THS7375 Datasheet, PDF(22/36 Page) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 5.6-V/V Gain

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THS7375

SBOS449 â SEPTEMBER 2008......................................................................................................................................................................................... www.ti.com

This mode of operation is recommended for use with

chroma (Câ), PâB, PâR, Uâ, Vâ, and non-sync R'G'Bâ

signals. This method can also be used with signals

with sync, if desired. The benefit of using the STC

function is that it maintains a constant back-porch

voltage as opposed to a back-porch voltage that

fluctuates depending on the video content. Because

the corner frequency of the input is a very low 2.5 Hz,

the corner frequency of the input is also a very low

2.5 Hz, which is respectable performance relative to a

STC configuration.

OUTPUT MODE OF OPERATION:

DC-COUPLED

The THS7375 incorporates a rail-to-rail output stage

that can be used to drive the line directly without the

need for large ac coupling capacitors, as shown in

Figure 50. This approach offers the best line tilt and

field tilt (or droop) performance because no ac

coupling occurs. Keep in mind that if the input is

ac-coupled, then the resulting tilt because of the input

ac coupling is seen on the output regardless of the

output coupling. The 80-mA output current drive

capability of the THS7375 was designed to drive two

video lines simultaneously (essentially, a 75-â¦ load)

while maintaining as wide an output dynamic range

as possible.

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 THS7375 has

a dc bias on the output. With two lines terminated to

ground, this configuration creates a dc current path

that results in a slightly decreased high output voltage

swing and an increase in power dissipation of the

THS7375. While the THS7375 was designed to

operate with a junction temperature of up to +125Â°C,

care must be taken to ensure that the junction

temperature does not exceed this level; otherwise,

long-term reliability could suffer. Although this

configuration only adds less than 10 mW of power

dissipation per channel, the overall low-power

dissipation of the THS7375 design minimizes

potential thermal issues even when using the TSSOP

package at high ambient temperatures.

Note that the THS7375 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

outputâtypically 75-â¦. This termination helps isolate

capacitive loading effects from the THS7375 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 THS7375 output

pins should be kept below 22-pF. The best way to

ensure this limit is maintained is to place the 75-â¦

series output resistor as close as possible to the

output pin. If an output capacitor is used, as

discussed in the next section, then it should be

placed after the resistor.

+1.8 V

CVBS

Y'/G'

DaVinci

DMxxxx

P'B/B'

P'R/R'

THS7375

1 CH1 IN CH1 OUT 14

2 CH2 IN CH2 OUT 13

3 CH3 IN CH3 OUT 12

4 CH4 IN CH4 OUT 11

5 GND

VS+ 10

6 DISABLE BYPASS 9

7 NC

NC 8

CVBS/Sync

75 W

Y'/G' Out

75 W

P'B/B' Out

75 W

To GPIO Controller

or GND

+3 V to +5 V

P'R/R' Out

75 W

Figure 50. Typical SDTV System with DC-Coupled Line Driving

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