Application Hints - NEC MultiSync FE700 Serviceanleitung

to the output emitter follower stage to reduce crossover distortion at low signal levels. Figure 2 shows a
typical test circuit for evaluation of the LM2439. This circuit is designed to allow testing of the LM2439 in a
50Ω environment without the use of an expensive FET probe. In this test circuit, two low inductance resistors
in series totaling 4.95 κΩ form a 100:1 wide band, low capacitance probe when connected to a 50Ω coaxial
cable and a 50Wload (such as a 50Ω oscilloscope input). The input signal from the generator is ac coupled
to the base of Q5.

Application Hints

INTRODUCTION
National Semiconductor (NSC) is committed to provide application information that assists our customers in
obtaining the best performance possible from our products. The following information is provided in order to
support this commitment. 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
component 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 performance.
IMPORTANT INFORMATION
The LM2439 performance is targeted for the VGA (640 x 480) to XGA (1024 x 768, 70 Hz 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 arcover are designed
specifically for the LM2439. If another member of the LM243X family is used, please refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2439 is a wide bandwidth amplifier, proper power supply bypassing is critical for optimum
performance. Improper power supply bypassing can result in large overshoot, ringing or oscillation. A 0.01
µF capacitor should be connected from the supply pin, V , to ground, as close to the supply and ground
CC
pins as is practical. Additionally, a 10 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 LM2439's
supply and ground pins. A 0.1 µF capacitor should be connected from the bias pin, V , to ground, as close
BB
as is practical to the part.
ARC PROTECTION
During normal CRT operation, internal arcing may occasionally 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 LM2439. This fast, high voltage, high energy pulse can damage the LM2439
output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the output of
the LM2439 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 or equivalent diodes for the clamp diodes. D1 and D2 should have short,
low impedance connections to VCC and 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
de-coupling capacitor should be very close to the LM2439 ground. This will significantly reduce the high
frequency volt-age transients that the LM2439 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 LM2439 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 LM2439 would be subjected to. The inductor will not only help protect the device but it
will also help maximize 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.
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