6

FN6142.4

March 15, 2007

Typically, this is accomplished with 220µF coupling

capacitor, a large and somewhat costly solution providing a

low frequency pole around 5Hz. If the size of this capacitor is

even slightly reduced we have found that the accompanying

phase shift in the 50Hz to 100Hz frequency range results in

field tilt, which results in a degraded video image.

The internal SAG network of the ISL59112 replaces the

220µF AC coupling capacitor with a network of two smaller

capacitors as shown, in Figure 12. Additionally, the network

is designed to place a zero in the ~30Hz range, providing a

small amount of peaking to compensate the phase response

associated with field tilt.

DC Output Coupling

The ISL59112 internal sync clamp makes it possible to DC

couple the output to a video load, eliminating the need for

any AC coupling capacitor, thereby saving board space and

additional expense for capacitors. Additionally, this solution

completely eliminates the issue of field tilt in the lower

frequency. The trade off is greater demand of supply current.

Typical load current for AC coupled is around 3mA

compared to typical 6mA used when DC coupling.

Output Drive Capability

The ISL59112 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 in transient applications, 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 ISL59112, 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:

Θ

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:

overheat.

SAG

NETWORK

AC COUPLING

CAPACITOR

FIGURE 12. SAG NETWORK AND AC COUPLING CAPACITORS

-

+

TELEVISION

OR VCR

ENABLE

FIGURE 13. DC COUPLE

PD

MAX

T

JMAX

T

AMAX

–

Θ

JA

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

=

(EQ. 1)

PD

MAX

V

S

I

SMAX

V

S

V

–

()

+

×

V

R

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

×

=

(EQ. 2)

PD

MAX

V

S

I

SMAX

V

–

()

+

×

I

×

=

(EQ. 3)

ISL59112