NSC LM2402T Datasheet

LM2402 Monolithic Triple 3 ns CRT Driver

General Description

The LM2402 is an integrated high voltage CRT driver circuit designed for use in high resolution color monitor applications. The IC contains three high input impedance, wide band amplifiers which directly drive the RGB cathodes of a CRT. Each channelhas its gain internally set to −14 and can drive CRT capacitive loads as well as resistive loads presented 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 on page 5.

Features

n Rise/fall times typically 3.0/2.8 ns with 8 pF load at

40 V

Schematic and Connection Diagrams

n Well matched with LM2202 video preamps n Output swing capability: 50 V 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

Applications

n CRT driver for color monitors with display resolutions up

to 1600 x 1200

n Pixel clock frequency up to 200 MHz

LM2402 Monolithic Triple 3 ns CRT Driver

August 1999

for V

80V

DS101016-1

FIGURE 1. Simplified Schematic Diagram

(One Channel)

Top View

Order Number LM2402T

DS101016-2

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.

Supply Voltage, V Bias Voltage, V Input Voltage, V

Storage Temperature Range, T Lead Temperature (Soldering,

−0.5V to V

−65˚C to +150˚C

STG

10 sec.) 300˚C

BIAS

+90V +16V

+ 0.5V

ESD Tolerance

Human Body Model 2 kV Machine Model 250V

Recommended Operating Conditions

OUT(VCC

80V, V

(Note 3)

12V) +17V to +72V

+60V to +85V

+8V to +15V

+1V to +5V

Case Temperature −20˚C to +100˚C Do not operate the part without a heat sink.

Electrical Characteristics

(See

Figure 2

Unless otherwise noted: V

for Test Circuit)

Symbol Parameter Conditions

I I V A ∆A

CC BB

OUT V

Supply Current Per Channel, No Output Load 22 27 32 mA Bias Current All Three Channels 40 50 60 mA DC Output Voltage V DC Voltage Gain −12 −14 −16

Gain Matching (Note 4) 1.0 dB LE Linear Error (Notes 4, 5) 3.5 t

Rise Time 10%to 90%,40VPPOutput (1 MHz) 3.0 ns

Fall Time 10%to 90%,40VPPOutput (1 MHz) 2.8 ns OS Overshoot 40 V

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 Note 6: Input from signal generator: t

+80V, V

r,tf

1 ns.

+12V, V

+3.3 V

DC,CL

8 pF, T

60˚C.

LM2402

Min Typ Max

1.9V 62 65 68 V

Output (1 MHz) 5

1.5V to V

5V.

Units

AC Test Circuit

FIGURE 2. Test Circuit (One Channel)

Figure 2

LM2402. This circuit is designed for testing the LM2402 with a FET probe. When calculating the total load capacitance,

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shows a typical test circuit for evaluation of the

DS101016-3

the Tektronix P6201 FET probe with a 100:1 divider is specified to have 1.5 pF. The total board capacitance should be

6.5 pF.

Typical Performance Characteristics

FIGURE 3. VINvs V

OUT

FIGURE 4. Speed vs Temp.

DS101016-4

DS101016-6

DS101016-5

FIGURE 6. Power Dissipation vs Frequency

DS101016-7

FIGURE 7. Speed vs Offset

FIGURE 5. Rise/Fall Time

DS101016-8

DS101016-9

FIGURE 8. Bandwidth

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Theory of Operation

The LM2402 is a high voltage monolithic three channel CRT driver suitable for very high resolution display applications, up to 1600 x 1200 at 85 Hz refresh rate. The LM2402 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 LM2402 is shown in

Figure 1

.APNP emitter follower,Q5, provides input buffering. This minimizes the current loading of the video pre-amp. R9 is used to turn off Q5 when there is no input. This will drive the output stage to the V power dissipation with no inputs. R6 is a pull-up resistor for

rail, minimizing the

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 LM2402. R1, R2 and R3 are all related when calculating the output voltage of the CRT driver. R Q1 and Q2 are in a cascade configuration. Q1 is a low volt-

limits the current through the base of Q2.

age and very fast transistor.Q2 is a higher voltage transistor. The cascade configuration gives the equivalent of a very fast and high voltage transistor. The two output transistors, 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 LM2402. 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.

Figure 2

shows a typical test circuit for evaluation of the LM2402. Due to the very wide bandwidth of the LM2402, it is necessary to use a FET probe that is DC coupled to the output for evaluation of the CRT driver’s performance. The 50Ω resistor is used to duplicate the required 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.

Application Hints

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 LM2402’s supply and ground pins. A 0.1 µF capacitor should be connected from the bias pin, V tical to the part.

, to ground, as close as is prac-

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 groundwill limit the maximum voltage, but to a value that is much higher than allowable on the LM2402. This fast, high voltage, high energy pulse can damage the LM2402 output stage. The application circuit shown in

Figure 9

is designed to help clamp the voltage at the output of the LM2402 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 respectively.The cathode of D1 should be located very close

and ground

to a separately decoupled bypass capacitor. The ground connection of the diode and the decoupling capacitor should be very close to the LM2402 ground.This will significantly reduce the high frequency voltage transients that the LM2402 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 LM2402 as well as the voltage stress at the outputs of the device. R2 should be a carbon type resistor. R1 can be a

⁄4W metal or carbon film

⁄2W solid

type resistor. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2402 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 43Ω, with L1=.049 µH.

INTRODUCTION

National Semiconductor (NSC) is committed to providing application information that assists our customers in obtaining the best performance possible fromour 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.

POWER SUPPLY BYPASS

Since the LM2402 is a very high bandwidth amplifier, proper power supply bypassing is critical for optimum performance. 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 the supply and ground pins as is practical. Additionally, a

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, to ground, as close to

DS101016-10

FIGURE 9. One Channel of the LM2402 with the

Recommended Arc Protection Circuit.

OPTIMIZING TRANSIENT RESPONSE

Figure 9

Referring to

, 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. Air core inductors from J.W. Miller Magnetics (part #75F518MPC) were used for optimizing the perfor-

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