Theory of Operation

The LM2462 is a high voltage monolithic three channel CRT

driver suitable for high resolution display applications. The

LM2462 operates with 80V and 12V power supplies. The

part is housed in the industry standard 11-lead TO-220

molded plastic power package.

The circuit diagram of the LM2462 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 changes in load capacitance. Q6

provides biasing to the output emitter follower stage to re-

duce crossover distortion at low signal levels.

Figure 2 shows a typical test circuit for evaluation of the

LM2462. This circuit is designed to allow testing of the

LM2462 in a 50

Ω environment without the use of an expen-

sive FET probe. In this test circuit, the two 2.49k

Ω resistors

form a 200:1 wideband, low capacitance probe when con-

nected to a 50

Ω coaxial cable and a 50Ω load (such as a

50

Ω oscilloscope input). The input signal from the generator

is ac coupled to the base of Q5.

Application Hints

INTRODUCTION

National Semiconductor (NSC) is committed to provide ap-

plication information that assists our customers in obtaining

the best performance possible from our products. The fol-

lowing information is provided in order to support this com-

mitment. 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 com-

ponent 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 per-

formance.

IMPORTANT INFORMATION

The LM2462 performance is targeted for the high end 19“

and 21“ market with resolutions up to 1600 x 1200 and an

85Hz refresh rate. The application circuits shown in this

document were specifically designed to optimize the perfor-

mance of the LM2462 as well as protect it from damage due

to a CRT arc-over. If another member of the LM246X family

is used, please refer to its datasheet.

POWER SUPPLY BYPASS

Since the LM2462 is a wide bandwidth amplifier, proper

power supply bypassing is critical for optimum performance.

Improper power supply bypassing can result in large over-

shoot, ringing or oscillation. 0.1 µF capacitors should be

connected from the supply pins, V

close to the LM2462 as is practical. Additionally, a 47 µF or

larger electrolytic capacitor should be connected from both

supply pins to ground reasonably close to the LM2462.

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 LM2462. This fast, high voltage, high energy pulse can

damage the LM2462 output stage. The application circuit

shown in

Figure 9 is designed to help clamp the voltage at

the output of the LM2462 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 capaci-

tance. 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 V

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 LM2462 ground. This

will significantly reduce the high frequency voltage transients

that the LM2462 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 LM2462 as

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

metal or carbon film type resistor. Having large value resis-

tors 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

LM2462 would be subjected to. 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 compo-

nents shown in

Figure 9.

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