FMS6646_09 Datasheet, PDF (9/12 Pages) Fairchild Semiconductor – Six Channel, SD/HD 1080p Video Filter Driver

The same method can be used for biased signals with
the addition of a pull-up resistor to make sure the clamp
never operates. The internal pull-down resistance is
800kΩ±20%, so the external resistance should be
7.5MΩ to set the DC level to 500mV. If a pull-up
resistance less than 7.5MΩ is desired, an external pull-
down can be added such that the DC input level is set
to 500mV.
External video
source must
7.5MΩ
be AC coupled ^0.1μF
75Ω
500mV +/-350mV
LCVF
75Ω
Bias
Input
Figure 15.Biased SCART with DC-Coupled Outputs
The same circuits can be used with AC-coupled outputs
if desired, as shown in Figure 16.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
Clamp
Inactive
75
220µF
Figure 16.DC-Coupled Inputs, AC-Coupled Outputs
DVD or
STB
SoC
DAC
Output
0V - 1.4V
0.1μF
LCVF
Clamp
Active
75Ω ^220μF
Figure 17.-Coupled Inputs, AC-Coupled Outputs
External video
source must
7.5MW
be AC coupled. ^0.1µF
75W
500mV +/-350mV
LCVF
Clamp
Active
75W ^220µF
Power Dissipation
The FMS6646 output drive configuration must be
considered when calculating overall power dissipation.
Care must be taken not to exceed the maximum die
junction temperature. The following example can be
used to calculate the FMS6646’s power dissipation and
internal temperature rise:
TJ = TA + Pd • θJA
where Pd = PCH1 + PCH2 + PCHx and
PCHx = VS • ICH - (VO2/RL)
where VO = 2VIN + 0.280V
ICH = (ICC / 6) + (VO/RL)
VIN = RMS value of input signal
ICC = 90mA , VS = 3.3V
RL = channel load resistance
Board layout can affect thermal characteristics. Refer to
the Layout Considerations section for more information.
Output Considerations
The FMS6646 outputs will be DC offset from the input
by 150mv therefore VOUT = 2*VIN DC+150mv. This offset
is required to obtain optimal performance from the
output driver and is held at the minimum value in order
to decrease the standing DC current into the load.
Since the FMS6646 has a 2x (6dB) gain, the output is
typically connected via a 75Ω series back-matching
resistor followed by the 75Ω video cable. Because of
the inherent divide by two of this configuration, the
blanking level at the load of the video signal is always
less then 1V. When AC-coupling the output ensure that
the coupling capacitor of choice will pass the lowest
frequency content in the video signal and that line time
distortion (video tilt) is kept as low as possible.
The selection of the coupling capacitor is a function of
the subsequent circuit input impedance and the leakage
current of the input being driven. In order to obtain the
highest quality output video signal the series
termination resistor must be placed as close to the
device output pin as possible. This greatly reduces the
parasitic capacitance and inductance effect on the
FMS6646 output driver. Recommend distance from
device pin to place series termination resistor should be
no greater than 0.1 inches.
Figure 18.Biased SCART with AC-Coupled Outputs
Note:
6. The video tilt or line time distortion is dominated by
the AC-coupling capacitor. The value may need to
be increased beyond 220μF to obtain satisfactory
operation in some applications.
© 2009 Fairchild Semiconductor Corporation
FMS6646 • Rev.1.0.2
9
Figure 19.Distance from Device Pin to Series
Termination Resistor
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