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LM2465
Monolithic Triple 5.5 ns High Gain CRT Driver
General Description
The LM2465 is an integrated high voltage CRT driver circuit
designed for use in color monitor applications. The IC
contains three high input impedance, wide band amplifiers
which directly drive the RGB cathodes of a CRT. Each
channel has its gain internally set to −20 and can drive CRT
capacitive loads as well as resistive loads present in other
applications, limited only by the package’s power dissipation.
The IC is packaged in an industry standard 9-lead TO-220
molded plastic package.
Features
n Higher gain to match LM126X CMOS preamplifiers
n 0V to 3.75V input range
n Stable with 0–20 pF capacitive loads and inductive
peaking networks
n Same pinout as LM2467/8/9, maintaining the standard
LM243X Family pinout for easy PCB layout
n Convenient TO-220 staggered lead package style
Applications
n Up to 1280 x 1024 at 75Hz
n Pixel clock frequencies up to 135 MHz
n Monitors using video blanking
Schematic and Connection Diagrams
DS200190-1
FIGURE 1. Simplified Schematic Diagram
(One Channel)
DS200190-2
Note: TabisatGND
Top View
Order Number LM2465TA
March 2001
LM2465 Monolithic Triple 5.5 ns High Gain CRT Driver
© 2001 National Semiconductor Corporation DS200190 www.national.com
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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
) +90V
Bias Voltage (V
BB
) +16V
Input Voltage (V
IN
) 0V to 4.5V
Storage Temperature Range (T
STG
) −65˚C to +150˚C
Lead Temperature
(Soldering,
<
10 sec.) 300˚C
ESD Tolerance, Human Body Model 2 kV
Machine Model 250V
Operating Ranges (Note 2)
V
CC
+60V to +85V
V
BB
+8V to +15V
V
IN
+0V to +3.75V
V
OUT
+15V to +75V
Case Temperature −20˚C to +100˚C
Do not operate the part without a heat sink.
Electrical Characteristics
(See
Figure 2
for Test Circuit)
Unless otherwise noted: V
CC
= +80V, VBB= +12V, CL= 8 pF, TC= 50˚C
DC Tests: V
IN
= 2.25VDC
AC Tests: Output = 40V
PP
(25V - 65V) at 1MHz
Symbol Parameter Conditions
LM2465
Units
Min Typical Max
I
CC
Supply Current All Three Channels, No Input Signal,
No Output Load
36 44 mA
I
BB
Bias Current All Three Channels 20 24 mA
V
OUT
DC Output Voltage No AC Input Signal, VIN= 1.25V 62 65 68 V
DC
A
V
DC Voltage Gain No AC Input Signal −18 −20 −22
∆A
V
Gain Matching (Note 4), No AC Input Signal 1.0 dB
LE Linearity Error (Notes 4, 5), No AC Input Signal 5 %
t
R
Rise Time (Note 6), 10% to 90% 5.5 ns
t
F
Fall Time (Note 6), 90% to 10% 6.0 ns
OS Overshoot (Note 6) 3 %
Note 1: Absolute Maximum Ratings indicate limits 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. Forguaranteedspecificationsand
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 3: All voltages are measured with respect to GND, unless otherwise specified.
Note 4: Calculated value from Voltage Gain test on each channel.
Note 5: Linearity Error is the variation in dc gain from V
IN
= 1.0V to VIN= 3.5V.
Note 6: Input from signal generator: t
r,tf
<
1 ns.
LM2465
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AC Test Circuit
Figure 2
shows a typical test circuit for evaluation of the LM2465. This circuit is designed to allow testing of the LM2465 in a 50Ω
environment without the use of an expensive FET probe. The two 2490Ω resistors form a 200:1 divider with the 50Ω resistor and
the oscilloscope. A test point is included for easy use of an oscilloscope probe.The compensation capacitor is used to
compensate the stray capacitance of the two 2490Ω resistors to achieve flat frequency response.
DS200190-3
Note: 8 pF load includes parasitic capacitance.
