LM2412A_技术文档

February 2001

LM2412A

Monolithic Triple 2.8 ns CRT Driver

n Faster Maximum Rise/Fall time than LM2412, 3.2nsec

vs 3.5nsec.

n Well matched with LM2202 video preamps

n Output swing capability: 50 V_PPfor V_CC= 80V

n 1V to 5V input range

n Stable with 0-20 pF capacitive loads and inductive

peaking networks

n Convenient TO-220 staggered lead package style

n Standard LM240X family pinout which is designed for

easy PCB layout

General Description

The LM2412A is an integrated high voltage CRT driver circuit

designed for use in high resolution color monitor applica-

tions. 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 −14 and can

drive CRT capacitive loads as well as resistive loads pre-

sented by other applications, limited only by the package’s

power dissipation.

The IC is packaged in an industry standard 11 lead TO-220

molded plastic power package. See thermal considerations

section for heat sinking requirements.

Applications

n CRT driver for color monitors with display resolutions up

to 1600 x 1200 with 85 Hz refresh rate

Features

n Rise/fall times typically 2.8 ns with 8 pF load at 40 V_PP

n Pixel clock frequency up to 200 MHz

Schematic and Connection Diagrams

DS200124-1

DS200124-2

FIGURE 1. Simplified Schematic Diagram

(One Channel)

Top View

Order Number LM2412ATA

See NS package Number

DS200124

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Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

ESD Tolerance

Human Body Model

Machine Model

2 kV

250V

Operating Ranges (Note 3)

Supply Voltage, V_CC

+90V

+16V

Bias Voltage, V_BB

V_CC

+60V to +85V

+8V to +15V

Input Voltage, V_IN

0V to 6V

V_BB

Storage Temperature Range, T_STG

−65˚C to +150˚C

300˚C

V_IN

+1V to +5V

<

Lead Temperature (Soldering, 10 sec.)

V_OUT(V_CC= 80V, V_BB= 12V)

Case Temperature

+15V to +75V

−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, V_BB= +12V, V_IN= +3.3 V_DC, C_L= 8 pF, T_C= 60˚C, no AC input.

LM2412A

Typ

21

Symbol

I_CC

Parameter

Supply Current

Conditions

Units

Min

16

Max

26

Per Channel, No Output Load

All Three Channels

V_IN= 1.9V

mA

I_BB

V_OUT

A_V

∆A_V

LE

t_r

Bias Current

27

42

57

DC Output Voltage

DC Voltage Gain

Gain Matching

62

65

68

V_DC

−12

−14

1.0

−16

(Note 4)

dB

%

Linearity Error

(Notes 4, 5)

3.5

Rise Time (Notes 6, 7)

Fall Time (Notes 6, 7)

Overshoot

10% to 90%, 40 V_PPOutput (1 MHz)

40 V_PPOutput (1 MHz)

2.8

3.2

ns

%

t_f

2.8

OS

5

Note 1: “Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaranteed. They are not meant to imply that the devices

should be operated at these limits. The table of “Electrical Characteristics” specifies conditions of device operation.

Note 2: All voltages are measured with respect to GND, unless otherwise specified.

Note 3: 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 4: Calculated value from voltage gain test on each channel.

Note 5: Linearity error is the variation in DC gain from V = 1.6V to V = 5.0V.

IN

<

f

Note 6: Input from signal generator: t , t

1 ns.

r

Note 7: 100% tested in production. These limits are not used to calculate outgoing quality levels.

AC Test Circuit

DS200124-3

FIGURE 2. Test Circuit (One Channel)

Figure 2 shows a typical test circuit for evaluation of the

LM2412A. This circuit is designed to allow testing of the

LM2412A in a 50Ω environment without the use of an ex-

pensive FET probe. The combined resitors of 4950Ω at the

output form a 200:1 voltage divider when connected to a

50Ω load. The test board supplied by NSC also offers the

option to test theLM2412A with a FET probe. C_Lis the total

capacitance at the LM2412A output, including the board

capacitance.

