NSC LM2476

LM2476
Monolithic Triple Channel 6.5 ns High Gain CRT Driver
and Bias Clamp
n Convenient TO-247 staggered lead package style
General Description
The LM2476 is a monolithic triple channel CRT driver and
triple bias clamp for low-cost color monitor applications. The
highly integrated IC contains three wide-band amplifiers for
driving the RGB cathodes of a CRT through external coupling capacitors and three low-band clamp amplifiers for DC
restoration and cutoff adjustment of the video outputs. The
CRT drivers have a high, fixed gain of -27 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 a 19-lead TO-247 molded plastic
package and must be operated with a properly chosen heat
sink. See the Package Mounting and Thermal Considerations sections for more information.
CRT Drivers
n High gain of –27 for up to 60VP-P output swing
n Stable with capacitive loads and inductive peaking
networks
Bias Clamps
n Gain of –17 for up to 60V DC output range
Applications
n 1024 x 768 displays up to 85 Hz refresh rate
n Pixel clock frequencies up to 95 MHz
n Monitors using video blanking
Features
n Well-matched to the LM123X/4X family of preamplifiers
n Operates with VCC = 60V to 90V
Pinout Diagram and Pin Descriptions
Pin Name
Pin Description
IND
Driver Input Pins (1, 3, 4)
INC
Clamp Input Pins (6, 7, 8)
VBB
Bias Voltage Pin (2)
VCC
Supply Voltage Pin (14)
GND*
Ground Pins (5, 10, 12, 16, 18)
OUTC
Clamp Output Pins (9, 11, 13)
OUTD
Driver Output Pins (15, 17, 19)
*Note: All GND pins should be connected together via low HF impedance
traces on the PCB.
20121901
Note: Tab is at GND
FIGURE 1. Pinout and Connection Diagram
© 2005 National Semiconductor Corporation
DS201219
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LM2476 Monolithic Triple Channel 6.5 ns High Gain CRT Driver and Bias Clamp
January 2004
LM2476
Schematic Diagrams
20121902
FIGURE 2. CRT Driver Simplified Schematic (One Channel)
20121915
FIGURE 3. Bias Clamp Simplified Schematic (One Channel)
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(Notes 1,
3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (VCC)
96V
Bias Voltage (VBB)
10V
Driver Input Voltage (VIND)
0V to 4.5V
Clamp Input Voltage (VINC)
0V to 5.0V
Storage Temperature Range (TSTG)
2 KV
Machine Model
200V
Operating Ranges (Note 2)
VCC
60V to 85V
VBB
7V to 9V
VIND
0V to 3.5V
VINC
0V to 4.0V
VOUTD (Driver Output Voltage)
−65˚C to +150˚C
Lead Temperature
(Soldering, < 10 sec.)
Human Body Model
300˚C
12V to VCC
VOUTC (Clamp Output Voltage)
12V to VCC
Case Temperature (device tab)
−20˚C to +100˚C
Do not operate the part without a heat sink.
ESD Tolerance
Electrical Characteristics
(See Figure 4 for Test Circuit)
Unless otherwise noted: VCC = 85V, VBB = 8V, CL = 8 pF, TC = 40˚C
DC Tests: VIND = 2.30V, VINC = 2.35V
AC Tests: VOUTD = 40VP-P (35V – 75V) at 1 MHz, VINC = 2.35V
Symbol
Parameter
Conditions
LM2476
Min
Typical
Max
Units
ICC
Supply Current
All Three Channels, No AC Input
Signal, No Output Load
34
45
mA
IBB
Bias Current
All Three Channels, No AC Input
Signal, No Output Load
21
30
mA
VOUTD, 1
Driver DC Output Voltage
No AC Input Signal, VIND = 2.30V
41
46
51
V
VOUTD, 2
Driver DC Output Voltage
No AC Input Signal, VIND = 1.15V
72
77
82
V
AV-OUTD
Driver DC Voltage Gain
No AC Input Signal
–24
–27
–30
∆AV-OUTD
Driver Gain Matching
(Note 4), No AC Input Signal
LEOUTD
Driver Linearity Error
(Notes 4, 5), No AC Input Signal
tR
Rise Time
tF
Fall Time
OS
Overshoot
(Note 6)
VOUTC
Clamp DC Output Voltage
VINC = 2.35V
VOUTC-RANGE
Clamp DC Output Voltage
Range
VINC-RANGE = 0.5V to 4.0V
AV-OUTC
Clamp DC Voltage Gain
No AC Input Signal
LEOUTC
Clamp Linearity Error
(Notes 4, 5), No AC Input Signal
1.0
dB
5
%
(Note 6), 10% to 90%
6.0
ns
(Note 6), 90% to 10%
6.7
ns
3
46
51
%
56
58
–14.5
–16.5
5
V
VDC
–18.5
%
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. 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: Calculated value from Voltage Gain test on each channel.
Note 5: Driver Linearity Error is the variation in dc gain from VIND = 1.1V to VIND = 3.6V.
Note 6: Input from signal generator: tr, tf < 1 ns.
