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| 型號(hào)： | LM1279A |
| 文件頁數(shù)： | 8/13頁 |
| 文件大?。?/td> | 511K |
| 代理商： | LM1279A |
Functional Description
(Continued)
channel has its own independent control pin with the 0V to
4V control range. An input of 4V give no attenuation, and an
input of 0V gives the full 12 dB attenuation.
The output of the drive attenuator stage goes to the inverting
input of A2. Since this is the second inversion stage, the out-
put of A2 will be the non-inverted video signal. Note that the
output of gm1 goes to the non-inverting input ofA2.Also note
that the output of A2 goes to the inverting input of gm1. This
is the feedback for the clamp circuitry. The output stage ofA2
is an exact duplicate of the video output through A3. If a
390
load impedance is used at the video output, then the
black level at the output stage will accurately track the output
ofA2. The other input to gm1 is the desired black level output
of the LM1279A. Since the LM1279A has a fixed black level
output, the non-inverting inputs to gm1 in all three channels
go to a fixed 1.35V internal reference. This sets the black
level output to a nominal 1.35V. gm1 acts like a sample and
hold amplifier. Once the clamp pulse exceeds 3.6V gm1 is
activated, driving the input of A2 to a level where the video
output will be 1.35V. For proper DC restoration it is important
that gm1 be activated only during the horizontal flyback time
when the video is at the black level. gm1 also charges the
clamp cap to the correct voltage to maintain a 1.35V black
level at the video output. When gm1 is turned off the voltage
stored on the clamp cap will maintain the correct black level
during the active video, thus restoring the DC level for a
1.35V black level.
The input ofA3 receives the output fromA2. The video chan-
nel of A3 is a duplication of the output stage to A2. As men-
tioned in the previous paragraph this is done so that the DC
restoration can be done at theA2 stage.A3 also receives the
OSD input for blanking the channels with no OSD input. By
doing DC restoration at the A2 stage, OSD can be activated
at the output stage during the time DC restoration is being
done at A2.
The OSD signal goes into a special interface circuit. The out-
put of this circuit will drive the output of A3 to either an OSD
black level or to about 2.4V above the video black level
(OSD white level). The OSD black level is about 300 mV be-
low the video black level. This guarantees that if the OSD
signal is not activated for a particular channel, then its output
will be slightly below the cutoff level. If an OSD input is re-
ceived in a particular channel, then the video output will be at
the OSD white level. The OSD mode is automatically acti-
vated if there is only one OSD signal to any of the video
channels. This OSD control circuit will allow any color, ex-
cept black, during the OSD mode. This also saves the need
for a special signal to switch into the OSD mode. Remember
that at least one OSD input must be high to enable the OSD
mode, therefore black can’t be used in the OSD window.
Applications of the LM1279A
Two demonstration boards are available to evaluate the
LM1279A. One board is the pre-amp demonstration board.
This board was used for testing and characterizing the
LM1279A. The schematic for this board is shown in Figure 4
and the printed circuit layout for this board is shown in Figure
6 The other board is a complete video channel neck board
that can be directly plugged into the CRT socket. The sche-
matic for this board is shown in Figure 9 and the printed cir-
cuit layout is shown in Figure 10 The CRT driver used on
this board is the LM2407. Any of National’s monolithic CRT
drivers can be used in this board, but the LM2407 is consid-
ered the best match to the LM1279A based on cost and per-
formance.
Some important notes on Figure 4 All three video inputs
have a 75
terminating resistor for a 75
video system. This
is the normal video impedance of the video from a computer
system. It is possible to also have a 50
system, then R1,
R3, and R5 would be changed to 50
. R2, R4, and R6 are in
series with the video inputs of the LM1279A. These three
30
resistors are required to protect the IC from any sudden
voltage surges that may result during the power up and
power down modes, or when connecting the monitor to other
equipment. The monitor designer must include these resis-
tors in his design for good monitor reliability. If additional pro-
tection against ESD at the video inputs is necessary, then
adding clamp diodes on the IC side of the 30
resistors is
recommended, one to V
and one to ground (see Figure
5). Sometimes a designer may want to increase the value of
the 30
resistors at the video inputs, for additional ESD pro-
tection. This is not recommended with the LM1279A. C5, C7,
and C9 are part of the DC restoration circuit. This circuit is
depending on a total maximum circuit resistance of about
110
; 30
input series resistor plus 75
for the video termi-
nation resistor. Increasing the value of the 30
resistors will
exceed the 110
limit. The excellent internal ESD protection
and the external clamp diodes (if needed) will provide excel-
lent ESD protection.
The 30
resistors in series with the OSD inputs are also
necessary if the OSD signals are external to the monitor, or
if these signals are present any time when the +8V is not
fully powered up. Interfacing to the OSD inputs is quite easy
since the signal processing necessary to match the OSD sig-
nals to the video levels is done internal by the LM1279A.
There is also no need for an OSD window signal. Any time
there is a high TTL signal at any of the three OSD inputs, the
LM1279A will automatically switch to the OSD mode. A high
TTL OSD signal will give a high video output for that color.
The OSD level is fixed, typically 2.3V above the video black
level. This will give a fixed brightness to the OSD window,
but not at maximum video brightness which could be un-
pleasant to the user. Figure 2 and Figure 3 show the timing
diagrams of the OSD signals for the LM1279A.
The recommended load impedance for the LM1279A is
390
. However, some changes in the load impedance can
be made. If the load impedance is reduced, the monitor de-
signer must confirm that the part is still operating in its proper
die temperature range, never exceeding a die temperature
of 150C. When changing the load impedance, the black
level shift is shown in the chart below. The measured V
P-P
output with under 1% distortion is also listed.
Load
430
390
330
270
V
P-P
3.62V
3.62V
3.58V
3.51V
Blk. Level Shift
+15 mV
0 mV
25 mV
45 mV
When using a lower load impedance, the LM1279A does go
into hard clipping more quickly. This does reduce the head-
room of the video output.
Board layout is always critical in a high frequency application
such as using the LM1279A. A poor layout can result in ring-
ing of the video waveform after sudden transitions, or the
part could actually oscillate. A good ground plane and proper
routing of the +8V are important steps to a good PCB layout.
The LM1279A does require very good coupling between
L
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