Rainbow Electronics MAX7470 - Manual (Page 15)

MAX7469/MAX7470
HDTV Continuously Variable
Anti-Aliasing Filters
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I 2 C Compatibility
The MAX7469/MAX7470 are compatible with existing
I
2 C systems supporting standard I 2 C 8-bit communica-
tions. The general call address is ignored, and CBUS
formats are not supported. The device’s address is
compatible with 7-bit I
2 C addressing protocol only; 10-
bit address formats are not supported.
Applications Information
Input Considerations
Use 0.1µF ceramic capacitors to AC-couple the inputs.
The inputs cannot be DC-coupled. The internal clamp
circuit stores a DC voltage across the input capacitors
to obtain the appropriate output DC voltage level.
Increasing the value of these capacitors to improve line-
time distortion is not necessary due to the extremely low
input leakage current yielding a very low line-time dis-
tortion performance.
The MAX7469/MAX7470 provide a high input imped-
ance to allow a nonzero source impedance to be used,
such as when the input is connected directly to a back-
matched video cable, ensuring the external resistance
determines the termination impedance.
Output Considerations
The MAX7469/MAX7470 outputs can be DC- or AC-
coupled. The MAX7470, with its +6dB gain, is typically
connected to a 75Ω series back-match resistor fol-
lowed by the video cable. Because of the inherent
divide-by-two of this configuration, the blanking level of
the video signal is always less than 1V, which complies
with digital TV requirements.
The MAX7469, with its 0dB gain, is typically connected
to an ADC or video decoder. This can be a DC or AC
connection. If a DC connection is used, ensure that the
DC input requirements of the ADC or video decoder
are compatible.
If an AC connection is used, choose an AC-coupling
capacitor value that ensures that the lowest frequency
content in the video signal is passed and the line-time
distortion is kept within desired limits. The selection of
this value is a function of the input impedance and,
more importantly, the input leakage of the circuit being
driven. Use a video clamp to reestablish the DC level, if
not already included in the subsequent circuit.
The outputs of the MAX7469/MAX7470 are fully protected
against a short-circuit condition either to ground or the
positive supply of the device.
Power-Supply Bypassing and Layout
Considerations
The MAX7469/MAX7470 operate from a single +5V ana-
log supply and a +3.3V digital supply. Bypass AV
DD to
GND with a 0.1µF capacitor and an additional 1µF
capacitor in parallel for additional low-frequency decou-
pling. Determine the proper power-supply bypassing
necessary by taking into account the desired distur-
bance level tolerable on the output, the power-supply
rejection of the MAX7469/MAX7470, and the amplitude
and frequency of the disturbance signals present in the
vicinity of the MAX7469/MAX7470. Use an extensive
ground plane to ensure optimum performance. The three
AV
DD pins (pins 7, 9, and 11) that supply the individual
channels can be connected together and bypassed as
one, provided the components are close to the pins.
Bypass DV
DD to DGND with a 0.1µF capacitor. All
ground pins (GND) must be connected to a low imped-
ance ground plane as close as possible to the device.
Place the input termination resistors as close as possi-
ble to the device. Alternatively, the terminations can be
placed further from the device if the PC board traces
are designed to be a controlled impedance of 75Ω.
Minimize parasitic capacitance as much as possible to
avoid performance degradation in the upper frequency
range possible with the MAX7469/MAX7470.
Refer to the MAX7469/MAX7470 evaluation kit for a
proven PC board layout.
Exposed Pad and Heat Dissipation
The MAX7469/MAX7470 TQFN package has an
exposed pad on its bottom. This pad is electrically con-
nected, internal to the device, to GND. Do not route any
PC board traces under the package.
The MAX7469/MAX7470 typically dissipate 900mW of
power, therefore, pay careful attention to heat disper-
sion. The use of at least a two-layer board with a good
ground plane is recommended. To maximize heat dis-
persion, place copper directly under the MAX7469/
MAX7470 package so that it matches the outline of the
plastic encapsulated area. Do the same thing on the
bottom ground plane layer and then place as many
vias as possible connecting the top and bottom layers
to thermally connect it to the ground plane.
Maxim has evaluated a four-layer board using FR-4
material and 1oz copper with equal areas of metal on
the top and bottom side coincident with the plastic
encapsulated area of the 20-pin TQFN package. The
two middle layers are used as power and ground