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參數(shù)資料
| 型號： | LM2476 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Monolithic Triple Channel 6.5 ns High Gain CRT Driver and Bias Clamp |
| 中文描述： | 單片6.5納秒三通道CRT顯示器驅(qū)動高增益和偏置夾 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 1235K |
| 代理商： | LM2476 |
Application Hints
(Continued)
will be increased. This increase in package stress will result
in reduced contact area with the heat sink increasing die
operating temperature and possible catastrophic IC failure.
TYPICAL APPLICATION
A typical application of the LM2476 is shown in
Figure 14
.
Used in conjunction with an LM123X/4X preamplfier, a com-
plete video channel from monitor input to CRT cathode can
be achieved. Performance is ideal for 1024 x 768 resolution
displays with pixel clock frequencies up to 95 MHz. Please
see the next two sections below for hints on how to properly
evaluate the LM123X/4X and LM2476 combination in a
monitor.
Figure 13
shows the typical cathode response for
this application. The peaking component values used are
shown in
Figure 12
and
Figure 14
.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, use single-point grounds sys-
tems with adequate ground planes, isolate between chan-
nels, apply good supply bypassing, and minimize unwanted
feedback and parasitic capacitance. Also, the length of the
video signal traces from the preamplifier to the LM2476 and
from the LM2476 to the CRT cathodes should be as short as
possible. The following references are recommended:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease,
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a monitor if feedback occurs around the video
channel through the chassis wiring. To prevent this, leads to
the video amplifier input circuit should be shielded, and input
circuit wiring should be spaced as far as possible from output
circuit wiring.
Robert
A.,
“Troubleshooting
Analog
Circuits”,
NSC DEMONSTRATION BOARD
Figure 14
is the schematic for the NSC LM1276/3X/4X_2476
Demonstration PCB that can be used to evaluate the
LM1276, LM123X, or LM124X preamp with the LM2476 in a
monitor.
Figure 15
shows the routing and component place-
ment on the NSC Demonstration PCB. This board provides a
good example of a layout that can be used as a guide for
future layouts. Note the location of the following compo-
nents:
C26—V
bypass capacitor, located very close to Pin 14
and GND Pins
C27—V
BB
bypass capacitors, located close to Pin 2 and
GND Pins
C28, C30, C33—V
bypass capacitors, near LM2476
and V
clamp diodes. These are very important for arc
protection.
The routing of the LM2476 driver outputs to the CRT is very
critical to achieving optimum performance.
Figure 16
shows
the routing and component placement from Pin 17 of the
LM2476 to the Red cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2476 to the Red cathode pin of the CRT connector. This
is done to minimize the length of the video path between
these two components for optimal performance and to mini-
mize EMI. Note also that L4, D9, D13 and R23, R19, D10 are
placed to minimize the size of the video nodes that they are
attached to. This minimizes parasitic capacitance in the
video path and also enhances the effectiveness of the pro-
tection diodes. The anode of protection diode D13 is con-
nected directly to a section of the ground trace that has a
short and direct path to CRT ground to shunt arcover current
away from the LM2476. The cathode of D9 is connected to
V
very close to decoupling capacitor C30 (see
Figure 16
)
which is connected to the same section of the ground trace
as D13. The diode placement and routing is very important
for minimizing the voltage stress on the LM2476 during an
arcover event. R37 and C29 are placed very close to the
clamp output on Pin 11. These components will help limit the
current and voltage stress to the clamp output. Lastly, notice
that S2 is placed very close to the Red cathode and is
connected directly to CRT ground.
20121916
40V
P-P
(35V – 75V)
FIGURE 13. Typical Cathode Response
L
www.national.com
8
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