NSC LM2451TB

LM2451
220V Monolithic Triple Channel 12 MHz DC Coupled CRT
DTV Driver
General Description
Features
The LM2451 is a triple channel high voltage DC coupled
CRT driver circuit designed for use in DTV applications. The
IC contains three high input impedance, wide band amplifiers which directly drive the RGB cathodes of a CRT. Each
amplifier has a summing input where the DC level of the
output is controlled by a low voltage DC input voltage. Normally the DC input voltage is from a DAC. Each channel has
its gain internally set to −54 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 15-lead TO-247 molded plastic
power package designed specifically to meet high voltage
spacing requirements. See Thermal Considerations section.
12 MHz bandwidth at 130VP-P
100V black level adjustment range using 0V to 5V input
Current output for IK feedback systems
Greater than 130VP-P output swing capability
0V to 5V input voltage range
Stable with 0 pF–20 pF capacitive loads and inductive
peaking networks
n Convenient TO-247 staggered thin lead package style
Connection Diagram
Schematic Diagram
n
n
n
n
n
n
Applications
n DC coupled DTV applications using the 480p format as
well as standard NTSC and PAL formats.
20144401
20144402
FIGURE 1. Top View
Order Number LM2451TB
See NS Package Number TB15A
© 2005 National Semiconductor Corporation
DS201444
FIGURE 2. Simplified Schematic Diagram
(One Channel)
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LM2451 220V Monolithic Triple Channel 12 MHz DC Coupled CRT DTV Driver
December 2005
LM2451
Absolute Maximum Ratings
Junction Temperature
(Notes 1,
150˚C
θJC (typ)
3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (VCC)
Operating Ratings (Note 2)
+250V
Bias Voltage (VBB)
+16V
Input Voltage (VIN)
−0.5V to VBB +0.5V
Storage Temperature Range (TSTG)
300˚C
2 kV
200V
+7V to +13V
+0V to +5V
+40V to +215V
Case Temperature
(10W max power)
110˚C
Do not operate the part without a heat sink and thermal
grease. Heat sink must have a thermal resistance under
5.0˚C/W. (Note 7)
ESD Tolerance,
Machine Model
+100V to +230V
VBB
VOUT
−65˚C to +150˚C
Human Body Model
VCC
VIN
Lead Temperature
(Soldering, < 10 sec.)
4.0˚C/W
Electrical Characteristics
(See Figure 3 for Test Circuit). Unless otherwise noted: VCC = +220V, VBB = +12V, VDAC = +0.5V, CL = 10 pF, TC = 50˚C. DC
Tests: VIN = +2.7VDC. AC Tests: Output = 130VPP (60V – 190V) at 1 MHz.
Symbol
Parameter
LM2451
Conditions
Min
Typ
Max
No Input Signal, No Video Input, No
Output Load
10
18
25
Units
ICC
Supply Current
IBB
Bias Current
18
26
34
mA
VOUT, 1
DC Output Voltage
No AC Input Signal, VIN = 2.7VDC
122
127
132
VDC
VOUT, 2
DC Output Voltage
No AC Input Signal, VIN = 1.2VDC
200
205
210
VDC
VOUT, 3
DC Output Voltage
No AC Input Signal, VIN = 1.2VDC,
VDAC = 1.2VDC
192
198
204
VDC
VOUT, 4
DC Output Voltage
No AC Input Signal, VIN = 1.2VDC,
VDAC = 2.7VDC
154
160
166
VDC
AV
DC Voltage Gain
No AC Input Signal
−51
−54
−57
V/V
ADAC
DAC Input DC Voltage Gain
No AC Input Signal
−23
−26
−29
V/V
∆AV
Gain Matching
(Note 4), No AC Input Signal
LE
Linearity Error
(Notes 4, 5), No AC Input Signal
8
%
tr
Rise Time
(Note 6), 10% to 90%
31
ns
10
%
(Note 6), 90% to 10%
34
ns
1.0
+OS
Overshoot
tf
Fall Time
−OS
Overshoot
(Note 6)
BWL
Large Signal Bandwidth
VOUT
BWM
Medium Signal Bandwidth
VOUT
BWS
Small Signal Bandwidth
VOUT
IkERROR
Current Output Error
Output Current = 0 µA to 200 µA
∆IkERROR
Current Output Difference
Between Channels
Output Current = 0 µA to 200 µA
AC
= 130 VP-P, VOUT
DC
= 125 V
AC
= 100 VP-P, VOUT
DC
= 125 V
AC
= 60 VP-P, VOUT
DC
= 125 V
mA
dB
3
%
12
MHz
14
MHz
16
MHz
−52
0
52
µA
0
NA
32
µA
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: Linearity Error is the variation in DC gain from VIN = 1.10V to VIN = 4.30V.
