Application Hints

POWER SUPPLY BYPASS

Since the LM2406 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

(preferably less than

from the supply pin) Additionally

a10 mF – 100 mF electrolytic capacitor should be connected

from the supply pin to ground The electrolytic capacitor

should also be placed reasonably close to the LM2406’s

supply pin A 01 mF capacitor should be connected from

the bias pin to ground as close as is practical to the

LM2406

The LM2406 is short circuit proof to momentary shorts to

ARC PROTECTION

During normal CRT operation internal arcing may occasion-

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

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

er than allowable on the LM2406 This fast high voltage

high energy pulse can damage the LM2406 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

LM2406 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

LM2406 and reduces the power dissipation of the output

transistors when the output is stressed beyond the supply

voltage Having large value resistors for R1 and R2 would

be desirable but this has the effect of increasing rise and

fall times

IMPROVING RISE AND FALL TIMES

Because of an emitter follower output stage the rise and fall

times of the LM2406 are relatively insensitive to capacitive

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 – 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 The inductor value is empirically determined and is

dependent on the load An inductor value of 01 mHisa

good starting value Note that peaking the amplifier’s fre-

quency response will increase the overshoot

THERMAL CONSIDERATIONS

Power supply current increases as the input signal increas-

es and consequently power dissipation also increases

The LM2406 cannot be used without heat sinking Typical

‘‘average’’ power dissipation with the device output voltage

at one half the supply voltage is 24W per channel for a total

dissipation of 72W package dissipation Under white screen

conditions ie

25V output dissipation increases to 35W

per channel or 105W total The LM2406 case temperature

must be maintained below 100 C If the maximum expected

ambient temperature is 50 C then a maximum heat sink

thermal resistance can be calculated

105W

e

476 CW

This example assumes a typical CRT capacitive load and is

without a resistive load

TYPICAL APPLICATION

A typical application of the LM2406 is shown in

Figure 8

Used in conjunction with an LM1207 a complete video

channel from monitor input to CRT cathode can be

achieved Performance is satisfactory for all applications up

to 1024 x 768 non-interlaced

TLH12327 – 8

4