8

FN6319.1

January 28, 2008

Application Information

The ISL59119 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

8MHz and slew rate of about 25V/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

ISL59119 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 ISL59119 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 page 1 shows the basic

operation of the ISL59119’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 at the input.

Figure 27 shows the setup for testing the clamp’s response

to a large step response at the input.

Once the signals are clamped at the input they are level

shifted by +65mV before being amplified by a gain of x2.

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 ISL59119, a three-pole roll-off at 8MHz. 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, with poles two and three generated

with a Sallen Key, creating a nice three-pole roll-off at 8MHz.

Output Coupling

The ISL59119 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 ISL59119’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

FIGURE 25. PACKAGE POWER DISSIPATION vs AMBIENT

TEMPERATURE

FIGURE 26. PACKAGE POWER DISSIPATION vs AMBIENT

TEMPERATURE

JEDEC JESD51-7 HIGH EFFECTIVE THERMAL

CONDUCTIVITY TEST BOARD

0

0.4

1.0

0.8

0.2

0.6

0

100

125

150

AMBIENT TEMPERATURE (°C)

50

25

75 85

435mW

909mW

θ

+23

0°C

/W

SO

T23

-6

JEDEC JESD51-3 LOW EFFECTIVE THERMAL

CONDUCTIVITY TEST BOARD

0

0.2

0.7

0.6

0.1

0.4

0.5

0.3

0

100

125

150

AMBIENT TEMPERATURE (°C)

50

25

75 85

391mW

625mW

θ

+2

56

°C

/W

SO

T2

3-6

75

Ω

75

Ω

500

Ω

0.1µF

NTSC VIDEO

1Hz SQUARE WAVE

150

Ω

OUTPUT

ISL59119

CH1

CH2

FIGURE 27. DC STEP RESPONSE CIRCUIT

ISL59119