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2

Typical Performance Characteristics

DS200124-5

DS200124-4

FIGURE 6. Power Dissipation vs Frequency

FIGURE 3. V_INvs V_OUT

DS200124-6

DS200124-7

FIGURE 4. Speed vs Temp.

FIGURE 7. Speed vs Offset

DS200124-8

DS200124-9

FIGURE 5. Rise/Fall Time

FIGURE 8. Bandwidth

3

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POWER SUPPLY BYPASS

Theory of Operation

Since the LM2412A is a very high bandwidth amplifier,

proper power supply bypassing is critical for optimum per-

formance. Improper power supply bypassing can result in

large overshoot, ringing and oscillation. A 0.1 µF capacitor

should be connected from the supply pin, V_CC, to ground, as

close to the supply and ground pins as is practical. Addition-

ally, a 10 µF to 100 µF electrolytic capacitor should be

connected from the supply pin to ground. The electrolytic

capacitor should also be placed reasonably close to the

LM2412A’s supply and ground pins. A 0.1 µF capacitor

should be connected from the bias pin, V_BB, to ground, as

close as is practical to the part.

The LM2412A is a high voltage monolithic three channel

CRT driver suitable for very high resolution display applica-

tions, up to 1600 x 1200 at 85 Hz refresh rate. The LM2412A

operates using 80V and 12V power supplies. The part is

housed in the industry standard 11-lead TO-220 molded

plastic power package.

The simplified circuit diagram of one channel of the

LM2412A is shown in Figure 1. A PNP emitter follower, Q5,

provides input buffering. This minimizes the current loading

of the video pre-amp. R9 is used to turn on Q5 when there is

no input. With Q5 turned on, Q1 will be almost completely

off, minimizing the current flow through Q1 and Q2. This will

drive the output stage near the V_CCrail, minimizing the

power dissipation with no inputs. R6 is a pull-up resistor for

Q5 and also limits the current flow through Q5. R3 and R2

are used to set the current flow through Q1 and Q2. The ratio

of R1 to R2 is used to set the gain of the LM2412A. R1, R2

and R3 are all related when calculating the output voltage of

the CRT driver. R_blimits the current through the base of Q2.

Q1 and Q2 are in a cascode configuration. Q1 is a low

voltage and very fast transistor. Q2 is a higher voltage

transistor. The cascode configuration gives the equivalent of

a very fast and high voltage transistor. The two output tran-

sistors, Q3 and Q4, form a class B amplifier output stage. R4

and R5 are used to limit the current through the output stage

and set the output impedance of the LM2412A. Q6, along

with R7 and R8 set the bias current through Q3 and Q4 when

there is no change in the signal level. This bias current

minimizes the crossover distortion of the output stage. With

this bias current the output stage now becomes a class AB

amplifier with a crossover distortion much lower than a class

B amplifier.

ARC PROTECTION

During normal CRT operation, internal arcing may occasion-

ally 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 LM2412A. This fast, high voltage, high energy pulse can

damage the LM2412A output stage. The application circuit

shown in Figure 9 is designed to help clamp the voltage at

the output of the LM2412A to a safe level. The clamp diodes

should have a fast transient response, high peak current

rating, low series impedance and low shunt capacitance.

FDH400 or equivalent diodes are recommended. D1 and D2

should have short, low impedance connections to V_CCand

ground respectively. The cathode of D1 should be located

very close to a separately decoupled bypass capacitor. The

ground connection of the diode and the decoupling capacitor

should be very close to the LM2412A ground. This will

significantly reduce the high frequency voltage transients

that the LM2412A 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 LM2412A as

well as the voltage stress at the outputs of the device. R2

Figure 2 shows a typical test circuit for evaluation of the

LM2412A. Due to the very wide bandwidth of the LM2412A,

it is highly recommended that the stand alone board suplied

by NSC be used for the evaluation of the CRT driver’s

performance. The 50Ω resistor is used to duplicate the re-

quired series resistor in the actual application. This resistor

would be part of the arc-over protection circuit. The input

signal from the generator is AC coupled to the input of the

CRT driver.

should be a ¹⁄

2

W solid carbon type resistor. R1 can be a ¹⁄

W

4

metal or carbon film type resistor. Inductor L1 is critical to

reduce the initial high frequency voltage levels that the

LM2412A would be subjected to during an arc-over. Having

large value resistors for R1 and R2 would be desirable, but

this has the effect of increasing rise and fall times. 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. The values of L1

and R1 may need to be adjusted for a particular application.

