5V, 6dB Video Buffer with Sync-Tip Clamp,

Output Sag Correction, and 150nA Shutdown Current

6

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Detailed Description

The MAX4032 5V, 6dB video buffer with sync-tip

clamp, output sag correction, and low-power shutdown

mode is available in tiny SOT23 and SC70 packages.

The sag-corrected output of the MAX4032 is designed

to drive AC-coupled, 150

Ω back-terminated video

loads in portable video applications such as digital still

cams, portable DVD players, digital camcorders,

PDAs, video-enabled cell phones, portable game sys-

tems, and notebook computers. The sag correction

feature introduces low-frequency compensation that

reduces the value of the normally bulky and expensive

330µF AC-coupling capacitor to two small, less expen-

sive 22µF capacitors. The input clamp positions the

video waveform at the output and allows the MAX4032

to be used as either an AC- or DC-coupled output driver.

The MAX4032 operates from a single 5V supply and

consumes only 6.5mA of supply current. The low-power

shutdown mode reduces the supply current to 150nA,

making the MAX4032 ideal for low-voltage, battery-

powered video applications.

The input signal to the MAX4032 is AC-coupled through

a capacitor into an active sync-tip clamp circuit, which

places the minimum of the video signal at approximate-

ly 0.38V. The output buffer amplifies the video signal

while still maintaining the 0.38V clamp voltage at the

(1V)) = 2.38V when SAG is shorted OUT.

There are two common output connections for the

MAX4032:

1) SAG is shorted to OUT and 150

Ω is directly con-

nected from OUT to ground (see Figure 2).

2) Two capacitors and 150

Ω are connected between

OUT, SAG, and ground (see Figure 3).

Sag Correction

Sag correction refers to the low-frequency compensa-

tion of the highpass filter formed by the 150

Ω load of a

back-terminated coax and the output-coupling capaci-

tor. This break point must be low enough in frequency

to pass the Vertical Sync Interval (<25Hz for PAL and

<30Hz for NTSC) to avoid Field Tilt. Traditionally, the

break point is made <3~5Hz, and the coupling capaci-

tor must be very large, typically >330µF. The MAX4032

reduces the value of this coupling capacitor, replacing

it with a pair of 22µF capacitors. This is done by putting

a resistor network in series with the feedback, raising

the gain, and creating a high-impedance node at the

SAG output. This node is AC-coupled to the load in par-

allel with the normal output, as shown in Figure 3. This

the interface caps and their cost while retaining the low-

frequency response.

The minimum value of the output-coupling capacitor is

a function of the acceptable Field Tilt. In Figure 1, the

Field Tilt is given for several values of capacitance from

10µF to 47µF for comparison. Although values lower

than 22µF may have acceptable Field Tilt, they are not

recommended, since tolerance, aging, and voltage

and temperature coefficients reduce the capacitance in

actual

applications.

Increasing

the

output-

coupling capacitors beyond 47µF does not improve

performance.

Shutdown Mode

The MAX4032 features a low-power shutdown mode

tions. Pulling the SHDN pin high enables the output.

Connecting the SHDN pin to ground (GND) disables

the output and places the MAX4032 into a low-power

shutdown mode.

Applications Information

Input Coupling the MAX4032

The MAX4032 input must be AC-coupled because the

input capacitor stores the clamp voltage. The MAX4032

requires a typical value of 0.1µF for the input clamp to

meet the Line Droop specification. A minimum of a

ceramic capacitor with an X7R temperature coefficient is

recommended to avoid temperature-related problems

with Line Droop. For extended temperature operation,

such as outdoor applications, or where the impressed

Figure 1. Field Tilt vs. Output-Coupling Capacitance

COUPLING CAPACITANCE (

µF)

100

20

40

60

80

10

20

30

40

50