FIGURE 2. Test Circuit (One Channel)
LM2465
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Typical Performance Characteristics (V
CC
= +80VDC,VBB= +12VDC,CL= 8pF, V
OUT
= 40V
PP
(25V−65V), Test Circuit -
Figure 2
unless otherwise specified)
DS200190-4
FIGURE 3. V
OUT
vs V
IN
DS200190-5
FIGURE 4. Speed vs Temperature
DS200190-6
FIGURE 5. LM2465 Pulse Response
DS200190-7
FIGURE 6. Power Dissipation vs Frequency
DS200190-8
FIGURE 7. Speed vs Offset
DS200190-9
FIGURE 8. Speed vs Load Capacitance
LM2465
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Theory of Operation
The LM2465 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2465 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 LM2465 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 −20. Emitter followers Q3 and Q4
isolate the high output impedance of the amplifier,
decreasing the sensitivity of the device to changes in load
capacitance. Q6 provides biasing 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
LM2465. This circuit is designed to allow testing of the
LM2465 in a 50Ω environment without the use of an
expensive FET probe. In this test circuit, two low inductance
resistors in series totaling 4.95kΩ form a 200:1 wideband,
low capacitance probe when connected to a 50Ω coaxial
cable and a 50Ω load (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 LM2465 performance is targeted for the 17“ and low end
19“ monitor market with resolutions up to 1280 X 1024 and a
75Hz refresh rate. 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
LM2465. If another member of the LM246X family is used,
please refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2465 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 0.1 µF capacitor should be
connected from the supply pin, V
CC
and VBB, to ground, as
close to the LM2465 as is practical. Additionally, a 47µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2465.
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 LM2465. This fast, high voltage, high
energy pulse can damage the LM2465 output stage. The
application circuit shown in
Figure 9
is designed to help
clamp the voltage at the output of the LM2465 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
CC
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 decoupling capacitor should be very close to the
LM2465 ground. This will significantly reduce the high
frequency voltage transients that the LM2465 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 LM2465 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 LM2465 would be
subjected to. The inductor will not only help protect the
device but it will also help optimize 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
.
LM2465
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Application Hints (Continued)
OPTIMIZING TRANSIENT RESPONSE
Referring to
Figure 9
, there are three components (R1, R2
and L1) that can be adjusted to optimize the transient
response of the application circuit. Increasing the values of
R1 and R2 will slow the circuit down while decreasing
overshoot. Increasing the value of L1 will speed up the circuit
as well as increase overshoot. It is very important to use
inductors with very high self-resonant frequencies,
preferably above 300MHz. Ferrite core inductors from J.W.
Miller Magnetics (part
#
78FR--k) were used for optimizing
the performance of the device in the NSC application board.
The values shown in
Figure 9
can be used as a good starting
point for the evaluation of the LM2465. Using a variable
resistor for R1 will simplify finding the value needed for
optimum performance in a given application. Once the
optimum value is determined, the variable resistors can be
replaced with fixed values.
EFFECT OF LOAD CAPACITANCE
Figure 8
shows the effect of increased load capacitance on
the speed of the device. This demonstrates the importance
of knowing the load capacitance in the application. The rise
time increased about 0.12nsec for an increase of 1pF in the
load capacitance. The fall time increased about 0.10 nsec for
a 1pF increase in the load capacitance.
EFFECT OF OFFSET
Figure 7
shows the variation in rise and fall times when the
output offset of the device is varied from 40 to 50 V
DC
. The
rise time increases less than 0.20nsec from its fastest point
near 45V. The fall time becomes faster as the offset voltage
increases, but the 45V offset is only 0.1nsec slower than the
fastest fall time.
THERMAL CONSIDERATIONS
Figure 4
shows the performance of the LM2465 in the test
circuit shown in
Figure 2
as a function of case temperature.
The figure shows that the rise time of the LM2465 increases
by approximately 13% as the case temperature increases
from 30˚C to 95˚C. This corresponds to a speed degradation
of 2% for every 10˚C rise in case temperature. The fall time
degrades around 0.3% for every 10˚C rise in case
temperature.
Figure 6
shows the maximum power dissipation of the
LM2465 vs. Frequency when all three channels of the device
are driving an 8pF load with a 40V
p-p
alternating one pixel
on, one pixel off signal. The graph assumes a 72% active
time (device operating at the specified frequency) which is
typical in a monitor application. The other 28% of the time
the device is assumed to be sitting at the black level (65V in
this case). This graph gives the designer the information
needed to determine the heat sink requirement for his
application. The designer should note that if the load
capacitance is increased, the AC component of the total
power dissipation will also increase.
