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參數(shù)資料
| 型號： | LM2459TE/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1 CHANNEL, VIDEO AMPLIFIER, PZFM7 |
| 封裝： | PLASTIC, TO-220, 7 PIN |
| 文件頁數(shù)： | 9/11頁 |
| 文件大?。?/td> | 578K |
| 代理商： | LM2459TE/NOPB |
Application Hints (Continued)
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 300V, connected from
the CRT cathodes to CRT ground will limit the maximum
voltage, but to a value that is much higher than allowable on
the LM2459. This fast, high voltage, high energy pulse can
damage the LM2459 output stage. The application circuit
shown in Figure 13 is designed to help clamp the voltage at
the output of the LM2459 to a safe level. The clamp diodes,
D1 and D2, should have a fast transient response, high peak
current rating, low series impedance and low shunt capaci-
tance. 1SS83 or equivalent diodes are recommended. D1
and D2 should have short, low impedance connections to
V
CC and ground respectively. The cathode of D1 should be
located very close to a separately decoupled bypass capaci-
tor (C3 in Figure 13). The ground connection of D2 and the
decoupling capacitor should be very close to the LM2459
ground. This will significantly reduce the high frequency
voltage transients that the LM2459 would be subjected to
during an arcover condition. Resistor R2 limits the arcover
current that is seen by the diodes while R1 limits the current
into the LM2459 as well as the voltage stress at the outputs
of the device. R2 should be a 12W solid carbon type resistor.
R1 can be a 14W metal or carbon film type resistor. Having
large value resistors for R1 and R2 would be desirable, but
this has the effect of increasing rise and fall times. Inductor
L1 is critical to reduce the initial high frequency voltage
levels that the LM2459 would be subjected to. The inductor
will not only help protect the device but it will also help
minimize rise and fall times as well as minimize EMI. For
proper arc protection, it is important to not omit any of the arc
protection components shown in Figure 13.
EFFECT OF LOAD CAPACITANCE
Figure 7 shows the effect of increased load capacitance on
the speed of the device. This demonstrates the importance
of knowing the load capacitance in the application.
EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
output offset of the device is varied from 95 to 105V
DC. The
rise time shows a variation of less than 7% relative to the
center data point (100V
DC). The fall time shows a variation of
18% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2459 in the test
circuit shown in Figure 3 as a function of case temperature.
The figure shows that the rise and fall times of the LM2459
increase by approximately 18% and 29%, respectively, as
the case temperature increases from 50C to 90C. This
corresponds to a speed degradation of 5% and 7% for every
10C rise in case temperature.
Figure 10 shows the maximum power dissipation of the
LM2459 vs. frequency when the device is driving a 10pF
load with a 130V
PP alternating one pixel on, one pixel off
signal. The graph assumes a 77% active time (device oper-
ating at the specified frequency), which is typical in a TV
application. The other 23% of the time the device is assumed
to be sitting at the black level (165V in this case). Table 1
also shows the typical power dissipation of the LM2459 for
various video patterns in the 480i and 480p video formats.
Figure 10, Figure 11, and Table 1 give the designer the
information needed to determine the heatsink requirement
for the LM2459. For example, if an HDTV application uses
the 480p format and "Vertical Lines 1 On 1 Off" is assumed
to be the worst-case pattern to be displayed, then the power
the maximum allowed case temperature is 134C when
3.8W is dissipated. If the maximum expected ambient tem-
perature is 70C, then a maximum heatsink thermal resis-
tance can be calculated:
This example assumes a capacitive load of 10pF and no
resistive load. The designer should note that if the load
capacitance is increased, then the AC component of the total
power dissipation will also increase.
Note: An LM126X preamplifier, with rise and fall times of
about 2 ns, was used to drive the LM2459 for these power
measurements. Using a preamplifier with rise and fall times
slower than the LM126X will cause the LM2459 to dissipate
less power than shown in Table 1.
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
20067810
FIGURE 13. Recommended Application Circuit
LM2459
www.national.com
7
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相關(guān)代理商/技術(shù)參數(shù) |
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| LM246 | 制造商:STMICROELECTRONICS 制造商全稱:STMicroelectronics 功能描述:PROGRAMMABLE QUAD BIPOLAR OPERATIONAL AMPLIFIERS |
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| LM2462 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Monolithic Triple 3 ns CRT Driver |
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