LM2460 National Semiconductor Corporation, LM2460 Datasheet - Page 5

• Manufacturer Part Number: LM2460
• Description: Monolithic Triple Channel High Swing CRT Driver
• Manufacturer: National Semiconductor Corporation

Theory of Operation
The LM2460 is a high voltage monolithic three channel CRT
driver with a higher output swing suitable for driving the new
high brightness CRTs. The LM2460 operates with 120V and
8V power supplies. The part is housed in the industry stan-
dard 9-lead TO-220 molded plastic power package.
The circuit diagram of the LM2460 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 −32. 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 shows a typical test circuit for evaluation of the
LM2460. This circuit is designed to allow testing of the
LM2460 in a 50Ω environment without the use of an expen-
sive FET probe. In this test circuit, two low inductance resis-
tors in series totaling 4.95 kΩ form a 200:1 wideband, low
capacitance probe when connected to a 50Ω coaxial cable
and a 50Ω load (such as a 50Ω oscilloscope input). The
input signal from the generator is AC coupled to the base of
Q5. V
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 may 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 also critical to achieving maximum per-
formance.
IMPORTANT INFORMATION
The LM2460 performance is targeted for the 15" and 17"
market with resolutions up to 1024 x 7684 and 75 Hz refresh
rate. 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 LM2460. If
another member of the LM246X family is used, please refer
to its datasheet.
POWER SUPPLY BYPASS
Since the LM2460 is a wide bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing or oscillation. A 0.1 µF capacitor should be
connected from the supply pin, V
the supply and ground pins as is practical. Additionally, a
22 µF to 100 µF electrolytic capacitor should be connected
from the supply pin to ground. The electrolytic capacitor
should also be placed reasonably close to the LM2460’s
BIAS
is used to adjust the DC level of the output.
CC
, to ground, as close to
5
supply and ground pins. A 0.1 µF 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 LM2460. This fast, high voltage, high energy pulse can
damage the LM2460 output stage. The application circuit
shown in Figure 9 is designed to help clamp the voltage at
the output of the LM2460 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 9). The ground connection of D2 and the decoupling
capacitor should be very close to the LM2460 ground. This
will significantly reduce the high frequency voltage transients
that the LM2460 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 LM2460 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 this has the
effect of increasing rise and fall times. Inductor L1 is critical
to reduce the initial high frequency voltage levels that the
LM2460 would be subjected to. The inductor will not only
help protect the device but it 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 9.
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 9, 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, perferably
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 9 can be used as a good starting
point for the evaluation of the LM2460. Using a variable
FIGURE 9. One Channel of the LM2460 with the
Recommended Application Circuit
BB
) to ground, as close as is
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CC
20082609
and