THS7376 Datasheet, PDF (46/61 Pages) Texas Instruments – 4-Channel Video Amplifier with One SD and Three HD 8th-Order Filters with 6-dB Gain

THS7376
SBOS692 – JUNE 2013
www.ti.com
OUTPUT MODE OF OPERATION: DC-COUPLED
The device 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 100-mA output current drive
capability of the device is designed to drive two video lines simultaneously (essentially a 75-Ω load) while
keeping the output dynamic range as wide as possible. Figure 134 shows the device driving two video lines while
keeping the output dc-coupled.
CVBS
R
Y’, G’
R
P’_B, B’
R
P’_R, R’
R
1 CVBS IN
CVBS OUT 14
2 HD CH1 IN HD CH1 OUT 13
3 HD CH2 IN HD CH2 OUT 12
4 ^{HD CH3 IN HD CH3 OUT} 11
5 GND
VS+ 10
6 DISABLE HD BYPASS 9
7 ^NC
NC ₈
75 W
CVBS 1 Out _{75 W}
Y’, G’ 1 Out
75 W
75 W
P’_B, B’ 1 Out
75 W
75 W
P’_R, R’ 1 Out
75 W
75 W
75 W
CVBS 2 Out _{75 W}
Y’, G’ 2 Out
75 W
75 W
P’_B, B’ 2 Out
75 W
75 W
P’_R, R’ 2 Out
75 W
75 W
To GPIO
+3 V to 5 V
Controller Or GND
Figure 134. 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 device output 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 device power dissipation. Although the device
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
device 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 device 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 device 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 device output pins should be kept below 20 pF for the fixed SD filter
channels and below 15 pF for the HD filter channels. One way to verify this condition is satisfied is to ensure the
75-Ω source resistor is placed within 0.5 inches, or 12.7 mm, of the device output pin. If a large ac-coupling
capacitor is used, the capacitor should be placed after this resistor.
There are many reasons why 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 at the video amplifier output. The THS7376 can easily meet this requirement without issue. However, in
Japan, the EIAJ CP-1203 specification stipulates a 0-V ± 0.1-V level with no signal. This requirement can be met
with the THS7376 in shutdown mode, but while active the device cannot meet this specification without output
ac-coupling. AC-coupling the output essentially ensures that the video signal functions with any system and any
specification. For many modern systems, however, dc-coupling can satisfy most needs.
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