LM2437 National Semiconductor Corporation, LM2437 Datasheet - Page 4

• Manufacturer Part Number: LM2437
• Description: LM2437 - Monolithic Triple 7.5 NS CRT Driver, Package: to 220, Pin Nb=9
• Manufacturer: National Semiconductor Corporation

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Theory of Operation
The LM2437 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2437 operates with 80V and 12V power supplies. The
part is housed in the industry standard 9-lead TO-220
molded plastic power package.
The circuit diagram of the LM2437 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 −14. Emitter followers Q3 and Q4 iso-
late the high output impedance of the cascode stage from
the capacitance of the CRT cathode which decreases the
sensitivity of the device to load capacitance. Q6 provides bi-
asing to the output emitter follower stage to reduce cross-
over distortion at low signal levels.
Figure 2 shows a typical test circuit for evaluation of the
LM2437. This circuit is designed to allow testing of the
LM2437 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 follow-
ing information is provided in order to support this commit-
ment. 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 pur-
poses. When working with high bandwidth circuits, good lay-
out practices are also critical to achieving maximum perfor-
mance.
IMPORTANT INFORMATION
The LM2437 performance is targeted for the XGA (1024 x
768, 85 Hz refresh) resolution market. The application cir-
cuits shown in this document to optimize performance and to
protect against damage from CRT arcover are designed spe-
cifically for the LM2437. If another member of the LM243X
family is used, please refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2437 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 con-
nected from the supply pins, V
close to the LM2437 as is practical. Additionally, a 47 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2437.
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 volt-
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
4
age, but to a value that is much higher than allowable on the
LM2437. This fast, high voltage, high energy pulse can dam-
age the LM2437 output stage. The application circuit shown
in Figure 9 is designed to help clamp the voltage at the out-
put of the LM2437 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 capacitance.
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
spectively. 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 LM2437 ground. This will signifi-
cantly reduce the high frequency voltage transients that the
LM2437 would be subjected to during an arcover condition.
Resistor R2 limits the arcover current that is seen by the di-
odes while R1 limits the current into the LM2437 as well as
the voltage stress at the outputs of the device. R2 should be
a
carbon film type resistor. Having large value resistors for R1
and R2 would be desirable, but this has the effect of increas-
ing rise and fall times. Inductor L1 is critical to reduce the ini-
tial high frequency voltage levels that the LM2437 would be
subjected to. The inductor will not only help protect the de-
vice but it will also help minimize rise and fall times as well as
minimize EMI. For proper arc protection, it is important to not
omit any of the arc protection components shown in Figure
9 .
1
⁄
2
W solid carbon type resistor. R1 can be a
CC
and ground re-
1
⁄
4
W metal or