lm2462ta National Semiconductor Corporation, lm2462ta Datasheet - Page 5

• Manufacturer Part Number: lm2462ta
• Description: Monolithic Triple 3 Ns Crt Driver
• Manufacturer: National Semiconductor Corporation

Theory of Operation
The LM2462 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2462 operates with 80V and 12V power supplies. The
part is housed in the industry standard 11-lead TO-220
molded plastic power package.
The circuit diagram of the LM2462 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 cascode stage from
the capacitance of the CRT cathode which decreases 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
LM2462. This circuit is designed to allow testing of the
LM2462 in a 50
sive FET probe. In this test circuit, the two 2.49k
form a 200:1 wideband, low capacitance probe when con-
nected 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 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 LM2462 performance is targeted for the high end 19“
and 21“ market with resolutions up to 1600 x 1200 and an
85Hz refresh rate. The application circuits shown in this
document were specifically designed to optimize the perfor-
mance of the LM2462 as well as protect it from damage due
to a CRT arc-over. If another member of the LM246X family
is used, please refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2462 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. 0.1 µF capacitors should be
connected from the supply pins, V
close to the LM2462 as is practical. Additionally, a 47 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2462.
oscilloscope input). The input signal from the generator
coaxial cable and a 50
environment without the use of an expen-
CC
and V
BB
load (such as a
, to ground, as
resistors
5
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 LM2462. This fast, high voltage, high energy pulse can
damage the LM2462 output stage. The application circuit
shown in Figure 9 is designed to help clamp the voltage at
the output of the LM2462 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 LM2462 ground. This
will significantly reduce the high frequency voltage transients
that the LM2462 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 LM2462 as
well as the voltage stress at the outputs of the device. R2
should be a
metal or carbon film type resistor. Having large value resis-
tors 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
LM2462 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 .
1
⁄
2
W solid carbon type resistor. R1 can be a
www.national.com
CC
1
and
⁄
4
W