THS7313 Datasheet, PDF (24/48 Pages) Texas Instruments – 3-Channel Low Power SDTV Video Amplifier with I2C Control, 6-dB Gain, SAG Correction, 2:1 Input MUX, and Selectable Input Bias Modes

THS7313
SLOS483 – NOVEMBER 2005
www.ti.com
APPLICATION INFORMATION (continued)
OUTPUT MODES OF OPERATION – DC COUPLED
The THS7313 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 is accomplished by connecting the output pin of each channel directly to
the SAG output pin of the corresponding channel as shown in Figure 52. This offers the best line tilt and field tilt
(or droop) performance since there is no ac coupling occurring. Keep in mind that if the input is ac coupled, then
the resulting tilt due to the input ac coupling is still seen on the output regardless of the output coupling. The
70-mA output current drive capability of the THS7313 is designed to drive two video lines simultaneously –
essentially a 75-Ω load – while keeping the output dynamic range as wide as possible.
One concern of dc coupling is if the line is terminated to ground. When the ac-bias input mode is selected, the
output of the THS7313 is at mid-rail. With 2 lines terminated to ground, this creates a dc current path to exist
which results in a slightly decreased high output voltage swing resulting in an increase in power dissipation of the
THS7313. While the THS7313 is 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.
Although this configuration adds less then 10 mW of power dissipation per channel, the overall low power
dissipation of the THS7313 design minimizes potential thermal issues even when using the TSSOP package at
high ambient temperatures.
3.3 V
DAC /
Encoder
CVBS
(THS8200)
R
DC + 135 mV
CVBS
Out
75 W
SDTV
CVBS
Y’
S-Video Y’
S-Video C’
480i
576i
G’B’R’
C’
CVBS
S-Video Y’
S-Video C’
External
Input
R
R
0.1 mF
75 W
0.1 mF
75 W
0.1 mF
75 W
1 NC
NC 20
DC + 135 mV
DC + 135 mV
AC STC
AC STC
AC Bias
2 CH.1 IN A
3 CH.2 IN A
4 CH.3 IN A
5 CH.1 IN B
6 CH.2 IN B
7 CH.3 IN B
CH.1 OUT 19
CH. 1 SAG 18
CH.2 OUT 17
CH. 2 SAG 16
CH.3 OUT 15
CH. 3 SAG ¹⁴
8 I 2C-A1
SCL 13
9 I 2C-A0
SDA 12
10 GND
V_S+ 11
0.01 mF
75 W
S-Video
75 W
+Vs
+
100 mF
2
IC
Controller
75 W
Y’
Out
C’
75 W
Out
Figure 52. Typical SDTV CVBS/Y'/C' System With DC-Coupled Line Driving
75 W
Note that the THS7313 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 pin – typically 75-Ω.
This helps isolate capacitive loading effects from the THS7313 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 THS7313 output pins should be kept below 25-pF for best performance.
When driving 2 video lines, each line should have its own 75-Ω source termination resistors to isolate the lines
from each other.
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