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參數(shù)資料
| 型號(hào)： | LM2451 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 220V Monolithic Triple Channel 12 MHz DC Coupled CRT DTV Driver |
| 中文描述： | 220單片三通道12 MHz的直流耦合顯像管電視驅(qū)動(dòng) |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大小： | 1101K |
| 代理商： | LM2451 |
Application Hints
(Continued)
THERMAL CONSIDERATIONS
Figure 9
shows the performance of the LM2451 in the test
circuit shown in
Figure 3
as a function of case temperature.
The figure shows that the rise time of the LM2451 increases
by about 7ns as the case temperature increases from 30C
to 110C. Over the same case temperature range the fall
time increased by about 9 ns.
Figure 10
shows the maximum power dissipation of the
LM2451 vs. Frequency when all three channels of the device
are driving into a 10 pF load with a 130V
P-P
alternating one
pixel on, one pixel off. Note that the frequency given in
Figure 10
is half of the pixel frequency. The graph assumes
an 80% active time (device operating at the specified fre-
quency), which is typical in a TV application. The other 20%
of the time the device is assumed to be sitting at the black
level (190V in this case).ATV picture will not have frequency
content over the whole picture exceeding 15 MHz. It is
important to establish the worst case condition under normal
viewing to give a realistic worst-case power dissipation for
the LM2451. One test is a 1 to 30 MHz sine wave sweep
over the active line. This would give a slightly lower power
than taking the average of the power between 1 and 30 MHz.
This average is 10.4 W. A sine wave will dissipate slightly
less power, probably about an even 10W of power dissipa-
tion. All of this information is critical for the designer to
establish the heat sink requirement for his application. The
designer should note that if the load capacitance is in-
creased the AC component of the total power dissipation will
also increase.
The LM2451 case temperature must be maintained below
110C given the maximum power dissipation estimate of
10W. If the maximum expected ambient temperature is 60C
and the maximum power dissipation is 10W then a maximum
heat sink thermal resistance can be calculated:
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if the load
capacitance is increased the AC component of the total
power dissipation will also increase. If the worst case power
dissipation is not 10W please refer to
Figure 11
for the
maximum case temperature for the expected maximum
power dissipation.
OPTIMIZING TRANSIENT RESPONSE
Referring to
Figure 13
, there are three components (R1, R2
and L1) that can be adjusted to optimize the transient re-
sponse of the application circuit. Increasing the values of R1
and R2 will slow the circuit down while decreasing over-
shoot. Increasing the value of L1 will speed up the circuit as
well as increase overshoot. It is very important to use induc-
tors with very high self-resonant frequencies, preferably
above 300 MHz. Ferrite core inductors from J.W. Miller
Magnetics (part # 78FR--K) were used for optimizing the
performance of the device in the NSC application board. The
values shown in
Figure 13
can be used as a good starting
point for the evaluation of the LM2451. Using a variable
resistor for R1 will simplify finding the value needed for
optimum performance in a given application. Once the opti-
mum value is determined the variable resistor can be re-
placed with a fixed value. Due to arc over considerations it is
recommended that the values shown in
Figure 13
not be
changed by a large amount.
Figure 12
shows the typical cathode pulse response with an
output swing of 130V
inside a modified production TV set
using the LM1237 pre-amp.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane, iso-
lation between channels, good supply bypassing and mini-
mizing unwanted feedback are necessary.Also, the length of
the signal traces from the signal inputs to the LM2451 and
from the LM2451 to the CRT cathode 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,
Robert
A.,
“Troubleshooting
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a TV 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.
Analog
Circuits”,
TYPICAL APPLICATION
A typical application of the LM2451 is shown in
Figure 14
.
Used in conjunction with a pre-amp with a 1.2V black level
output no buffer transistors are required to obtain the correct
black level at the cathodes. If the pre-amp has a black level
closer to 2V, then an NPN transistor should be used to drop
the video black level voltage closer to 1.2V. When using only
one NPN transistor as an emitter follower, a jumper needs to
be added in each channel. In the red channel a jumper
needs to be added between C7 and R25. With just one
transistor neither of these components would be installed.
In addition to the video inputs are the DAC inputs. These
inputs are used to vary the LM2451 output black level by a
DAC. in the past when a driver was used with a CMOS AVP
there was not enough range on the video output to vary the
black level.Aclamp circuit had to be used in conjunction with
the AVP and the driver. The DAC inputs of the LM2451 are
driven in the same way the clamp circuit had been driven,
eliminating the need for a clamp circuit.
Figure 4
shows the
variation in the black level as the DAC input voltage is
changed. This is shown for both V
IN
= 1.2V and V
IN
= 2.1V.
The neck board in
Figure 14
has two transistors in each
channel enabling this board to work with pre-amps with a
black level output as high as 2.5V. Each transistor stage has
a gain of 1. This setup still gives the two diode drop at the
driver input; however, now additional peaking can be done
on the video signal before reaching the driver inputs. Some
popular AVPs do have a black level of 2.5V. For lower black
levels either one or both transistors would not be used.
It is important that the TV designer use component values for
the driver output stage close to the values shown in
Figure
14
. These values have been selected to protect the LM2451
from arc over. Diodes D1,D8, D9, and D13–D15 must also
be used for proper arc over protection. The NSC demonstra-
tion board can be used to evaluate the LM2451 in a TV.
L
www.national.com
7
相關(guān)PDF資料 |
PDF描述 |
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| LM2451TB | 220V Monolithic Triple Channel 12 MHz DC Coupled CRT DTV Driver |
| LM2481NA | 145V Triple Bias Clamp |
| LM25005 | 42V, 2.5A STEP-DOWN SWITCHING REGULATOR |
| LM25005MH | 42V, 2.5A STEP-DOWN SWITCHING REGULATOR |
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2451TB | 制造商:Rochester Electronics LLC 功能描述: 制造商:Texas Instruments 功能描述: |
| LM2451TB/NOPB | 功能描述:顯示驅(qū)動(dòng)器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel |
| LM2452 | 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:220V Monolithic Triple Channel 17 MHz DC Coupled CRT DTV Driver |
| LM2452TB | 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:220V Monolithic Triple Channel 17 MHz DC Coupled CRT DTV Driver |
| LM2452TB/NOPB | 功能描述:顯示驅(qū)動(dòng)器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel |
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