LM2481
145V Triple Bias Clamp
General Description
The LM2481 driver is an Integrated 145V triple bias clamp
circuit for DC recovery of each of the AC coupled outputs of
a HDTV CRT driver. It is well matched with the DAC outputs
of the LM126X family of pre-amplifiers. Each amplifier has its
gain internally set to -20. The LM2481 is packaged in an
industry standard 14 lead molded plastic dual-in-line pack-
age.
Features
n
Wide range integrated triple bias clamp
n
High input impedance
n
Matched to LM2421 HDTV Driver
Applications
n
AC coupled HDTV applications using the 1080i and
720p formats
Schematic and Connection Diagrams
20039833
Top View
Order Number LM2481NA
20039832
FIGURE 1. Simplified Schematic Diagram (One Channel)
June 2002
LM2481
145V
T
riple
Bias
Clamp
2002 National Semiconductor Corporation
DS200398
www.national.com
Absolute Maximum Ratings
(Notes 1,
3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, V
CC
+155V
Bias Voltage, V
BB
+15V
Input Voltage, V
IN
-0.5V to V
BB
+0.55V
Storage Temperature Range, T
STG
-65C to +150C
Lead Temperature (Soldering,
<
10sec.)
300C
ESD Tolerance
Machine Model
200V
Human Body Model
2KV
Max Junction Temperature
150C
JA
(Typ) (Note 5)
70C
Limits of Operating Ranges
(Note 3)
V
CC
130V to 150V
V
BB
7V to 13V
V
IN
1V to 5V
V
OUT
25V to 140V
Ambient Temperature Range, T
A
0C to 70C
DC Clamp Electrical Characteristics
(See Figure 2 for Test Circuit)
Unless otherwise noted: V
CC
= +145V, V
BB
= 8V, V
IN
= +2.5V, T
A
= 30C
Symbol
Parameter
Conditions
Min
Typ
Max
Units
I
CC
Supply Current
All three channels, includes 100K
pull-up resistor current
4.5
7
mA
I
BB
Bias Supply Current
All channels
2
mA
V
OUT
DC Output Voltage
100
108
115
V
DC
V
OUT-Range
Output Voltage Range
V
IN
Range = +1.5V to 4.5V
60
V
A
V
DC Voltage Gain
-18
-20
-22
V/V
LE
Linearity Error
See Note 4
5
%
Note 1: Absolute Maximum Ratings indicate beyond which damage to the device may occur.
Note 2: Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and
the test conditions, see the Electrical Characteristics. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. The guaranteed
specifications apply only for the test conditions listed. Some performance characteristics may change when the device is not operated under the listed test
conditions.
Note 3: All voltages are measured with respect to GND, unless otherwise specified.
Note 4: Linearity Error is the variation in dc gain from V
IN
= 1.5 volts to V
IN
= 4.5 volts.
Note 5: Measured with 1in
2
copper on PCB connected to pins 3, 4, 5, 10, 11, 12. See Thermal Considerations Section.
Test Circuit
Figure 2 shows the test circuit for evaluation of the LM2481 Clamp Amplifier. A high impedance Voltmeter (
>
100M
) is used for
DC measurements at the output.
20039834
FIGURE 2. Test Circuit
LM2481
www.national.com
2
Typical Performance Characteristics
(V
CC
= +145V, V
BB
= +8V, V
IN
= +2.5V, Test Circuit - Figure 2)
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 (
>
100M
) is
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 F 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 F 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):
The traces connecting the LM126X preamp DACs and
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.
C43, C44, and C45 should be located close to the
LM126X preamp.
R44, R45, and R46 should be located close to the
LM2481.
C21, C24, and C25 on the output pins of the 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.
C9 and C36 on the supply lines (pins 2 and 7) should be
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.
The ground of the LM2481 should have a short direct
connection to the ground of the LM2421.
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 150C 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
CC
=
+145V, and each channel of the LM2481 needs to sink an
average of 1mA, the LM2481 will use:
0.4W + 3 x (145V x 1mA ) = 0.835W
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 70C.
20039835
FIGURE 3. V
out
vs V
in
20039836
FIGURE 4. Maximum Power Dissipation vs. Ambient
Temperature
LM2481
www.national.com
3
PDF 
ZIP
