Datasheet

THS7360
www.ti.com SLOS674 –JUNE 2010
The THS7360 allowable input range is approximately and the input capacitor size. Lastly, the input
0 V to (V
S+
– 1.5 V), allowing for a very wide input capacitor forms a high-pass filter with the parallel
voltage range. As such, the input dc bias point is very impedance of the pull-up resistor and the 800-kΩ
flexible, with the output dc bias point being the resistor. In general, it is good to have this high-pass
primary factor. For example, if the output dc bias filter at approximately 3 Hz to minimize any potential
point on a SD channel is desired to be 1.6 V on a droop on a P’
B
or P’
R
signal. A 0.1-mF input capacitor
3.3-V supply, then the input dc bias point should be with a 9.31-MΩ pull-up resistor equates to
(1.6 V – 120 mV)/5.6 = 0.264 V. Thus, the pull-up approximately a 2.2-Hz high-pass corner frequency.
resistor calculates to approximately 9.31 MΩ,
This mode of operation is recommended for use with
resulting in 0.261 V. If the output dc-bias point is
chroma (C’), P’
B
, P’
R
, U’, and V’ signals. This method
desired to be 1.6 V with a 5-V power supply, then the
can also be used with sync signals if desired. The
pull-up resistor calculates to approximately 14.3 MΩ.
benefit of using the STC function over the ac-bias
Keep in mind that the internal 800-kΩ resistor has configuration on embedded sync signals is that the
approximately a ±20% variance. As such, the STC maintains a constant back-porch voltage as
calculations should take this variance into account. opposed to a back-porch voltage that fluctuates
For the 0.261-V example above, using an ideal depending on the video content. Because the
9.31-MΩ resistor, the input dc bias voltage is high-pass corner frequency is a very low 2.2 Hz, the
approximately 0.261 V ± 0.05 V. impact on the video signal is negligible relative to the
STC configuration.
The value of the output bias voltage is very flexible
and is left to each individual design. It is important to
ensure that the signal does not clip or saturate the
OUTPUT MODE OF OPERATION:
video signal. Thus, it is recommended to ensure the
DC-COUPLED
output bias voltage is between 0.9 V and (V
S+
– 1 V).
The THS7360 incorporates a rail-to-rail output stage
For 100% color saturated CVBS or signals with
that can be used to drive the line directly without the
Macrovision
®
, the CVBS signal can reach up to
need for large ac-coupling capacitors. This design
1.23 V
PP
at the input, or 2.46 V
PP
at the output of the
offers the best line tilt and field tilt (droop)
THS7360. In contrast, other signals are typically
performance because no ac-coupling occurs. Keep in
1 V
PP
or 0.7 V
PP
at the input, which translate to an
mind that if the input is ac-coupled, then the resulting
output voltage of 2 V
PP
or 1.4 V
PP
, respectively. The
tilt as a result of the input ac-coupling continues to be
output bias voltage must account for a worst-case
seen on the output, regardless of the output coupling.
situation, depending on the signals involved.
The 80-mA output current drive capability of the
One other issue that must be taken into account is
THS7360 is designed to drive two video lines
the dc-bias point is a function of the power supply. As
simultaneously—essentially, a 75-Ω load—while
such, there is an impact on system PSRR. To help
keeping the output dynamic range as wide as
reduce this impact, the input capacitor combines with
possible. Figure 47 shows the THS7360 driving two
the pull-up resistance to function as a low-pass filter.
video lines while keeping the output dc-coupled.
Additionally, the time to charge the capacitor to the
final dc bias point is a function of the pull-up resistor
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