The recommended minimum value for R1 is 75Ω, with L1 =

.049 µH.

Application Hints

INTRODUCTION

National Semiconductor (NSC) is committed to providing

application information that assists our customers in obtain-

ing 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 optimiza-

tion 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.

DS200124-10

FIGURE 9. One Channel of the LM2412A with the

Recommended Arc Protection Circuit.

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4

If the monitor designer chooses to calculate the power dis-

sipation for the LM2412A using an active video time different

from 72%, then he needs to use the following steps when

using a 1.9V input black level:

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 re-

sponse of the application circuit. Increasing the values of R1

and R2 will slow the circuit down while decreasing over-

shoot. Increasing the value of L1 will speed up the circuit as

well as increase overshoot. It is very important to use induc-

tors with very high self-resonant frequencies, preferably

above 300 MHz. Air core inductors from J.W. Miller Magnet-

ics (part #75F518MPC) were used for optimizing the perfor-

mance 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 LM2412A.

1. Multiply the black level power dissipation, 2.7W, by 0.28,

the result is 0.8W.

2. Choose the maximum frequency to be used. A typical

application would use 100 MHz, or a 200 MHz pixel

clock. The power dissipation is 13.8W.

3. Subtract the 0.8W from the power dissipation from Fig-

ure 6. For 100 MHz this would be 13.8 – 0.8 = 13.0W.

4. Divide the result from step 3 by 0.72. For 100 MHz, the

result is 18.1W.

5. Multiply the result in 4 by the new active time percent-

age.

Effect of Load Capacitance

6. Multiply 2.7W by the new inactive time.

The output rise and fall times as well as overshoot will vary

as the load capacitance varies. The values of the output

circuit (R1, R2 and L1 in Figure 9) should be chosen based

on the nominal load capacitance. Once this is done the

performance of the design can be checked by varying the

load based on what the expected variation will be during

production.

7. Add together the results of steps 5 and 6. This is the

expected power dissipation for the LM2412A in the de-

signer’s application.

The LM2412A case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

70˚C and the maximum power dissipation is 13.8W (from

Figure 6. 100MHz) then a maximum heat sink thermal resis-

tance can be calculated:

Effect of Offset

Figure 7 shows the variation in rise and fall times when the

output offset of the device is varied from 35 to 55 V_DC. The

rise and fall times show about the same overall variation.

The slightly slower fall time is fastest near the center point of

45V, making this the optimum operating point. At the low and

high output offset range, the characteristic of rise/fall time is

slower due to the saturation of Q3 and Q4. The recovery

time of the output transistors takes longer coming out of

saturation thus slows down the rise and fall times.

TYPICAL APPLICATION

A typical application of the LM2412A is shown in Figure 10.

Used in conjunction with three LM2202s, a complete video

channel from monitor input to CRT cathode can be achieved.

Performance is excellent for resolutions up to 1600 x 1200

and pixel clock frequencies at 200 MHz. Figure 10 is the

schematic for the NSC demonstration board that can be

used to evaluate the LM2202/LM2412A combination in a

monitor.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2412A in the test

circuit shown in Figure 2 as a function of case temperature.

Figure 4 shows that both the rise and fall times of the

LM2412A become slightly longer as the case temperature

increases from 40˚C to 125˚C. In addition to exceeding the

safe operating temperature, the rise and fall times will typi-

cally exceed 3 nsec. Please note that the LM2412A is never

to be operated over a case temperature of 100˚C. In

addition to exceeding the safe operating temperature, the

rise and fall times will typically exceed 3 nsec.

