Detailed Description
The primary function of the MAX7453 is to filter and
buffer the encoder DAC outputs that contain digital
video information in applications such as set-top boxes,
HDRs, DVD players, and digital VCRs. This device
reconstructs and cleans up analog video signals. The
MAX7453 consists of a lowpass filter and a +6dB-gain
output video buffer. The luma (YIN) and chroma (CIN)
video inputs are summed internally to provide the com-
posite (CVOUT) video output.
These devices operate from a single +5V supply. The
filters have a nominal cutoff frequency optimized for
NTSC, PAL, and SDTV.
Filter
Filter Response
The reconstruction filter consists of two 2nd-order
Sallen-Key stages. The Butterworth-type response fea-
tures a maximally flat passband for NTSC and PAL
bandwidths. The stopband offers 42dB (typ) of attenua-
tion at sampling frequencies of 27MHz and above (see
the Typical Operating Characteristics).
Output Buffer
The YOUT and COUT buffers have a fixed +6dB gain
capable of driving one standard 150Ω video load with a
2V
P-P
signal. The CVOUT buffer has a fixed +6dB gain,
capable of driving two 150Ω video loads. The MAX7453
can drive an AC load or drive the video load directly
without using the large output capacitor. The output
buffer of all the channels drives a DC load with an out-
put blanking level of less than 1V.
Output Clamp Level
When sync pulses in the luma signal (Y) are detected,
the DC restore loop is activated. The function of the
loop is to set the sync tip of the video signal to the
desired DC level of 0.8V for YOUT and COUT, and to
set the average DC voltage of COUT at 1.6V.
Applications Information
Input Considerations
Use a 0.1µF ceramic capacitor to AC-couple the input
to the MAX7453. This input capacitor stores a DC level
such that the outputs are clamped to the appropriate
DC voltage level.
Output Considerations
The outputs of the MAX7453 are typically connected to
a 75Ω series back-match resistor followed by the video
cable. Because of the inherent divide-by-two of this con-
figuration, the blanking level of the video cable is always
less than 1V, which complies with industry-standard
video requirements. The video buffer can also drive an
AC-coupled video load. Good video performance is
achieved with an output capacitor as low as 220µF.
Power-Supply Bypassing and Layout
The MAX7453 operates from a single +5V supply.
Bypass V
CC
to GND with a 0.1µF capacitor. Place all
external components as close to the device as possible.
The maximum power dissipation listed in the Absolute
Maximum Ratings section assumes that the PC board is
a multilayer board with ground and power planes.
Chip Information
TRANSISTOR COUNT: 4100
PROCESS: BiCMOS
