MAX4090 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX4090

3V/5V, 6dB Video Buffer with Sync-Tip Clamp and 150nA Shutdown Current

Maxim Integrated 

3V/5V, 6dB Video Buffer with Sync-Tip Clamp

and 150nA Shutdown Current

Detailed Description

The MAX4090 3V/5V, 6dB video buffer with sync-tip

clamp and low-power shutdown mode is available in tiny

SOT23 and SC70 packages. The MAX4090 is designed

to drive DC-coupled, 150Ω 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 systems, and note-

book computers. The input clamp positions the video

waveform at the output and allows the MAX4090 to be

used as a DC-coupled output driver.

The MAX4090 operates from a single 2.7V to 5.5V sup-

ply and consumes only 6.5mA of supply current. The

low-power shutdown mode reduces the supply current

to 150nA, making the MAX4090 ideal for low-voltage,

battery-powered video applications.

The input signal to the MAX4090 is AC-coupled

through a capacitor into an active sync-tip clamp cir-

cuit, which places the minimum of the video signal at

approximately 0.38V. The output buffer amplifies the

video signal while still maintaining the 0.38V clamp volt-

age at the output. For example, if VIN = 0.38V, then

VOUT = 0.38V. If VIN = (0.38V + 1V) = 1.38V, then VOUT

= (0.38V + 2 X (1V)) = 2.38V. The net result is that a 2V

video output signal swings within the usable output

voltage range of the output buffer when VCC = 3V.

Shutdown Mode

The MAX4090 features a low-power shutdown mode

(ISHDN = 150nA) for battery-powered/portable applica-

tions. Pulling the SHDN pin high enables the output.

Connecting the SHDN pin to ground (GND) disables

the output and places the MAX4090 into a low-power

shutdown mode.

Applications Information

Input Coupling the MAX4090

The MAX4090 input must be AC-coupled because the

input capacitor stores the clamp voltage. The MAX4090

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 prob-

lems with Line Droop. For extended temperature opera-

tion, such as outdoor applications, or where the

impressed voltage is close to the rated voltage of the

capacitor, a film dielectric is recommended. Increasing

the capacitor value slows the clamp capture time.

Values above 0.5µF should be avoided since they do

not improve the clamp’s performance.

The active sync-tip clamp also requires that the input

impedance seen by the input capacitor be less than

100Ω typically to function properly. This is easily met

by the 75Ω input resistor prior to the input-coupling

capacitor and the back termination from a prior stage.

Insufficient input resistance to ground causes the

MAX4090 to appear to oscillate. Never operate the

MAX4090 in this mode.

Using the MAX4090 with the

Reconstruction Filter

In most video applications, the video signal generated

from the DAC requires a reconstruction filter to smooth

out the signal and attenuate the sampling aliases. The

MAX4090 is a direct DC-coupled output driver, which

can be used after the reconstruction filter to drive the

video signal. The driving load from the video DAC can

be varied from 75Ω to 300Ω. A low input impedance

(<100Ω) is required by the MAX4090 in normal opera-

tion, special care must be taken when a reconstruction

filter is used in front of the MAX4090.

For standard video signal, the video passband is about

6MHz and the system oversampling frequency is at

27MHz. Normally, a 9MHz BW lowpass filter can be

used for the reconstruction filter. This section demon-

strates the methods to build simple 2nd- and 3rd-order

passive butterworth lowpass filters at the 9MHz cutoff

frequency and the techniques to use them with the

MAX4090 (Figures 1 and 4).

2nd-Order Butterworth Lowpass Filter Realization

Table 1 shows the normalized 2nd-order butterworth

LPF component values at 1rad/s with a source/load

impedance of 1Ω.

With the following equations, the L and C can be calcu-

lated for the cutoff frequency at 9MHz. Table 2 shows

the appropriated L and C values for different source/

load impedance, the bench measurement values for

the -3dB BW and attenuation at 27MHz. There is

approximately 20dB attenuation at 27MHz, which effec-

tively attenuates the sampling aliases. The MAX4090

requires low input impedance for stable operation and

it does not like the reactive input impedance. For R1/R2

greater than 100Ω, a series resistor RIS (Figure 1)

Table 1. 2nd-Order Butterworth Lowpass

Filter Normalized Values

Rn1 = Rn2 (Ω)

1

Cn1 (F)

1.414

Ln1 (H)

1.414

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