lm2468ta National Semiconductor Corporation, lm2468ta Datasheet - Page 4

• Manufacturer Part Number: lm2468ta
• Description: Monolothic Triple 14ns High Gain Crt Driver
• Manufacturer: National Semiconductor Corporation

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THEORY OF OPERATION
The LM2468 is a high voltage monolithic three channel CRT
driver suitable for color monitor applications. The LM2468
operates with 80V and 12V power supplies. The part is
housed in the industry standard 9-lead TO-220 molded plas-
tic power package.
The circuit diagram of the LM2468 is shown in Figure 1 . The
PNP emitter follower, Q5, provides input buffering. Q1 and
Q2 form a fixed gain cascode amplifier with resistors R1 and
R2 setting the gain at -20. Emitter followers Q3 and Q4
isolate the high output impedance of the amplifier from the
capacitive load on the output of the amplifier, decreasing the
sensitivity of the device to changes in load capacitance. Q6
provides biasing to the output emitter follower stage to re-
duce crossover distortion at low signal levels.
Figure 2 gives the pinout of the LM2468.
Figure 3 shows a typical test circuit for evaluation of the
LM2468. This circuit is designed to allow testing of the
LM2468 in a 50
sive FET probe. In this test circuit, two low inductance resis-
tors in series totalling 4.95K
capacitance probe when connected to a 50
50
is AC coupled to the base of Q5.
APPLICATION HINTS
INTRODUCTION
National Semiconductor (NSC) is committed to provide ap-
plication information that assists our customers in obtaining
the best performance possible from our products. The fol-
lowing information is provided in order to support this com-
mitment. The reader should be aware that the optimization of
performance was done using a specific printed circuit board
designed at NSC. Variations in performance can be realized
due to physical changes in the printed circuit board and the
application. Therefore, the designer should know that com-
ponent value changes might be required in order to optimize
performance in a given application. The values shown in this
document can be used as a starting point for evaluation
purposes. When working with high bandwidth circuits, good
layout practices are always critical to achieving maximum
performance.
IMPORTANT INFORMATION
The LM2468 performance is targeted for the VGA (640 x
480) to XGA (1024 x 768, 60Hz refresh) resolution market. It
is designed to be a replacement for discrete CRT drivers.
The application circuits shown in this document to optimize
performance and to protect against damage from CRT arc-
over are designed specifically for the LM2468. If another
member of the LM246X family is used, please refer to its
datasheet.
POWER SUPPLY BYPASS
Since the LM2468 is a wide bandwith amplifier, proper power
supply bypassing is critical for optimum performance. Im-
proper power supply bypassing can result in large over-
shoot, ringing or oscillation. A 0.1uF capacitor should be
connected from the supply pin, V
the supply and ground pins as is practical. Additionally, a
10uF to 100uF electrolytic capacitor should be connected
from the supply pin to ground. The electrolytic capacitor
should also be placed reasonably close to the LM2468’s
oscilloscope input). The input signal from the generator
environment without the use of an expen-
form a 200:1 wideband, low
CC
, to ground, as close to
load (such as
4
supply and ground pins. A 0.1uF capacitor should be con-
nected from the bias pin (V
practical to the part.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 200V, 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 LM2468. This fast, high voltage, high energy pulse can
damage the LM2468 output stage. The application circuit
shown in Figure 10 is designed to help clamp the voltage at
the output of the LM2468 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. FDH400 or equivalent diodes are recommended. Do
not use 1N4148 diodes for the clamp diodes. D1 and D2
should have short, low impedance connections to V
ground respectively. The cathode of D1 should be located
very close to a separately decoupled bypass capacitor (C3 in
Figure 10 ). The ground connection of D2 and the decoupling
capacitor should be very close to the LM2468 ground. This
will significantly reduce the high frequency voltage transients
that the LM2468 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 LM2468 as
well as the voltage stress at the outputs of the device. R2
should be a 1/2W solid carbon type resistor. R1 can be a
1/4W metal or carbon film type resistor. Having large value
resistors for R1 and R2 would be desirable, but has the
effect of increasing rise and fall times. Inductor L1 is critical
to reduce the initial high frequency voltage levels that the
LM2468 would be subjected to. The inductor will not only
help protect the device, but will also help optimize rise and
fall times as well as minimize EMI. For proper arc protection,
it is important to not omit any of the arc protection compo-
nents shown in Figure 10 .
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 10 , 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. Up to a point, increasing the value of L1 will speed up
the circuit as well as increase overshoot. It is very important
to use inductors with very high self-resonant frequencies,
preferably above 300 MHz. Ferrite core inductors from J.W.
Miller Magnetics (part # 78FR39K) were used for optimizing
the performance of the device in the NSC application board.
The values shown in Figure 10 can be used as a good
starting point for the evaluation of the LM2468. If needed,
additional damping can be provided by adding a resistor in
parallel with L1 to help control overshoot. Using variable
FIGURE 10. One Video Channel of the LM2468 with the
Recommended Arc Protection Circuit
bb
) to the ground, as close as is
DS200135-10
cc
and