FMS6646 Datasheet, PDF (8/12 Pages) Fairchild Semiconductor – Six Channel, SD/HD (1080p) Video Filter Driver

Applications Information
Functional Description
The FMS6646 Low-Cost Video Filter (LCVF) provides
6dB gain from input to output. In addition, the input is
slightly offset to optimize the output driver performance.
The offset is held to the minimum required value to
decrease the standing DC current into the load. Typical
voltage levels are shown in Figure 10.
1.0 -> 1.02V
0.65 -> 0.67V
0.3 -> 0.32V
0.0 -> 0.02V
V_IN
2.28V
1.58V
0.88V
0.28V
V_OUT
Driven by:
DC-Coupled DAC Outputs
AC-Coupled and Clamped
Y, CV, R, G, B
There is a 280mV offset from the DC input level to the
DC output level. V_OUT = 2 • V_IN + 280mV.
0.65V
Y_IN
Y_OUT
Driver
800k
Figure 11. Input Clamp Circuit
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs,
use the configuration shown in Figure 12.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75Ω
Clamp
Inactive
0.85V
0.5V
0.15V
1.98V
1.28V
0.58V
V_IN
Driven by:
AC-Coupled and Biased
U, V, Pb, Pr, C
V_OUT
Figure 10.Typical Voltage Levels
The FMS6646 provides an internal diode clamp to
support AC-coupled input signals. If the input signal
does not go below ground, the input clamp does not
operate. This allows DAC out puts to directly drive the
FMS6646 without an AC coupling capacitor. The worst-
case sync tip compression due to the clamp does not
exceed 7mV. The input level set by the clamp,
combined with the internal DC offset, keeps the output
within its acceptable range. When the input is AC-
coupled, the diode clamp sets the sync tip (or lowest
voltage) just below ground.
For symmetric signals like C, U, V, Cb, Cr, Pb, and Pr;
the average DC bias is fairly constant and the inputs
can be AC-coupled with the addition of a pull-up resistor
to set the DC input voltage. DAC outputs can also drive
these same signals without the AC coupling capacitor.
A conceptual illustration of the input clamp circuit is
shown in Figure 11.
Figure 12.DC-Coupled Inputs and Outputs
If the DAC’s average DC output level causes the signal
to exceed the range of 0V to 1.4V, it can be AC-coupled
as shown in Figure 13.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
0.1μF
LCVF
75Ω
Clamp
Active
Figure 13.AC-Coupled Inputs, DC-Coupled Outputs
When the FMS6646 is driven by an unknown external
source or a SCART switch with its own clamping
circuitry, the inputs should be AC-coupled as shown in
Figure 14.
0V - 1.4V
External video
source must
be AC coupled
0.1μF
75Ω
LCVF
75Ω
Clamp
Active
Figure 14.SCART Configuration with DC-Coupled
Outputs
© 2009 Fairchild Semiconductor Corporation
FMS6646 • Rev.1.0.1
8
www.fairchildsemi.com