THS7365 Datasheet, PDF (38/53 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7365
SBOS467 – MARCH 2009.................................................................................................................................................................................................. www.ti.com
CVBS
S-Video Y’
S-Video C’
Y'/G'
P'_B/B'
P'_R/R'
THS7365
1 SD1 IN
SD1 OUT 20
R
2 SD2 IN
SD2 OUT 19
3 SD3 IN
SD3 OUT 18
4 NC
R
+2.7 V to
+5 V
5 V_S+
6 NC
Disable SD 17
GND 16
Disable HD 15
Disable SD
Disable HD
7 HD1 IN
HD1 OUT 14
R
8 HD2 IN
HD2 OUT 13
9 HD3 IN
HD3 OUT 12
10 Bypass SD Bypass HD 11
R
Bypass
SD LPF
R
Bypass
HD LPF
R
CVBS 1 Out
75 W
S-Video Y' 1 Out
75 W
S-Video C' 1 Out
75 W
Y'/G' 1 Out
75 W
P’_B/B' 1 Out
75 W
P'_R/R' 1 Out
75 W
CVBS 1 Out
75 W
75 W
S-Video Y' 1 Out
75 W
75 W S-Video C' 1 Out
75 W
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
75 W
75 W
75 W
75 W
75 W
Figure 120. Typical Six-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 THS7365 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 THS7365. While the THS7365 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 THS7365 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 utilize RMS power and not
instantaneous power when evaluating the thermal
performance.
Note that the THS7365 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 THS7365 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 THS7365 output
pins should be kept below 220 pF for the 9.5-MHz
filter channels and below 15 pF for the 36-MHz filter
channels. One way to help ensure this condition is
satisfied is to make sure the 75-Ω source resistor is
placed next to each THS7365 output pin. If a large
ac-coupling capacitor is used, the capacitor should be
placed after this resistor.
38
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