THS7372 Datasheet, PDF (35/49 Pages) Texas Instruments – 4-Channel Video Amplifier with One CVBS and Three Full-HD Filters with 6-dB Gain

THS7372
www.ti.com
SBOS578 – AUGUST 2011
OUTPUT MODE OF OPERATION: DC-COUPLED
The THS7372 incorporates a rail-to-rail output stage that can be used to drive the line directly without the need
for large ac-coupling capacitors. This design offers the best line tilt and field tilt (droop) performance because no
ac-coupling occurs. Keep in mind that if the input is ac-coupled, then the resulting tilt as a result of the input
ac-coupling continues to be seen on the output, regardless of the output coupling. The 80-mA output current
drive capability of the THS7372 is designed to drive two video lines simultaneously—essentially, a 75-Ω
load—while keeping the output dynamic range as wide as possible. Figure 88 shows the THS7372 driving two
video lines while keeping the output dc-coupled.
CVBS
Y'/G'
P’_B/B'
P’_R/R'
37.4 W
+2.7 V to
+5 V
37.4 W
37.4 W
THS7372
1 CVBS IN
2 NC
3 ^VS+
4 NC
5 FHD1 IN
6 FHD2 IN
7 FHD3 IN
CVBS OUT 14
DIS CVBS 13
GND 12
DIS FHD 11
FHD1 OUT 10
FHD2 OUT 9
FHD3 OUT 8
Disable
CVBS
Disable
FHD
37.4 W
CVBS 1 Out
75 W
75 W
CVBS 2 Out
75 W
75 W
Y'/G’ 1 Out
75 W
75 W
Y'/G’ 2 Out
75 W
75 W
P'_B/B’ 1 Out
75 W
75 W
P'_B/B’ 2 Out
75 W
75 W
P'_R/R’ 1 Out
75 W
75 W
P'_R/R’ 2 Out
75 W
75 W
Figure 88. Typical Four-Channel System with DC-Coupled Line Driving and Two Outputs Per Channel
One concern of dc-coupling, however, arises if the line is terminated to ground. If the ac-bias input configuration
is used, the output of the THS7372 has a dc bias on the output, such as 1.6 V. With two lines terminated to
ground, this configuration allows a dc current path to flow, such as 1.6 V/75-Ω = 21.3 mA. The result of this
configuration is a slightly decreased high output voltage swing and an increase in power dissipation of the
THS7372. While the THS7372 was designed to operate with a junction temperature of up to +125°C, care must
be taken to ensure that the junction temperature does not exceed this level or else long-term reliability could
suffer. Using a 5-V supply, this configuration can result in an additional dc power dissipation of (5 V – 1.6 V) ×
21.3 mA = 72.5 mW per channel. With a 3.3-V supply, this dissipation reduces to 36.2 mW per channel. The
overall low quiescent current of the THS7372 design minimizes potential thermal issues even when using the
TSSOP package at high ambient temperatures, but power and thermal analysis should always be examined in
any system to ensure that no issues arise. Be sure to use RMS power and not instantaneous power when
evaluating the thermal performance.
Note that the THS7372 can drive the line with dc-coupling regardless of the input mode of operation. The only
requirement is to make sure the video line has proper termination in series with the output (typically 75 Ω). This
requirement helps isolate capacitive loading effects from the THS7372 output. Failure to isolate capacitive loads
may result in instabilities with the output buffer, potentially causing ringing or oscillations to appear. The stray
capacitance appearing directly at the THS7372 output pins should be kept below 20 pF for the fixed SD filter
channels and below 15 pF for the FHD filter channels. One way to help ensure this condition is satisfied is to
make sure the 75-Ω source resistor is placed within 0.5 inches, or 12.7 mm, of the THS7372 output pin. If a large
ac-coupling capacitor is used, the capacitor should be placed after this resistor.
There are many reasons dc-coupling is desirable, including reduced costs, printed circuit board (PCB) area, and
no line tilt. A common question is whether or not there are any drawbacks to using dc-coupling. There are some
potential issues that must be examined, such as the dc current bias as discussed above. Another potential risk is
whether this configuration meets industry standards. EIA/CEA-770 stipulates that the back-porch shall be 0 V ± 1
V as measured at the receiver. With a double-terminated load system, this requirement implies a 0-V ± 2-V level
Copyright © 2011, Texas Instruments Incorporated
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