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參數(shù)資料
| 型號(hào)： | LM2432TE |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 220V Monolithic Single Channel 37 MHz HDTV CRT Driver |
| 中文描述： | 1 CHANNEL, VIDEO AMPLIFIER, PZFM7 |
| 封裝： | PLASTIC, POWER PACK, TO-220, 7 PIN |
| 文件頁數(shù)： | 8/13頁 |
| 文件大?。?/td> | 859K |
| 代理商： | LM2432TE |
Application Hints
(Continued)
Tips for Reducing Power Dissipation
The following methods can be used to reduce the power
dissipated by the LM2432 in order to optimize heatsink size
and cost:
Use a lower V
CC
supply voltage while maintaining suffi-
cient operating range for cutoff, brightness, and drive
adjustments.
Reduce the input bandwidth to the LM2432 while main-
taining acceptable picture performance.
Lower the maximum V
swing while maintaining accept-
able picture contrast and brightness.
Minimize capacitive load on the LM2432 output by using
good PCB layout practices.
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 16
can be used as a good starting
point for the evaluation of the LM2432. 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.
Figure 12
shows a typical cathode pulse response with an
output swing of 110V
using a RGB video processor that
provides input speeds with 12 ns rise and fall times.
Note:
The RGB processor’s sharpness feature adds emphasis
(preshoots and overshoots) to the rising and falling edges of
the input pulse, which consequently adds emphasis to the
cathode pulse response.
CATHODE CURRENT OUTPUT FOR IK FEEDBACK
SYSTEMS
IK Feedback Systems
IK feedback was developed to accurately bias the CRT and
continuously calibrate it to the correct cut-off and/or drive
levels over the useful life of the CRT. RGB video processors
that use IK feedback to automatically adjust only cut-off, or
black level, are realized by a 1-point calibration system. A
few trade names for this system are Auto Kine Bias (AKB)
and Black Current Stabilization (BCS). RGB processors that
can automatically adjust both cut-off and drive, or white
level, are realized by a 2-point calibration system. This is
commonly known as Continuous Cathode Calibration
(CCC). For convenience, some 2-point RGB processors may
be programmed to 1-point operation if drive calibration is not
required. The LM2432 is compatible with both 1- and 2-point
systems.
To be compatible with various RGB processors, an interface
circuit may be needed in the feedback path between the
LM2432 IK output and the processor’s IK input. This feed-
back circuit depends on the RGB processor and feedback
topology (voltage or current) used. Because each processor
has its own IK input signal and topology requirements, it is
outside the scope of this data sheet to describe each feed-
back circuit in detail. For more information, please refer to
the RGB processor data sheet or contact your local National
Semiconductor Sales Office with your specific application
requirements.
Feedback Topologies
RGB processors that use
voltage feedback
require the
LM2432 IK current to be converted to voltage via a resistor
(R
IK
) to ground. This IK voltage, V
IK
, will be fed back to the IK
input of the RGB processor through an interface circuit,
which will be AC or DC coupled depending on the proces-
sor’s IK input requirement. For proper feedback operation,
some processors may require an emitter follower to isolate
the IK input from the high impedance of the resistor. During
the closed-loop IK measurement interval, the IK input volt-
age will be sampled and compared with the processor’s
internal reference voltage to automatically calibrate the video
levels for the next field. The value of R
is crucial, since it
establishes the IK voltage and consequently, the operating
point of the CRT. Once a stable operating point is estab-
lished with a properly chosen resistor, this point can be
fine-tuned using the adjustment range of the processor’s
RGB cut-off and/or gain controls via the I
2
C-bus. After the IK
measurement interval (usually at the end of blanking), nor-
mal video will resume and high currents will flow out of the IK
output. These high video currents will produce large IK volt-
ages across the resistor that can exceed the maximum
voltage rating for V
IK
. Therefore, it is recommended to use a
high-speed diode (D
) to clamp the LM2432 IK output to
a safe level (preferably V
or a lower supply). If a zener
diode is used instead, it may be necessary for the RGB
processor to have IK leakage compensation for the leakage
current attributed to the zener. Lastly, it is possible to use a
single R
IK
resistor to set the IK voltage for all three LM2432s.
See a simplified voltage feedback interface circuit in
Figure
14
.
RGB processors that use
current feedback
do not require
voltage conversion. The LM2432 IK current can be fed back
directly to the IK input of the RGB processor, although some
protection circuitry will be needed to protect the RGB pro-
cessor and LM2432. During the closed-loop IK measure-
ment interval, the voltage of the RGB processor’s IK pin will
be internally clamped, and the IK current will be sampled and
compared with the processor’s internal reference current to
calibrate the video levels. The operating point of the CRT
can be fine-tuned using the adjustment range of the proces-
20137724
FIGURE 14. Simplified IK Interface for Voltage
Feedback Systems
L
www.national.com
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