LM2479
120V Triple Bias Clamp
General Description
The LM2479 is an Integrated 120V triple bias clamp circuit
for DC recovery of each of the AC coupled outputs of a 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 -18. The IC is packaged in an industry
standard 8 lead molded DIP package.
Features
n
Wide range integrated triple bias clamp
n
High input impedance
n
Single supply operation
n
Matched to the LM126X family of preamplifiers
Recommended Applications
n
CRT monitors requiring DC restoration at the cathodes
Block Diagrams
Package Pinout
DS101391-1
FIGURE 1. Simplified Schematic (One Channel)
DS101391-2
FIGURE 2. LM2479 Package Pinout
Order Number LM2479NA
NS Package Number: N08E
October 2000
LM2479
120V
T
riple
Bias
Clamp
2000 National Semiconductor Corporation
DS101391
www.national.com
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, V
CC
+130V
Input Voltage, V
IN
0V to 5V
Storage Temperature Range, T
STG
-65C to +150C
Lead Temperature (Soldering,
<
10sec.)
300C
ESD Tolerance, Machine Model
200V
Limits of Operating Ranges
(Note 3)
V
CC
110 to 125
V
OUT
, V
CC
= 120V
50 to 120V
Ambient Temperature Range, T
A
0 to 70C
DC CLAMP ELECTRICAL CHARACTERISTICS TARGETS AND LIMIT
Unless otherwise noted: V
CC
= +120V, V
IN
= 2.25V
DC
, T
A
= 25C.
Symbol
Spec Parameter
Conditions
Min
Typ
Max
Units
I
CC
Supply Current
All channels
2.3
3.5
mA
V
OUT
DC Output Voltage
83
87
91
V
DC
V
OUT-Range
Output Voltage Range
V
IN
Range = 1.0V - 4.0V
53
V
A
V
DC Voltage Gain
-16
-18
-20
LE
Linearity Error
See Note 1
5
%
R
IN
Input Resistance
24K
Note 1: Linearity Error is the variation in DC gain from V
IN
= 1.0V to V
IN
= 4.0V.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.
Note 3: 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. 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 4: All voltages are measured with respect to GND, unless otherwise specified.
Test Circuit
Figure 3 shows the test circuit for evaluation of the LM2479 Clamp Amplifier. A high impedance VM (
>
100M
) is used for DC
measurements at the output.
DS101391-3
FIGURE 3. Test Circuit (One Channel)
LM2479
www.national.com
2
Typical Performance Characteristics
(V
CC
= +120V), Test Circuit - Figure 3 unless otherwise specified.
THEORY OF OPERATION
The circuit diagram of the LM2479 is shown in Figure 1. The
DC clamp circuit amplifies the input signal by -18 and the
gain is set by the resistor ratio of 18R and R. The output
requires pull-up resistor to 120V. Figure 2 shows the test
circuit used for evaluation of the LM2479 Clamp Amplifier. A
high impedance voltmeter (100M
) is used for DC measure-
ments at the output. The DC transfer function is shown in
Figure 4.
APPLICATION HINTS
Power Supply Bypass
The LM2479 should have proper power supply bypassing for
optimum performance. A 0.1F capacitor should be con-
nected from the supply pin, V
cc
, to ground, as close to the
supply and ground pins as is practical. Additionally, a 1.0F
electrolycelectrolytictic capacitor should be connected from
the supply pin to ground. The electrolytic capacitor should
also be placed reasonably close to the LM2479's supply and
ground pins.
Application Circuit
The application circuit shown in Figure 5 is designed to help
clamp the voltage at the output of the driver to the desired
level. Capacitor C
4
stabilizes the entire node at the anode of
the clamp diode, D
3
, by creating a low impedance at high
frequencies. Figure 5 also shows the standard application
circuit topology when used with an LM246X CRT driver. It
shows all the components necessary to optimize perfor-
mance as well as to protect against damage from a CRT arc
event. No additional components are required to protect the
LM2479 from arc damage.
Demonstration Hardware
National Semiconductor has designed a demonstration
neckboard for the LM126X, LM246X, and the LM2479
chipset. To obtain demonstration boards contact the National
Semiconductor Sales Office in your region.
DS101391-4
FIGURE 4. V
out
vs V
in
DS101391-7
FIGURE 5. One Channel of the LM2479 and LM246X
Application Circuit
LM2479
www.national.com
3
Physical Dimensions
inches (millimeters) unless otherwise noted
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into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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Note: Information contained in this data sheet is preliminary and may be subject to change without notice.
Molded Dual-In-Line Package (N)
NS Package Number N08E
Order NumbernLM2479NA
LM2479
120V
T
riple
Bias
Clamp
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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