Note 7: Clamp Linearity Error is the variation in dc gain from VINC = 1.0V to VINC = 4.0V.
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LM2476
Absolute Maximum Ratings
LM2476
LM2476 Test Circuits
20121903
Note: 8 pF load includes parasitic capacitance.
FIGURE 4. CRT Driver and Bias Clamp Test Circuits (One Channel)
Figure 4 shows a typical test circuit to evaluate the LM2476 CRT Driver and Bias Clamp electrical characteristics. The driver test
circuit is designed to allow for testing the transient response in a 50Ω environment without the use of an expensive FET probe.
An input from a 50Ω pulse generator output can be AC coupled and biased with an external supply via the VBIAS input. The two
2.49 kΩ resistors form a 200:1 divider with the 50Ω resistor and the oscilloscope. The clamp test circuit is designed to allow for
testing the clamp outputs. A clamp input can be biased with an external supply via the VDC input and a high impedance voltmeter
( > 100MΩ) can be used to measure the DC voltage at the clamp outputs. Test points can included to accommodate voltmeter or
oscilloscope probes.
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20121908
20121904
FIGURE 8. Speed vs Offset
FIGURE 5. VOUTD vs VIND
20121909
20121917
FIGURE 9. Speed vs Load Capacitance
FIGURE 6. VOUTC vs VINC
20121906
20121905
FIGURE 7. LM2476 Pulse Response
FIGURE 10. Speed vs Temperature
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LM2476
Typical Performance Characteristics (VCC = 85V, VBB = 8V, CL = 8 pF, VOUTD = 40VP-P (35V −
75V), VINC = 2.35V, Test Circuit - Figure 4 unless otherwise specified)
LM2476
Typical Performance
Characteristics (VCC = 85V, VBB = 8V, CL = 8
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.
pF, VOUTD = 40VP-P (35V − 75V), VINC = 2.35V, Test
Circuit - Figure 4 unless otherwise specified) (Continued)
IMPORTANT INFORMATION
The LM2476 performance is targeted for the VGA (640 x
480) to XGA (1024 x 768, 85Hz) resolution market. The
application circuits shown in this document to optimize performance and to protect against damage from CRT arcover
are designed specifically for the LM2476. If another member
of the NSC CRT Driver or Bias Clamp family is used, please
refer to its data sheet.
POWER SUPPLY BYPASS
Since the LM2476 contains wide bandwidth video amplifiers,
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 LM2476 as is practical. Additionally,
a 47 µF or larger electrolytic capacitor should be connected
from both supply pins to ground reasonably close to the
LM2476. For optimal supply bypassing, the bypass capacitors should have the shortest connections between the supply and ground pins of the LM2476.
20121907
FIGURE 11. Power Dissipation vs Frequency
ARC PROTECTION
During normal CRT operation, internal arcing may occasionally occur. A spark gap SG1 – in the range of 200V –
connected from each of the CRT cathodes to CRT ground
will limit the maximum voltage, but to a value that is much
higher than allowable on the LM2476. This fast, high voltage,
high energy pulse can damage the LM2476 driver and/or
clamp output stages. The application circuit shown in Figure
12 is designed to help clamp the voltage at the outputs of the
LM2476 to a safe level. The arc protection 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 (C1 in
Figure 12). The ground connection of D2 and the decoupling
capacitor should be closest to the CRT ground. This will
significantly reduce the high frequency voltage transients
that the LM2476 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 LM2476 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
LM2476 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. Current-limiting resistor
R3 and bypass capacitor C2 should be placed very close to
the clamp output pins to protect the LM2476 against damage
during an arcover condition. The ground connection of C2
should have a short return path to CRT ground to shunt
arcover currents away from the LM2476. For proper arc
protection, it is important to not omit any of the arc protection
components shown in Figure 12.
Theory of Operation
The LM2476 is a high voltage monolithic three channel CRT
driver and triple bias clamp suitable for low-cost color monitor applications. The LM2476 operates with 85V and 8V
power supplies. The part is housed in the 19-lead TO-247
molded plastic power package. The pinout and internal connection diagram is shown in Figure 1.
The CRT Driver circuit diagram 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 −27. 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. The typical driver DC
transfer function is shown in Figure 5.
The Bias Clamp circuit diagram is shown in Figure 3. The
clamp circuit amplifies the DC inputs, VINC, by the internally
fixed gain of –16.5. Each clamp output, VOUTC, will require a
pull-up resistor to VCC. The typical clamp DC transfer function is shown in Figure 6.
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
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LM2476
Application Hints
(Continued)
20121910
FIGURE 12. One Channel of the LM2476 with the Recommended Application Circuit
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 (75V 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 LM2476 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is about 6W (from
Figure 11, 95 MHz bandwidth), then a maximum heat sink
thermal resistance can be calculated:
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 12, 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 14 can be used as a good starting
point for the evaluation of the LM2476. 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 resistor can be replaced with a fixed value.
EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
output offset of the device is varied from 50 to 60VDC. The
rise time shows a maximum variation of 13% relative to the
center data point (55 VDC). The fall time shows a maximum
variation of less than 3% relative to the center data point.
PACKAGE MOUNTING CONSIDERATIONS
Mounting of the TO-247 package to a heat sink must be
done such that there is sufficient pressure from the mounting
screws to insure good contact with the heat sink for efficient
heat flow. The surface of the heat sink should be free of
contaminants before mounting to insure good contact. Incorrect mounting may lead to both thermal and mechanical
problems. Over tightening the mounting screws will cause
the package to warp, reducing contact area with the heat
sink. Less contact with the heat sink will increase the thermal
resistance from the package case to the heat sink (θCS)
resulting in higher operating die temperatures. Extreme over
tightening of the mounting screws will cause severe physical
stress resulting in a cracked or chipped package and possible catastrophic IC failure.
The recommended mounting screw size is M3 with a maximum torque of 50 N-cm. It is best to use fiber washers under
the screws to distribute the force over a wider area. Additionally, if the mounting screws are used to force the package into correct alignment with the heat sink, package stress
EFFECT OF LOAD CAPACITANCE
Figure 9 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.
THERMAL CONSIDERATIONS
Figure 10 shows the performance of the LM2476 in the test
circuit shown in Figure 4 as a function of case temperature
(on the device tab). The figure shows that the rise and fall
times of the LM2476 increase by approximately 10% and
5%, respectively, as the case temperature increases from
50˚C to 100˚C. This corresponds to a speed degradation of
2% and 1%, respectively, for every 10˚C rise in case temperature.
Figure 11 shows the maximum power dissipation of the
LM2476 vs. Frequency when all three channels of the device
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LM2476
Application Hints
Pease, Robert A., “Troubleshooting Analog Circuits”,
Butterworth-Heinemann, 1991.
(Continued)
will be increased. This increase in package stress will result
in reduced contact area with the heat sink increasing die
operating temperature and possible catastrophic IC failure.
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.
TYPICAL APPLICATION
A typical application of the LM2476 is shown in Figure 14.
Used in conjunction with an LM123X/4X preamplfier, 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. Please
see the next two sections below for hints on how to properly
evaluate the LM123X/4X and LM2476 combination in a
monitor. Figure 13 shows the typical cathode response for
this application. The peaking component values used are
shown in Figure 12 and Figure 14.
NSC DEMONSTRATION BOARD
Figure 14 is the schematic for the NSC LM1276/3X/4X_2476
Demonstration PCB that can be used to evaluate the
LM1276, LM123X, or LM124X preamp with the LM2476 in a
monitor. Figure 15 shows the routing and component placement on the NSC Demonstration PCB. 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:
• C26 — VCC bypass capacitor, located very close to Pin 14
and GND Pins
• C27 — VBB bypass capacitors, located close to Pin 2 and
GND Pins
• C28, C30, C33 — VCC bypass capacitors, near LM2476
and VCC clamp diodes. These are very important for arc
protection.
The routing of the LM2476 driver outputs to the CRT is very
critical to achieving optimum performance. Figure 16 shows
the routing and component placement from Pin 17 of the
LM2476 to the Red cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2476 to the Red cathode pin of the CRT connector. This
is done to minimize the length of the video path between
these two components for optimal performance and to minimize EMI. Note also that L4, D9, D13 and R23, R19, D10 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 D13 is connected directly to a section of the ground trace that has a
short and direct path to CRT ground to shunt arcover current
away from the LM2476. The cathode of D9 is connected to
VCC very close to decoupling capacitor C30 (see Figure 16)
which is connected to the same section of the ground trace
as D13. The diode placement and routing is very important
for minimizing the voltage stress on the LM2476 during an
arcover event. R37 and C29 are placed very close to the
clamp output on Pin 11. These components will help limit the
current and voltage stress to the clamp output. Lastly, notice
that S2 is placed very close to the Red cathode and is
connected directly to CRT ground.
20121916
40VP-P (35V – 75V)
FIGURE 13. Typical Cathode Response
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, use single-point grounds systems with adequate ground planes, isolate between channels, apply good supply bypassing, and minimize unwanted
feedback and parasitic capacitance. Also, the length of the
video signal traces from the preamplifier to the LM2476 and
from the LM2476 to the CRT cathodes 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.
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Application Hints
(Continued)
FIGURE 14. NSC Demonstration PCB Schematic
20121911
LM2476
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LM2476
Application Hints
(Continued)
20121913
FIGURE 15. NSC Demonstration PCB Layout
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LM2476
Application Hints
(Continued)
20121914
FIGURE 16. Trace Routing and Component Placement for Red Channel Output
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LM2476 Monolithic Triple Channel 6.5 ns High Gain CRT Driver and Bias Clamp
Physical Dimensions
inches (millimeters) unless otherwise noted
CONTROLLING DIMENSION IS INCH
VALUES IN [
] ARE MILLIMETERS
NS Package Number TB19A
Order Number LM2476
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.
For the most current product information visit us at www.national.com.
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