LM2432TE/NOPB National Semiconductor, LM2432TE/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM2432TE/NOPB
• Manufacturer: National Semiconductor

Specifications of LM2432TE/NOPB

• Lead Free Status / RoHS Status: Compliant

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Application Hints
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:
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
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
• Use a lower V
• Reduce the input bandwidth to the LM2432 while main-
• Lower the maximum V
• Minimize capacitive load on the LM2432 output by using
cient operating range for cutoff, brightness, and drive
adjustments.
taining acceptable picture performance.
able picture contrast and brightness.
good PCB layout practices.
CC
supply voltage while maintaining suffi-
PP
using a RGB video processor that
PP
swing while maintaining accept-
(Continued)
8
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
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
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
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
high-speed diode (D
a safe level (preferably V
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
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
IK
) to ground. This IK voltage, V
FIGURE 14. Simplified IK Interface for Voltage
IK
resistor to set the IK voltage for all three LM2432s.
IK
Feedback Systems
PROT
. Therefore, it is recommended to use a
) to clamp the LM2432 IK output to
BB
or a lower supply). If a zener
IK
, will be fed back to the IK
IK
2
C-bus. After the IK
is crucial, since it
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