circuitry. A short-circuit current of 80mA sourcing and 150mA

sinking for the output is connected half way between the rails

with a 10

 resistor. If the output is shorted indefinitely, the

power dissipation could easily increase such that the part will

be destroyed. Maximum reliability is maintained if the output