Note 6: Input from signal generator: tr, tf < 10 ns.
Note 7: Running the 1 MHz to 30 MHz test pattern at 1080i this part will dissipate approximately 10 W. This is the commonly accepted test pattern that is
representative of the worst case high frequency content for normal television viewing. This is the pattern used to estimate the worst case power dissipation of the
LM2451 in its normal application. It is recommended to use a heat sink with a thermal resistance of 5.0˚C/W or better.
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2
LM2451
AC Test Circuit
20144403
Note: 10 pF load includes parasitic capacitance.
FIGURE 3. Test Circuit (One Channel)
Figure 3 shows a typical test circuit for evaluation of the LM2451. This circuit is designed to allow testing of the LM2451 in a 50Ω
environment without the use of an expensive FET probe. The two 4990Ω resistors form a 400:1 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 network to achieve flat frequency response.
3
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LM2451
Typical Performance Characteristics
(VCC = +220VDC, VBB = +12VDC, CL = 10 pF, VOUT = 130VPP
(60V – 190V), TC = 50˚C, Test Circuit — Figure 3 unless otherwise specified)
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FIGURE 4. VOUT vs VIN
FIGURE 7. Speed vs Load Capacitance
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20144408
FIGURE 5. LM2451 Pulse Response
FIGURE 8. Speed vs Offset
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20144409
FIGURE 6. Bandwidth
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FIGURE 9. Speed vs Case Temperature
4
(VCC = +220VDC, VBB = +12VDC, CL = 10 pF, VOUT = 130VPP
(60V – 190V), TC = 50˚C, Test Circuit — Figure 3 unless otherwise specified)
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20144411
FIGURE 10. Power Dissipation vs Frequency
FIGURE 11. Safe Operating Area
20144412
FIGURE 12. LM2451 Cathode Response
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LM2451
Typical Performance Characteristics
LM2451
ARC PROTECTION
During normal CRT operation, internal arcing may occasionally occur. This fast, high voltage, high-energy pulse can
damage the LM2451 output stage. The application circuit
shown in Figure 13 is designed to help clamp the voltage at
the output of the LM2451 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. 1SS83 or equivalent diodes are recommended. 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 13). The ground connection of D2 and the
decoupling capacitor should be very close to the LM2451
ground. This will significantly reduce the high frequency
voltage transients that the LM2451 would be subjected to
during an arc over condition. Resistor R2 limits the arc over
current that is seen by the diodes while R1 limits the current
into the LM2451 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 LM2451 would be subjected to before the
clamp diodes have a chance to became activated. 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 13. NOTE: Do
not use spark gaps with the LM2451 when designing DC
coupled system. If a spark gap is triggered it shorts out the
output of the LM2451 and will destroy the part.
Theory of Operation
The LM2451 is a high voltage monolithic three channel CRT
driver suitable for DTV applications. The LM2451 operates
with 220V and 12V power supplies. The part is housed in a
15-lead TO-247 molded plastic power package with thin
leads for improved metal-to-metal spacing.
The circuit diagram of the LM2451 is shown in Figure 2. 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 −54. An additional cascode amplifier is
formed by Q7 and Q2. Gain of this stage is set to — 25 by
resistors R1 and R10. Q8 provides the input buffering for this
input. Q2 now becomes the summing point for both VIN and
VDAC. 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.
Figure 3 shows a typical test circuit for evaluation of the
LM2451. This circuit is designed to allow testing of the
LM2451 in a 50Ω environment without the use of an expensive FET probe. In this test circuit, the two 4.99 kΩ resistors
form a 400:1 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 video inputs of the LM2451.
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.
20144413
FIGURE 13. One Channel of the LM2451 with the
Recommended Application Circuit
IMPORTANT INFORMATION
The LM2451 performance is targeted for the HDTV market.
