MAX4090
3V/5V, 6dB Video Buffer with Sync-Tip Clamp
and 150nA Shutdown Current
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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 V
IN
= 0.38V, then
V
OUT
= 0.38V. If V
IN
= 1.38V, then V
OUT
= 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 V
CC
= 3V.
Shutdown Mode
The MAX4090 features a low-power shutdown mode
(I
SHDN
= 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 R
IS
(Figure 1)
