Application information TSH343
3.1 Using the TSH343 to drive Y-Pb-Pr video components
Figure 25.
Shapes of video signals coming from DACs
White (100 IRE)
1.030V
300mV
GND
10mV
27ns
(2t)
54ns
(4t)
590ns
(44t)
27ns
(2t)
590ns
(44t)
Synchronization tip
300mV 700mV
14.8
µ s (1100t): 1920*1080i
24.3
µ s (1800t): 1280*720i
Black (30 IRE)
0.330V
(0 IRE)
0.030V
time
•Fclock=74.25MHz
•t=1/Fclock=13.5ns
Amplitude
1Vp-p
Frequency
30MHz
Figure 26.
TSH343 in single supply for any DAC output video outputs
Y,G
(+synchro)
DAC
DAC
Pb,B
DAC
Pr,R
+5V
TSH343
SO8
75 Ω
75
Ω
75
Ω
LPF
LPF
LPF
Cable
Cable
Cable
HDTV video outputs
DAC
R
DAC
G
DAC
B
+5V
75 Ω
LPF
TSH344
SO8
75 Ω
75 Ω
LPF
LPF
Digital synchro
Cable
Cable
Cable
1.
See the TSH344 datasheet on st.com for more information. It is possible to drive RGB signals with the
TSH344.
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TSH343 Application information
Figure 27.
Detailed view of one TSH343 channel
+5V
STB TV
DAC
140 Ω
600mV
DC
+ 1/3 TSH343
(gain=2)
75 Ω
470nH
68pF 68pF video line
5Volt
-5V
0V
3,22V
5Volt
1,01V
10mV
0Volt
1,22V (10mV+600mV)*gain
0Volt
75 Ω
5Volt
1,61V
610mV
0Volt
Because of the shape of the signal shown in Figure 25
, we use a very low output rail triple high-speed buffer. The TSH343 supplied in 5V single power supply, features a low output rail of 40mV on 150-ohm load. The TSH343 is used to drive high definition video signals up to
30MHz on 75-ohm video lines. It is dedicated to driving YPbPr signals where the synchronization tip—close to zero volt—is included in the Y signal.
Figure 27
shows a solution used on the STMicroelectronics reference design of STi7100 or
STi7200 where the DAC output is loaded by 140
Ω
and the bottom of the synchronization tip is set at 10mV. Using the TSH343, an internal input DC value of 600mV is added to the video signal in order to shift the bottom from 10mV to 610mV. The shift is not based on the average of the signal, but is an analog summation of a DC component to the video signal.
Therefore, no input capacitors are required which provides a real advantage in terms of cost and board space.
The internal gain of 2 obtained makes it possible to remove two resistors on the BOM. To avoid any perturbation on matching from the DACs output impedance along a large band of
30MHz in HD, a discrete reconstruction filtering is implemented after the driver. This filter is matched on 75-ohms. Note that the TSH343 cannot be AC output coupled (it cannot sink an output current, therefore it is not possible to implement an output series capacitor).
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