PC Board Layout Considerations

For optimum performance, an adequate ground plane, iso-

lation between channels, good supply bypassing and mini-

mizing unwanted feedback are necessary. Also, the length of

the signal traces from the preamplifier to the LM2412A and

from the LM2412A to the CRT cathode should be as short as

possible. The red video trace from the buffer transistor to the

LM2412A input is about the absolute maximum length one

should consider on a PCB layout. If possible the traces

should actually be shorter than the red video trace. The

following references are recommended for video board de-

signers:

Figure 6 shows the total power dissipation of the LM2412A

vs. Frequency when all three channels of the device are

driving an 8 pF load. Typically the active time is about 72% of

the total time for one frame. Worst case power dissipation is

when a one on, one off pixel is displayed over the active time

of the video input. This is the condition used to measure the

total power disspation of the LM2412A at different input

frequencies. Figure 6 gives all the information a monitor

designer normally needs for worst case power dissipation.

However, if the designer wants to calculate the power dissi-

pation for an active time different from 72%, this can be done

using the information in Figure 14. The recommended input

black level voltage is 1.9V. From Figure 14, if a 1.9V input is

used for the black level, then power dissipation during the

inactive video time is 2.7W. This includes both the 80V and

12V supplies.

Ott, Henry W., “Noise Reduction Techniques in Electronic

Systems”, John Wiley & Sons, New York, 1976.

“Guide to CRT Video Design”, National Semiconductor Ap-

plication Note 861.

“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.

5

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cathode. Note that the components are placed so that they

almost line up from the output pin of the LM2412A to the blue

cathode pin of the CRT connector. This is done to minimize

the length of the video path between these two components.

The direct video path is shown in by a dark gray line through

the components and the PCB traces. Note also that D24,

D25, R58 and D19 are placed to keep the size of the video

nodes to a minimum (R58 is located under D19). This mini-

mizes parasitic capacitance in the video path and also en-

hances the effectiveness of the protection diodes. The traces

in the video nodes to these components are shown by the

white line. The anode of protection diode D25 is connected

directly to the ground plane giving a short and direct path to

the LM2412A ground pins. The cathode of D24 is connected

to V_CCvery close to decoupling capacitor C78 (Figure 13)

which is connected to the same section of the ground plane

as D25. The diode placement and routing is very important

for minimizing the voltage stress on the LM2412A during an

arc-over event. Lastly, notice that S3 is placed very close to

the blue cathode and is tied directly to CRT ground.

Application Hints (Continued)

NSC Demonstration Board

Figures 11, 12 show routing and component placement on

the NSC LM2202/2412 demonstration board. The schematic

of the board is shown in Figure 10. 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 compo-

nents:

•

C47 - V_CCbypass capacitor, located very close to pin 6

and ground pins. (Figure 12)

C49 - V_BBbypass capacitor, located close to pin 10 and

ground. (Figure 12)

C46 and C77 - V_CCbypass capacitors, near LM2412A

and V_CCclamp diodes. Very important for arc protection.

(Figure 11)

The routing of the LM2412A outputs to the CRT is very

critical to achieving optimum performance. Figure 13 shows

the routing and component placement from pin 1 to the blue

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6

Application Hints (Continued)

7

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Application Hints (Continued)

DS200124-13

FIGURE 11. PCB Top Layer

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8

Application Hints (Continued)

DS200124-14

FIGURE 12. PCB Bottom Layer

9

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Application Hints (Continued)

DS200124-15

FIGURE 13. PCB CRT Driver, Blue Channel Output

DS200124-16

FIGURE 14. I_CCand I_BBvs V_IN

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10

Physical Dimensions inches (millimeters) unless otherwise noted

11 Lead Molded TO-220

NS Package Number TA11B

Order Number LM2412ATA

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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

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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Tel: 1-800-272-9959

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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.


型号：	LM2412A
厂家：	National Semiconductor
描述：	Monolithic Triple 2.8 ns CRT Driver 单片三重2.8纳秒的CRT驱动器驱动器
文件：	总11页 (文件大小：818K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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