The LM2465 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 7.6W (from
Figure 6
, 75MHz bandwidth) then a maximum heat sink
thermal resistance can be calculated:
This example assumes a capacitive load of 8pF and no
resistive load.
TYPICAL APPLICATION
A typical application of the LM2465 is shown in
Figure 10
and
Figure 11
. Used in conjunction with an LM1267 and a
LM2479/2480bias clamp, a complete video channel from
monitor input to CRTcathode can be achieved. Performance
is ideal for 1280 x 1024 resolution displays with pixel clock
frequencies up to 135 MHz.
Figure 10
and
Figure 11
are the
schematic for the NSC demonstration board that can be
used to evaluate the LM1267/2465 /2480 combination in a
monitor.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane,
isolation between channels, good supply bypassing and
minimizing unwanted feedback are necessary. Also, the
length of the signal traces from the preamplifier to the
LM2465 and from the LM2465 to the CRT cathode should be
as short as possible. The following references are
recommended:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease, Robert A., “Troubleshooting Analog Circuits”,
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a monitor if feedback occurs around the video
channel through the chassis wiring. To prevent this, leads to
the video amplifier input circuit should be shielded, and input
circuit wiring should be spaced as far as possible from output
circuit wiring.
DS200190-10
FIGURE 9. One Channel of the LM2465 with the Recommended Arc Protection Circuit
LM2465
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Application Hints (Continued)
NSC DEMONSTRATION BOARD
Figure 12
shows the routing and component placement on
the NSC LM126X/246X/LM2479/80 demonstration board.
The schematic of the board is shown in
Figure 10
and
Figure
11
. This board provides a good example of a layout that can
be used as a guide for future layouts. Note the location of the
following components:
•
C16, C19—VCCbypass capacitor, located very close to
pin 4 and ground pins
•
C17, C20—VBBbypass capacitors, located close to pin 8
and ground
•
C46, C47, C48 —VCCbypass capacitors, near LM2465
and V
CC
clamp diodes. Very important for arc protection.
The routing of the LM2465 outputs to the CRT is very critical
to achieving optimum performance.
Figure 13
shows the
routing and component placement from pin 3 of the LM2465
to the blue cathode. Note that the components are placed so
that they almost line up from the output pin of the LM2465 to
the blue cathode pin of the CRT connector. This is done to
minimize the length of the video path between these two
components. Note also that D8, D9, R24 and D6 are placed
to minimize the size of the video nodes that they are
attached to. This minimizes parasitic capacitance in the
video path and also enhances the effectiveness of the
protection diodes. The anode of protection diode D8 is
connected directly to a section of the the ground plane that
has a short and direct path to the LM2465 ground pins. The
cathode of D9 is connected to V
CC
very close to decoupling
capacitor C48 (see
Figure 13
) which is connected to the
same section of the ground plane as D8. The diode
placement and routing is very important for minimizing the
voltage stress on the LM2465 during an arcover event.
Lastly,notice that S3 is placed very close to the blue cathode
and is tied directly to CRT ground.
LM2465
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Application Hints (Continued)
DS200190-11
FIGURE 10. LM126X/LM246X Demonstration Board Schematic
LM2465
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Application Hints (Continued)
DS200190-15
FIGURE 11. LM126X/LM246X Demonstration Board Schematic (continued)
LM2465
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Application Hints (Continued)
DS200190-13
FIGURE 12. LM126X/LM246X Demo Board Layout
LM2465
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Application Hints (Continued)
DS200190-14
FIGURE 13. Trace Routing and Component Placement for Blue Channel Output
LM2465
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Physical Dimensions inches (millimeters) unless otherwise noted
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accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
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can be reasonably expected to cause the failure of
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CONTROLLING DIMENSION IS INCH
VALUES IN [ ] ARE MILLIMETERS
NS Package Number TA09A
Order Number LM2465TA
LM2465 Monolithic Triple 5.5 ns High Gain CRT Driver
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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