NSC LM2466TA Monolithic triple 7.5 ns crt driver Datasheet

LM2466
Monolithic Triple 7.5 ns CRT Driver
General Description
Features
The LM2466 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.
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 Convenient TO-220 staggered lead package style
n Maintains standard LM243X Family pinout which is
designed for easy PCB layout
The IC is packaged in an industry standard 9-lead TO-220
molded plastic power package. See the Thermal Considerations section for more information.
Applications
n 1024 x 768 displays up to 85 Hz refresh
n Pixel clock frequencies up to 95 MHz
n Monitors using video blanking
Schematic Diagram
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FIGURE 1. Simplified Schematic Diagram
(One Channel)
© 2002 National Semiconductor Corporation
DS200436
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LM2466 Monolithic Triple 7.5 ns CRT Driver
June 2002
LM2466
Connection Diagram
20043602
Note: Tab is at GND
Top View
Order Number LM2466TA
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Machine Model
(Notes 1,
250V
3)
Operating Ranges (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (VCC)
+90V
Bias Voltage (VBB)
+16V
Input Voltage (VIN)
0V to 4.5V
Storage Temperature Range (TSTG)
VCC
+8V to +15V
VIN
+0V to +3.75V
VOUT
+15V to +75V
Case Temperature
−65˚C to +150˚C
−20˚C to +100˚C
Do not operate the part without a heat sink.
Lead Temperature
(Soldering, < 10 sec.)
+60V to +85V
VBB
300˚C
ESD Tolerance, Human Body
Model
2 kV
Electrical Characteristics
(See Figure 2 for Test Circuit) Unless otherwise noted: VCC = +80V, VBB = +12V, CL = 8 pF, TC = 50˚C DC Tests: VIN =
2.25VDC AC Tests: Output = 40VPP(25V - 65V) at 1MHz
Symbol
Parameter
ICC
Supply Current
Conditions
LM2466
Min
All Three Channels, No Input Signal,
No Output Load
Typical
Max
30
mA
IBB
Bias Current
All Three Channels
VOUT
DC Output Voltage
No AC Input Signal, VIN = 1.25V
62
65
68
AV
DC Voltage Gain
No AC Input Signal
−18
−20
−22
18
∆AV
Gain Matching
(Note 4), No AC Input Signal
LE
Linearity Error
(Notes 4, 5), No AC Input Signal
tR
Rise Time
(Note 6), 10% to 90%
1.0
Units
mA
VDC
dB
5
%
7.5
ns
tF
Fall Time
(Note 6), 90% to 10%
8
ns
OS
Overshoot
(Note 6)
1
%
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. For guaranteed specifications and
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 VIN = 1.0V to VIN = 3.5V.
Note 6: Input from signal generator: tr, tf < 1 ns.
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LM2466
Absolute Maximum Ratings
LM2466
AC Test Circuit
20043603
Note: 8 pF load includes parasitic capacitance.
FIGURE 2. Test Circuit (One Channel)
Figure 2 shows a typical test circuit for evaluation of the LM2466. This circuit is designed to allow testing of the LM2466 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.
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(VCC = +80 VDC, VBB = +12 VDC, CL = 8 pF, VOUT = 40 VPP
(25V−65V), Test Circuit - Figure 2 unless otherwise specified)
20043607
20043604
FIGURE 6. Power Dissipation vs Frequency
FIGURE 3. VOUT vs VIN
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20043605
FIGURE 7. Speed vs Offset
FIGURE 4. Speed vs Temp.
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20043609
FIGURE 5. LM2466 Pulse Response
Speed vs Load Capacitance
FIGURE 8.
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LM2466
Typical Performance Characteristics
LM2466
performance and to protect against damage from CRT arcover are designed specifically for the LM2466. If another
member of the LM246X family is used, please refer to its
datasheet.
Theory of Operation
The LM2466 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2466 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 LM2466 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 cascode stage from
the capacitance of the CRT cathode which decreases the
sensitivity of the device to load capacitance. Q6 provides
biasing to the output emitter follower stage to reduce crossover distortion at low signal levels.
POWER SUPPLY BYPASS
Since the LM2466 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. 0.1 µF capacitors should be
connected from the supply pins, VCC and VBB, to ground, as
close to the LM2466 as is practical. Additionally, a 47 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2466.
ARC PROTECTION
Figure 2 shows a typical test circuit for evaluation of the
LM2466. This circuit is designed to allow testing of the
LM2466 in a 50Ω environment without the use of an expensive FET probe. In this test circuit, the two 2.49kΩ resistors
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.
