Theory of Operation

The LM2409 is a high voltage monolithic three channel CRT

driver suitable for high resolution display applications. The

LM2409 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 LM2409 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 −14. Emitter followers Q3 and Q4 iso-

late 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 bi-

asing to the output emitter follower stage to reduce cross-

over distortion at low signal levels.

Figure 2 shows a typical test circuit for evaluation of the

LM2409. This circuit is designed to allow testing of the

LM2409 in a 50

Ω environment without the use of an expen-

sive FET probe. In this test circuit, two low inductance resis-

tors in series totaling 4.95 k

Ω form a 100:1 wideband, low

capacitance probe when connected to a 50

Ω coaxial cable

anda50

Ω load (such as a 50Ω oscilloscope input). The in-

put 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 follow-

ing information is provided in order to support this commit-

ment. 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 pur-

poses. When working with high bandwidth circuits, good lay-

out practices are also critical to achieving maximum perfor-

mance.

IMPORTANT INFORMATION

The LM2409 performance is targeted for the VGA (640 x

480) to XGA (1024 x 768, 70 Hz refresh) resolution market.

It is designed to be a replacement for discrete CRT drivers.

The application circuits shown in this document to optimize

performance and to protect against damage from CRT arc-

over are designed specifically for the LM2409. If another

member of the LM240X family is used, please refer to its

datasheet.

POWER SUPPLY BYPASS

Since the LM2409 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. A 0.01 µF capacitor should be

connected from the supply pin, V

the supply and ground pins as is practical. Additionally, 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 LM2409’s

supply and ground pins. A 0.1 µF capacitor should be con-

nected from the bias pin, V

tical to the part.

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 volt-

age, but to a value that is much higher than allowable on the

LM2409. This fast, high voltage, high energy pulse can dam-

age the LM2409 output stage. The application circuit shown

in

Figure 9 is designed to help clamp the voltage at the out-

put of the LM2409 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.

FDH400 or equivalent diodes are recommended. Do not use

1N4148 or equivalent diodes for the clamp diodes. D1 and

D2 should have short, low impedance connections to V

CC

and ground respectively. The cathode of D1 should be lo-

cated very close to a separately decoupled bypass capacitor

(C3 in

Figure 9). The ground connection of D2 and the de-

coupling capacitor should be very close to the LM2409

ground. This will significantly reduce the high frequency volt-

age transients that the LM2409 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

LM2409 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 lev-

els that the LM2409 would be subjected to. The inductor will

not only help protect the device but it will also help maximize

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.

DS100838-10

FIGURE 9. One Channel of the LM2409 with the Recommended Arc Protection Circuit

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