FMS6408_07 Datasheet, PDF (9/11 Pages) Fairchild Semiconductor – Precision Triple Video Filter Driver for RGB and YUV Signals

Application Notes
Introduction
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 condition, but can 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. To calculate the total
power dissipation the typical output voltages and the
loading must be known.
The highest power dissipation occurs for YUV video
signals DC-coupled into dual video loads (refer to
Figure 3).
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
(1)
The device dissipation due to this load is the
internal voltage drop multiplied by the load current:
P_D(Y) = (5V − 1.55V) • 20.6mA = 71mW
(2)
+5V
V_CC
I
Y
Driver
+ VI_Y -
I_U
Driver
+ VI_U -
I_V
Driver
+ VI -
V
YOUT
U
OUT
V
OUT
2.25V
1.55V
0.85V
0.25V
1.825V
1.125V
0.425V
11..882255VV
1.125V
0.425V
The average DC level for the U and V channels is set
by the clamp circuit to 1.125V. The signal is
symmetrical about this voltage, therefore:
I_LOAD (U) = 1.125V / 75Ω = 15mA
(3)
The device dissipation due to this load is the internal
voltage drop multiplied by the load current:
P_D (U) = (5V − 1.125V) • 15mA = 58.125mW (4)
Since the U and the V power dissipation are
approximately the same, the total dissipation due to
load can be estimated by:
P_D (load) = P(Y) + 2 • P(U) =
(5)
71mW + 2 • 58.125mW = 187.55mW
This brings 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 θJA, thermal resistance value.
Field Time Distortion
In applications with AC-coupled outputs, the AC-
coupling capacitors dominate the field time distortion.
Performance is specified with 220µF coupling
capacitors; if better performance is desired, the
capacitors may be increased or the outputs may be
DC-coupled.
75Ω
75Ω Video Cables
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
Figure 12. YUV Video Signals that are DC-Coupled into Dual Video Loads
© 2004 Fairchild Semiconductor Corporation
FMS6408 Rev. 3.0.0
9
www.fairchildsemi.com