lm2481 National Semiconductor Corporation, lm2481 Datasheet - Page 3

• Manufacturer Part Number: lm2481
• Description: 145v Triple Bias Clamp
• Manufacturer: National Semiconductor Corporation

Typical Performance Characteristics
(V
Theory of Operation
The simplified circuit diagram of the LM2481 is shown in
Figure 1. The DC clamp circuit amplifies the input signal by
-20 and the gain is set by the resistor ratio of 20R and R. The
output will require a pull-up resistor to 145V.
Figure 2 shows the test circuit for evaluation of the LM2481
Clamp Amplifier. A high impedance voltmeter (
used for DC measurements at the output.
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.
POWER SUPPLY BYPASS
The LM2481 should have proper power supply bypassing for
optimum performance. A 0.1 µF capacitor should be con-
nected from the supply pins, VCC and VBB, to ground, as
close to the supply and ground pins as is practical. Addition-
ally, a 1 µF electrolytic capacitor should be connected from
the supply pins to ground. The electrolytic capacitor should
also be placed reasonably close to the LM2481’s supply and
ground pins.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. To protect the LM2481 against arcing the follow-
ing steps should be done (See Figure 7):
CC
= +145V, V
BB
FIGURE 3. V
= +8V, V
IN
= +2.5V, Test Circuit - Figure 2)
out
vs V
in
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3
Thermal Considerations
The package the LM2481 is in uses the ground pins (3, 4, 5,
10, 11, and 12) to conduct heat from the LM2481. These pins
should be connected to a ground plane that acts as a heat
sink. See the example ground plane connected to these pins
in the National Semiconductor PCB shown in Figure 7.
Figure 4 is used to determine the size of ground plane heat
sink. The lines in Figure 4 represent a junction temperature
of 150˚C when the size of heat sink noted by the line is used.
The area under the line is the safe operating area. Using
Figure 4, the following example shows how to determine the
size of the ground plane.
The LM2481 uses about 0.4W of quiescent power. If V
+145V, and each channel of the LM2481 needs to sink an
average of 1mA, the LM2481 will use:
Figure 4 shows that the LM2481 dissipating 0.835W with
one sq. in. of copper ground plane heat sink is in the safe
operating range for any ambient temperature up to 70˚C.
• The traces connecting the LM126X preamp DACs and
• C43, C44, and C45 should be located close to the
• R44, R45, and R46 should be located close to the
• C21, C24, and C25 on the output pins of the LM2481
• C9 and C36 on the supply lines (pins 2 and 7) should be
• The ground of the LM2481 should have a short direct
FIGURE 4. Maximum Power Dissipation vs. Ambient
0.4W + 3 x (145V x 1mA ) = 0.835W
the LM2481 input pins (pins 1, 13, and 14) should be run
around the top of the LM2421. They should not go
through the output circuit of the LM2421.
LM126X preamp.
LM2481.
(pins 6, 8, and 9) should be located close to the LM2481.
The grounds of these capacitors should have a short
direct return to GND (pins 3, 4, 5, 10, 11, and 12) of the
LM2481.
located close to the LM2481. The grounds of these ca-
pacitors should have a short direct return to GND (pins 3,
4, 5, 10, 11, and 12) of the LM2481.
connection to the ground of the LM2421.
Temperature
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CC
=