Theory of Operation

The LM2405 is a high voltage monolithic triple CRT driver

suitable for SVGA and XGA display applications. The

LM2405 features +80V operation and low power dissipation.

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

molded plastic power package.

The circuit diagram of the LM2405 is shown in

Figure 1.A

PNP emitter follower, Q5, provides input buffering. Q1 and

Q2 form a fixed gain cascode amplifier, with a gain of −14.

Emitter followers Q3 and Q4 isolate the high output imped-

ance of the amplifier from the capacitance of the CRT cath-

ode, and make the circuit relative insensitive to load capaci-

tance. Q6 provides biasing to the output emitter follower

stage to reduce crossover distortion at low signal levels.

Figure 2 shows a typical test circuit for evaluation of the

LM2405. This circuit is designed to allow testing of the

LM2405 in a 50

Ω environment, such as a pulse generator

and a scope, or a network analyzer. In this test circuit, two

low inductance resistors in series totaling 4.95 k

Ω form a

100:1 wideband low capacitance probe when connected to a

50

Ω cable and load. The input signal from the generator is

AC coupled to the base of Q5.

Application Hints

INTRODUCTION

National Semiconductor is committed to providing applica-

tion 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 perfor-

mance was done using a specific printed circuit board de-

signed at National. Variations in performance can be realized

due to physical changes in the printed circuit board and the

application. Therefore, the designer should be aware that

component value changes may be required in order to opti-

mize performance in a given application. The values shown

in this document can be used as a starting point for evalua-

tion purposes. When working with high bandwidth circuits,

good layout practices are also critical to achieving maximum

performance.

POWER SUPPLY BYPASS

Since the LM2405 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 and oscillation. A 0.01 µF capacitor should be

connected from the supply pin, V

DS012682-4

FIGURE 3. V

DS012682-5

FIGURE 4. Power Dissipation vs V

CC

DS012682-6

FIGURE 5. Large Signal Frequency Response

DS012682-7

FIGURE 6. Pulse Response

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