9

FN6432.1

April 4, 2007

Application Information

The ISL59123 is a single-supply rail-to-rail triple video low-

pass filter and amplifier with a -3dB bandwidth of 18MHz. It

provides anti-aliasing for component, s-video, and

composite video signals. Its small size and low power make

the ISL59123 ideal for portable video applications.

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 ISL59123, a three-pole roll-off at 18MHz. 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 (including the impedance of the

source driving the ISL59123), with poles two and three

generated by a Sallen Key, creating a three-pole roll-off

characteristic at 18MHz.

Output Coupling

The ISL59123 can be AC or DC coupled to its output. When

AC coupled, a 220µF coupling capacitor is recommended to

ensure that low frequencies are passed, preventing video

“tilt” or “droop” across a line.

Output Drive Capability

The ISL59123 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

Ω

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 ISL59123, 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 Equation 1:

Where:

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 use Equation 2:

for sinking use Equation 3:

Where:

Power Supply Bypassing Printed Circuit Board

Layout

As with any modern operational amplifier, a good printed

circuit board layout is necessary for optimum performance.

Lead lengths should be as short as possible. The power

supply pin must be well bypassed to reduce the risk of

oscillation. For normal single supply operation, a single

4.7µF tantalum capacitor in parallel with a 0.1µF ceramic

Printed Circuit Board Layout

For good AC performance, parasitic capacitance should be

kept to minimum. Use of wire wound resistors should be

avoided because of their additional series inductance. Use

of sockets should also be avoided if possible. Sockets add

parasitic inductance and capacitance that can result in

compromised performance.

PD

MAX

T

JMAX

T

AMAX

–

θ

JA

---------------------------------------------

=

(EQ. 1)

PD

MAX

V

S

I

SMAX

V

S

V

OUT

–

()

+

×

V

OUT

R

L

----------------

×

=

(EQ. 2)

PD

MAX

V

S

I

SMAX

V

OUT

V

S

–

()

+

×

I

LOAD

×

=

(EQ. 3)

ISL59123