FMS6408 Datasheet, PDF (7/9 Pages) Fairchild Semiconductor – Triple Video Filter Driver for RGB and YUV Signals

FMS6408
DATA SHEET
Application Notes
Output Drive Capability
The FMS6408 can drive dual 75Ω loads where each load
consists of a 75Ω resistor in series with a 75Ω termination
resistor in the driven device. This presents a 150Ω load to
the output so two similar loads in parallel look like 75Ω from
the output to ground. In some cases it may be desirable to
drive a single load on one or more outputs with a dual load
on the remaining outputs. This is an acceptable loading con-
dition but might cause a slight degradation in gain matching.
Device Power Dissipation
The FMS6408 specifications provide a quiescent no-load
supply current of 52mA (typical). With a nominal 5V
supply, this results in a power dissipation of 260mW. The
overall power dissipation can be significantly affected by the
applied load, particularly in DC-coupled applications. In
order to calculate the total power dissipation the typical
output voltages and the loading must be known.
The highest power dissipation will occur for YUV video sig-
nals that are DC-coupled into dual video loads. Refer to the
the diagram in Figure 3 below.
Assume a video signal on the Y channel that averages 50%
luminance with an output voltage of 1.55V then calculate the
load current:
I_load (Y) = 1.55V/75Ω = 20.6mA
The device dissipation due to this load will be the internal
voltage drop multiplied by the load current:
P_diss (Y) = (5V - 1.55V) * 20.6mA = 71mW
The average DC level for the U and V channels is set by the
clamp circuit to 1.125V. The signal will be symmetrical
about this voltage so:
I_load (U) = 1.125V/75Ω = 15mA
The device dissipation due to this load will be the internal
voltage drop multiplied by the load current:
P_diss (U) = (5V - 1.125V) * 15mA = 58.125mW
Since the U and V power dissipation are approximately the
same, the total dissipation due to the load can be estimated
by:
P_diss (load) = P (Y) + 2 * P (U) = 71mW +
(2 * 58.125mW) = 187.55mW
This will bring the typical total device power dissipation to
260mW (quiescent power) + 187.55mW (load power) or
447.55mW. It is advisable to calculate the highest possible
power dissipation using worst-case quiescent supply current
and the maximum allowable power supply voltage. This
result should be used when calculating the die temperature
rise with the supplied θ_JA, thermal resistance value.
Field Time Distortion
In applications with AC-coupled outputs, the AC-coupling
capacitors will dominate the field time distortion. Perfor-
mance is specified with 220µF coupling capacitors; if better
performance is desired, the capacitors may be increased or
the outputs may be DC-coupled.
+5V
V_CC
IY
Driver
+ VIY -
YOUT
IU
Driver
+ VIU -
UOUT
IV
Driver
+ VIV -
VOUT
2.25V
1.55V
0.85V
0.25V
1.825V
1.125V
0.425V
1.825V
1.125V
0.425V
75Ω
75Ω Video Cables
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
Figure 3. YUV Video Signals that are DC-Coupled into Dual Video Loads
REV. 2C August 31, 2004
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