THS7353_15 Datasheet, PDF (27/57 Pages) Texas Instruments – 3-Channel Low Power Video Buffer with I2C Control, Selectable Filters, External Gain Control, 2:1 Input MUX, and Selectable Input Modes

THS7353
www.ti.com
SLOS484B – NOVEMBER 2005 – REVISED AUGUST 2012
OUTPUT MODES OF OPERATION – DC COUPLED
The THS7353 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 shown in Figure 64. 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 THS7353 drives 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 THS7353 is +V_S ÷ 3.3. This allows a dc current path to exist which results in a decreased high
output voltage swing culminating in an increase in power dissipation of the THS7353. The THS7353 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 long term reliability could suffer. Although this configuration adds less
than 10 mW of power dissipation per channel, the overall low power dissipation of the THS7353 design
minimizes potential thermal issues when using the TSSOP package at high ambient temperatures.
3.3 V
G’
DAC /
Encoder
B’
(THS8200)
R’
G’
B’
R’
R
R
R
0.1 mF
75 W
1 mF
75 W
1 mF
75 W
DC + 250 mV
1 NC
NC ₂₀
2 CH.1 IN A CH.1 OUT 19
DC + 250 mV
DC + 250 mV
3 CH.2 IN A
4 CH.3 IN A
CH1.
GAIN ADJ
18
CH.2 OUT 17
AC STC
AC Bias
5 CH.1 IN B
CH2.
GAIN ADJ
16
6 CH.2 IN B CH.3 OUT 15
AC Bias
7
CH.3 IN B
CH3.
GAIN ADJ
14
8 ^{I 2C-A1}
I2C-SCL 13
9 I 2C-A0
I2C-SDA ₁₂
10 GND
V_{S+ 11}
0.01 mF
G’
75 W ^Out
2.2 pF
750 W
2.2 pF
750 W
2.2 pF
750 W
B’
75 W ^Out
R’
75 W ^Out
+Vs
+
100 mF
2
IC
Controller
75 W
75 W
75 W
External
Input
Figure 64. G'B'R' (R'G'B') System with 6-dB Gain and DC-Coupled Driving
Note that the THS7353 drives the line with dc coupling regardless of the input mode of operation. The only
caution is when driving capacitive loads. This capacitive loading includes both the PCB stray capacitance and
possibly an ADC input capacitance which is typically between 5-pF and 10-pF. The THS7353 is designed to drive
up to 15-pF loads without any issues. But, if the total capacitive loading is 20-pF or more, then it is
recommended to place a series resistance at the output of the THS7353. The value of the resistor depends on
the capacitive load and can vary from 10 Ω to 75 Ω. If the THS7353 is used to drive a video line, then the
obvious use of a 75-Ω resistor ensures stability. Failure to isolate large capacitive loads may result in instabilities
with the output buffer potentially causing ringing or oscillations to appear.
Copyright © 2005–2012, Texas Instruments Incorporated
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