LMH1981MT/NOPB National Semiconductor, LMH1981MT/NOPB Datasheet - Page 14

• Manufacturer Part Number: LMH1981MT/NOPB
• Description: IC VIDEO SYNC SEPARATOR 14-TSSOP
• Manufacturer: National Semiconductor
• Type: Video Syncr

Specifications of LMH1981MT/NOPB

• Applications: SDTV/HDTV
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• Supply Voltage (max): 5 V
• Supply Voltage (min): 3.3 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• For Use With: LMH1981EVAL - BOARD EVALUATION FOR LMH1981
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMH1981MT

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OPTIONAL CONSIDERATIONS
Optional Input Filtering
An external filter may be necessary if the video signal has
considerable high-frequency noise or has large chroma am-
plitude that extends near the sync tip. A simple RC low-pass
filter with a series resistor (R
can be used to improve the overall signal-to-noise ratio and
sufficiently attenuate chroma such that minimum peak of its
amplitude is above the 50% sync slice level. To achieve the
desired filter cutoff frequency, it’s advised to vary C
R
voltage drop across R
increase the propagation delay from the input to the outputs.
In applications where the chroma filter needs to be disabled
when non-composite video (ie: ED/HD video) is input, it is
possible to use a transistor to switch open C
ground as shown in Figure 18. This transistor can be switched
off/on by logic circuitry to decode the lines-per-field data out-
put (VFOUT). As shown in Table 1, NTSC and PAL both have
1 (logic high) for the 3rd bit of VFOUT. If the logic circuitry
detects 0 (logic low) for this bit, indicating non-composite
video, the transistor can be turned off to disable the chroma
filter.
AC-Coupled Video Sources
An AC coupled video source typically has a 100 µF or larger
output coupling capacitor (C
the DC bias of the amplifier output from the video signal.
When the video source is load terminated, the average value
of the video signal will shift dynamically as the video duty cycle
varies due to the averaging effect of the C
resistors. The average picture level or APL of the video con-
tent is closely related to the duty cycle.
For example, a significant decrease in APL such as a white-
to-black field transition will cause a positive-going shift in the
sync tips characterized by the source’s RC time constant, t
OUT
have difficulty stabilizing the input signal under this type of
shifting; consequently, the unstable signal at V
missing sync output pulses to result, unless a proper value
for C
To avoid this potential problem when interfacing AC-coupled
sources to the LMH1981, it’s necessary to introduce a voltage
droop component via C
ing related to changes in the APL. This can be accomplished
by selecting C
LMH1981’s input circuit, t
The effective time constant of the input circuit can be approx-
imated
R
FIGURE 18. External Chroma Filter with Control Circuit
S
S
small (ie. 100Ω) to minimize sync tip clipping due to the
= 150Ω, R
(150Ω*C
IN
is chosen.
as:
OUT
I
IN
t
RC-IN
= 4000Ω (input resistance), T
). The LMH1981’s input clamp circuitry may
such that the effective time constant of the
S
=
IN
. Note that using an external filter will
to compensate for video signal shift-
RC-IN
(R
OUT
S
S
) and a capacitor (C
+R
, is less than t
) for protection and to remove
I
)*C
IN
*T
LINE
OUT
/T
F
RC-OUT
’s connection to
LINE
and termination
CLAMP
IN
F
∼
may cause
) to ground
F
.
,
and keep
64 μs for
20174518
where
RC-
14
NTSC, and T
white-to-black field transition in NTSC video through C
exhibit the maximum sync tip shifting due to its long line period
(T
C
worst-case condition.
For instance, t
ensure t
ing C
coupled video sources using C
PCB LAYOUT CONSIDERATIONS
LMH1981 IC Placement
The LMH1981 should be placed such that critical signal paths
are short and direct to minimize PCB parasitics from degrad-
ing the high-speed video input and logic output signals.
Ground Plane
A two-layer, FR-4 PCB is sufficient for this device. One of the
PCB layers should be dedicated to a single, solid ground
plane that runs underneath the device and connects the de-
vice GND pins together. The ground plane should be used to
connect other components and serve as the common ground
reference. It also helps to reduce trace inductances and min-
imize ground loops. Try to route supply and signal traces on
another layer to maintain as much ground plane continuity as
possible.
Power Supply Pins
The power supply pins should be connected together using
short traces with minimal inductance. When routing the sup-
ply traces, be careful not to disrupt the solid ground plane.
For high frequency bypassing, place 0.1 µF SMD ceramic by-
pass capacitors with very short connections to power supply
and GND pins. Two or three ceramic bypass capacitors can
be used depending on how the supply pins are connected
together. Place a 4.7 µF SMD tantalum bypass capacitor
nearby all three power supply pins for low frequency supply
bypassing.
R
The R
sistor. Place R
connect to pin 1 and the ground plane using the shortest pos-
sible connections. All input and output signals must be kept
away from this pin to prevent unwanted signals from coupling
into this pin.
Video Input
The input signal path should be routed using short, direct
traces between video source and input pin. Use a 75Ω input
termination and a SMD capacitor for AC coupling the video
input to pin 4.
Output Routing
The output signal paths should be routed using short, direct
traces to minimize parasitic effects that may degrade these
high-speed logic signals. All output signals should have a re-
sistive load of about 10 kΩ and capacitive load of less than
10 pF, including parasitic capacitances for optimal signal
quality. This is especially important for the horizontal sync
output, in which it is critical to minimize timing jitter. Each out-
put can be protected by current limiting with a small series
resistor, like 100Ω.
IN
EXT
LINE
can be calculated to ensure proper operation under this
IN
Resistor
). By setting t
EXT
= 0.01 μF, the LMH1981 will function properly with AC-
RC-IN
resistor should be a 10 kΩ 1% SMD precision re-
< 33 ms, C
CLAMP
RC-OUT
EXT
as close as possible to the device and
RC-IN
= 250 ns (internal clamp duration). A
is about 33 ms for C
IN
< t
must be less than 31 nF. By choos-
RC-OUT
OUT
≥
, the maximum value of
220 μF.
OUT
= 220 µF. To
OUT
will