The application circuits shown in this document to optimize
performance and to protect against damage from CRT arc
over are designed specifically for the LM2451. If another
member of the LM245X family is used, please refer to its
datasheet.
EFFECT OF LOAD CAPACITANCE
Figure 7 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. Increasing the load capacitance from 10 pF to 20 pF adds about
14 ns to the rise time and about 18 ns to the fall time. It is
very important to keep the board capacitance as low as
possible to maximize the speed of the driver.
POWER SUPPLY BYPASS
Since the LM2451 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 LM2451 as is practical. Additionally, a 22 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2451.
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EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
black level of the device is varied from 180V to 200VDC. The
rise time increases only about 3 ns as the offset is increased
in voltage and the fall time decreases by about 7 ns with the
same offset adjustment.
6
placed with a fixed value. Due to arc over considerations it is
recommended that the values shown in Figure 13 not be
changed by a large amount.
(Continued)
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2451 in the test
circuit shown in Figure 3 as a function of case temperature.
The figure shows that the rise time of the LM2451 increases
by about 7ns as the case temperature increases from 30˚C
to 110˚C. Over the same case temperature range the fall
time increased by about 9 ns.
Figure 10 shows the maximum power dissipation of the
LM2451 vs. Frequency when all three channels of the device
are driving into a 10 pF load with a 130VP-P alternating one
pixel on, one pixel off. Note that the frequency given in
Figure 10 is half of the pixel frequency. The graph assumes
an 80% active time (device operating at the specified frequency), which is typical in a TV application. The other 20%
of the time the device is assumed to be sitting at the black
level (190V in this case). A TV picture will not have frequency
content over the whole picture exceeding 15 MHz. It is
important to establish the worst case condition under normal
viewing to give a realistic worst-case power dissipation for
the LM2451. One test is a 1 to 30 MHz sine wave sweep
over the active line. This would give a slightly lower power
than taking the average of the power between 1 and 30 MHz.
This average is 10.4 W. A sine wave will dissipate slightly
less power, probably about an even 10W of power dissipation. All of this information is critical for the designer to
establish 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 LM2451 case temperature must be maintained below
110˚C given the maximum power dissipation estimate of
10W. If the maximum expected ambient temperature is 60˚C
and the maximum power dissipation is 10W then a maximum
heat sink thermal resistance can be calculated:
Figure 12 shows the typical cathode pulse response with an
output swing of 130VPP inside a modified production TV set
using the LM1237 pre-amp.
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 signal inputs to the LM2451 and
from the LM2451 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 TV 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 LM2451 is shown in Figure 14.
Used in conjunction with a pre-amp with a 1.2V black level
output no buffer transistors are required to obtain the correct
black level at the cathodes. If the pre-amp has a black level
closer to 2V, then an NPN transistor should be used to drop
the video black level voltage closer to 1.2V. When using only
one NPN transistor as an emitter follower, a jumper needs to
be added in each channel. In the red channel a jumper
needs to be added between C7 and R25. With just one
transistor neither of these components would be installed.
In addition to the video inputs are the DAC inputs. These
inputs are used to vary the LM2451 output black level by a
DAC. in the past when a driver was used with a CMOS AVP
there was not enough range on the video output to vary the
black level. A clamp circuit had to be used in conjunction with
the AVP and the driver. The DAC inputs of the LM2451 are
driven in the same way the clamp circuit had been driven,
eliminating the need for a clamp circuit. Figure 4 shows the
variation in the black level as the DAC input voltage is
changed. This is shown for both VIN = 1.2V and VIN = 2.1V.
The neck board in Figure 14 has two transistors in each
channel enabling this board to work with pre-amps with a
black level output as high as 2.5V. Each transistor stage has
a gain of −1. This setup still gives the two diode drop at the
driver input; however, now additional peaking can be done
on the video signal before reaching the driver inputs. Some
popular AVPs do have a black level of 2.5V. For lower black
levels either one or both transistors would not be used.
It is important that the TV designer use component values for
the driver output stage close to the values shown in Figure
14. These values have been selected to protect the LM2451
from arc over. Diodes D1,D8, D9, and D13–D15 must also
be used for proper arc over protection. The NSC demonstration board can be used to evaluate the LM2451 in a TV.
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if the load
capacitance is increased the AC component of the total
power dissipation will also increase. If the worst case power
dissipation is not 10W please refer to Figure 11 for the
maximum case temperature for the expected maximum
power dissipation.