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 LM2466. This fast, high voltage, high energy pulse can
damage the LM2466 output stage. The application circuit
shown in Figure 9 is designed to help clamp the voltage at
the output of the LM2466 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 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 decoupling
capacitor should be very close to the LM2466 ground. This
will significantly reduce the high frequency voltage transients
that the LM2466 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 LM2466 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
LM2466 would be subjected to. The inductor will not only
help protect the device but it will also help minimize 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.
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 LM2466 performance is targeted for the VGA (640 x
480) to XGA (1024 x 768, 85 Hz refresh) resolution market.
The application circuits shown in this document to optimize
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LM2466
Application Hints
(Continued)
20043610
FIGURE 9. One Channel of the LM2466 with the Recommended Application Circuit
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.
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 300 MHz. 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 11 and Figure 12 can be used as a
good starting point for the evaluation of the LM2466. Using
variable resistors for R1 and the parallel resistor will simplify
finding the values needed for optimum performance in a
given application. Once the optimum values are determined
the variable resistors can be replaced with fixed values.
The LM2466 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature inside
the monitor is 70˚C and the power dissipation is 4.4W (from
Figure 6, 50 MHz max. video frequency), then a maximum
heat sink thermal resistance can be calculated:
This example assumes a capacitive load of 8 pF and no
resistive load.
TYPICAL APPLICATION
A typical application of the LM2466 is shown in Figure 11 and
Figure 12. Used in conjunction with an LM1267, a complete
video channel from monitor input to CRT cathode can be
achieved. Performance is ideal for 1024 x 768 resolution
displays with pixel clock frequencies up to 95 MHz. Figure 11
and Figure 12 are the schematic for the NSC demonstration
board that can be used to evaluate the LM1267/2466 combination in a monitor, and Figure 10 shows the typical response at the red cathode for this application. The input
video rise time is 3.2ns, and the peaking component values
are those recommended in Figure 12. Table 1 shows the
typical cathode response of all three channels.
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.
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 VDC. The
rise time shows a maximum variation relative to the center
data point (45 VDC) less than 5%. The fall time shows a
variation of less than 8% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2466 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2466 increases
by approximately 10% as the case temperature increases
from 50˚C to 100˚C. This corresponds to a speed degradation of 2% for every 10˚C rise in case temperature. The fall
time increases by approximately 7% as the case temperature increases from 50˚C to 100˚C.
Figure 6 shows the maximum power dissipation of the
LM2466 vs. Frequency when all three channels of the device
are driving an 8 pF load with a 40 Vp-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
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LM2466
Application Hints
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.
(Continued)
NSC DEMONSTRATION BOARD
Figure 13 shows the routing and component placement on
the NSC LM1267/2466 demonstration board. The schematic
of the board is shown in Figure 11 and Figure 12. 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:
• C19 — VCC bypass capacitor, located very close to pin 4
and ground pins
• C20 — VBB bypass capacitors, located close to pin 8 and
ground
• C46, C47, C48 — VCC bypass capacitors, near LM2466
and VCC clamp diodes. Very important for arc protection.
20043616
FIGURE 10. Red Cathode Response
The routing of the LM2466 outputs to the CRT is very critical
to achieving optimum performance. Figure 14 shows the
routing and component placement from pin 1 of the LM2466
to the blue cathode. Note that the components are placed so
that they almost line up from the output pin of the LM2466 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 LM2466 ground pins. The cathode of
D9 is connected to VCC very close to decoupling capacitor
C19 (see Figure 14) 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
LM2466 during an arcover event. Lastly, notice that S3 is
placed very close to the blue cathode and is tied directly to
CRT ground.
TABLE 1. LM2466 Cathode Response
Channel
tr/OS
tf/OS
Red
6.3ns / 7%
6.3ns / 3%
Green
6.1ns / 6%
5.9ns / 4%
Blue
6.5ns / 3%
6.4ns / 2%
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 LM2466 and
from the LM2466 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.
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Application Hints
(Continued)
FIGURE 11. LM126X/LM246X Demonstration Board Schematic
20043617
LM2466
9
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Application Hints
(Continued)
FIGURE 12. LM126X/LM246X Demonstration Board Schematic (continued)
20043618
LM2466
LM2466
Application Hints
(Continued)
20043619
FIGURE 13. LM126X/LM246X Demo Board Layout
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LM2466
Application Hints
(Continued)
20043614
FIGURE 14. Trace Routing and Component Placement for Blue Channel Output
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LM2466
Physical Dimensions
inches (millimeters)
unless otherwise noted
CONTROLLING DIMENSION IS INCH
VALUES IN [
] ARE MILLIMETERS
NS Package Number TA09A
Order Number LM2466TA
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LM2466 Monolithic Triple 7.5 ns CRT Driver
Notes
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