Theory of Operation

The LM2408 is a high voltage monolithic triple CRT driver

suitable for SVGA and XGA display applications

The

tion The part is housed in the industry standard 11-Lead

TO-220 molded plastic power package

The simplified circuit diagram of the LM2408 is shown in

Figure 1 A PNP emitter follower Q1 provides input buffer-

ing Q2 and Q3 form a high gain amplifier Feedback around

this amplifier through the 15 kX resistor working with the

Q5 and Q6 isolate the high output impedance of the amplifi-

er from the capacitance of the CRT cathode and make the

circuit relative insensitive to load capacitance

Figure 2 shows a typical test circuit for evaluation of the

LM2408 This circuit is designed to allow testing of the

LM2408 in a 50X environment such as a pulse generator

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

low inductance resistors in series totaling 495 kX form a

1001 wideband low capacitance probe when connected to

a50X cable and load The input signal from the generator is

AC coupled to the base of Q1

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 per-

formance was done using specific printed circuit boards de-

signed at National Variations in performance can be real-

ized due to physical changes in the printed circuit board and

the application Therefore the designer should be aware

that component value and board layout changes may be

required to optimize performance in a given application The

values shown in this document can be used as a staring

point for testing and evaluation purposes When working

with high bandwidth circuits good layout practices are also

critical to achieving maximum performance

POWER SUPPLY BYPASS

Since the LM2408 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 001 mF capacitor should be

the supply pin as is practical (preferably less than

from

the supply pin) Additionally a 10 mFto100 mF electrolytic

capacitor should be connected from the supply pin to

ground The electrolytic capacitor should also be placed

reasonably close to the LM2408’s supply pin A 01 mF ca-

ground as close as is practical to the part

ARC PROTECTION

During normal CRT operation internal arcing may occasion-

ally occur Spark gaps of 200V to 300V at the cathodes will

limit the maximum voltage but to a value that is much high-

er than allowable on the LM2408 This fast high voltage

high energy pulse can damage the LM2408 output stage

The addition of clamp diodes D1 and D2 (as shown in

Fig-

ure 7 ) will help clamp the voltage at the output of the

LM2408 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 equiva-

lent diodes are recommended Resistor R2 in

Figure 7 limits

the arcover current while R1 limits the current into the

LM2408 and reduces the power dissipation of the output

transistors when the output is stressed beyond the supply

voltage Peaking inductor Lp also helps protect the LM2408

from CRT arcover and is part of the arc protection circuit

Having large value resistors for R1 and R2 would be desir-

able but this has the effect of increasing rise and fall times

For proper arc protection it is important to not omit any of

the arc protection components shown in

Figure 7

TLH12683 – 8

FIGURE 7 One Section of the LM2408 with Arc

There are also ESD protection diodes built into the part To

avoid damaging these diodes do not apply an input voltage

are held at ground potential

IMPROVING RISE AND FALL TIMES

Because of an emitter follower output stage the rise and fall

times of the LM2408 are relatively insensitive to capactive

loading However the series resistors R1 and R2 (see

Fig-

ure 7 ) will increase the rise and fall times when driving the

CRT’s cathode which appears as a capacitive load The ca-

pacitance at the cathode typically ranges from 8 pF to

12 pF

To improve the rise and fall times at the cathode a small

inductor is often used in series with the output of the amplifi-

cy response at the cathode thus improving rise and fall

times It also acts with the output load capacitance to form a

low pass filter which reduces the amplitudes of high fre-

quency harmonics of the video signal to lower radiated

electromagnetic interference The inductor value is empiri-

cally determined and is dependent on the load An inductor

value of 01 mH is a good starting value Note that excessive

peaking of the amplifier’s frequency response will increase

the overshoot Choosing the correct values for R1 R2 and

Lp will provide arc protection and the fastest rise and fall

times without excessive peaking

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