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 13, 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 13 can be used as a good starting
point for the evaluation of the LM2451. 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 re-
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LM2451
Application Hints
LM2451
Application Hints
This feedback system consists of the preamp, LM2451, and
interface circuit, forming a closed loop to automatically adjust the black level of the drive signals to the cutoff point of
the RGB cathodes. Following is a description of the interface
circuit operation used for AVPs that have a voltage input for
their IK sense input.
The output at pin 8 of the LM2451 is filtered of high frequency noise by C14. D7 is used to limit the peak voltage at
pin 8. Without this clamp diode the voltage would easily
exceed 12V during active video when the cathode currents
are much greater than the small currents being detected
during vertical blanking. Exceeding 12V could damage Q4
and result in improper operation of the driver.
(Continued)
NSC DEMONSTRATION BOARD
Figure 15 shows the routing and component placement on
the NSC LM2451 demonstration board. 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 12
and ground pins
• C27 — VBB bypass capacitor, located close to pin 7 and
ground
• C28, C30, C33 — VCC bypass capacitors, near LM2451
and VCC clamp diodes. Very important for arc protection.
R35 is essential to convert the IK current to voltage. Choosing the value of R35 sets the gain of the feedback voltage,
and consequently, the operating point of the tube. Once a
stable operating point is established, this point can be finetuned using the adjustment range of the feedback system or
standard preamp controls. Changing the value of R35 will
change the cutoff voltage at the cathode. A smaller value of
R35 requires more IK current to maintain the feedback loop.
The cutoff voltage set at the cathode will be lower to adjust to
the higher IK current. This additional current must come from
the cathode; therefore, the cathode voltage is set lower to
meet higher current requirement. A higher value of R35 will
do the opposite, raising the cathode voltage because less IK
current is needed to maintain the same voltage at R35.
The emitter follower, Q7, isolates R35 from the input impedance of the preamp. R21 and R39 bias the emitter of Q7 to
limit the maximum voltage to the preamp. These resistor
values should be chosen to limit the maximum voltage at the
emitter and protect the preamp from any large voltages that
would otherwise occur during active video. C9 is used to AC
couple the IK signal to the preamp. The advantage of AC
coupling is that any DC component (leakage current from the
driver) of the IK signal is not detected by the IK sense input
of the preamp.
Some AVPs do have a direct current input for their IK sense
input. For interfacing to these AVPs the only components to
be used in the IK sense section are D7 and R41. To complete the signal path a jumper must be used to replace R34,
C9 and the base-emitter junction of Q7. C14 can still be used
for high frequency filtering.
The routing of the LM2451 outputs to the CRT is very critical
to achieving optimum performance. Figure 16 shows the
routing and component placement from pin 13 (VOUT3) of the
LM2451 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2451 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 D1, D8 and R36 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 D1 is connected directly to a section of the ground plane that has a
short and direct path to the CRT ground and the LM2451
ground pins. The cathode of D8 is connected to VCC very
close to decoupling capacitor C28 which is connected to the
same area of the ground trace as D1. The diode placement
and routing is very important for minimizing the voltage
stress on the LM2451 during an arc over event.
This demonstration board uses large PCB holes to accommodate socket pins, which function to allow for multiple
insertions of the LM2451 in a convenient manner. To benefit
from the enhanced LM2451 package with thin leads, the
device should be secured in small PCB holes to optimize the
metal-to-metal spacing between the leads.
CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS
The LM2451 can be used in DTV applications that use an IK
feedback system. Figure 14 shows an example of an interface circuit used to feed back the IK output of LM2451 to a
preamplifier with an ac coupled IK input.
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Application Hints
(Continued)
FIGURE 14. LM2451 DTV Applications Circuit
20144414
LM2451
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LM2451
Application Hints
(Continued)
20144416
FIGURE 15. LM2451 DTV Demonstration Board Layout
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10
LM2451
Application Hints
(Continued)
20144417
FIGURE 16. Trace Routing and Component Placement for Blue Channel Output
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LM2451 220V Monolithic Triple Channel 12 MHz DC Coupled CRT DTV Driver
Physical Dimensions
inches (millimeters) unless otherwise noted
NOTE: Available only with lead free plating
NS Package Number TB15A
Order Number LM2451TB
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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