POWER SUPPLY BYPASS

Since the LM2429 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. 0.1µF capacitors should be con-

nected from the supply pins, V

close to the LM2429 as is practical. Additionally, a 22µF or

larger electrolytic capacitor should be connected from both

supply pins to ground reasonably close to the LM2429.

ARC PROTECTION

During normal CRT operation, internal arcing may occasion-

ally occur. Spark gaps, in the range of 300V, 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 LM2429. This fast, high voltage, high energy pulse can

damage the LM2429 output stage. The application circuit

shown in Figure 13 is designed to help clamp the voltage at

the output of the LM2429 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. 1SS83 or equivalent diodes are recommended. D1

and D2 should have short, low impedance connections to

V

located very close to a separately decoupled bypass capaci-

tor (C3 in Figure 13). The ground connection of D2 and the

decoupling capacitor should be very close to the LM2429

ground. This will significantly reduce the high frequency

voltage transients that the LM2429 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 LM2429 as well as the voltage stress at the outputs

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

levels that the LM2429 would be subjected to. The inductor

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

minimize 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 13.

EFFECT OF LOAD CAPACITANCE

Figure 7 shows the effect of increased load capacitance on

the speed of the device. This demonstrates the importance

of knowing the load capacitance in the application.

EFFECT OF OFFSET

Figure 8 shows the variation in rise and fall times when the

output offset of the device is varied from 95V to 105V

rise time shows a variation of less than 8% relative to the

center data point (100V

less than 9% relative to the center data point.

THERMAL CONSIDERATIONS

Figure 9 shows the performance of the LM2429 in the test

circuit shown in Figure 3 as a function of case temperature.

The figure shows that the rise and fall times of the LM2429

increase by approximately 15% and 30%, respectively, as

the case temperature increases from 50˚C to 90˚C. This

corresponds to a speed degradation of 3.75% and 7.5% for

every 10˚C rise in case temperature.

Figure 10 shows the power dissipation of the LM2429 vs.

Frequency when all three channels of the device are driving

an 8pF load with a 130V

off signal. The graph assumes a 72% active time (device

operating at the specified frequency) which is typical in a TV

application. The other 28% of the time the device is assumed

to be sitting at the black level (165V in this case). Table 1

also shows the typical power dissipation of the LM2429 for

various video patterns in the 480i and 480p video formats.

Figure 10, Figure 11, and Table 1 give the designer the

information needed to determine the heatsink requirement

for the LM2429. For example, if an HDTV application uses

the 480p format and "Vertical Lines 2 On 2 Off" is assumed

to be the worst-case pattern to be displayed, then the power

dissipated will be 11.4W (from Table 1). Figure 11 shows that

the maximum allowed case temperature is 117˚C when

11.4W is dissipated. If the maximum expected ambient tem-

perature is 70˚C, then a maximum heatsink thermal resis-

tance can be calculated:

This example assumes a capacitive load of 8pF and no

resistive load. The designer should note that if the load

capacitance is increased the AC component of the total

power dissipation will also increase.

Note: A LM126X preamplifier, with rise and fall times of about

2 ns, was used to drive the LM2429 for these power mea-

surements. Using a preamplifier with rise and fall times

slower than the LM126X will cause the LM2429 to dissipate

less power than shown in Table 1.

OPTIMIZING TRANSIENT RESPONSE

In Figure 13, there are three components (R1, R2 and L1)

that can be adjusted to optimize the transient response of

the application circuit. Increasing the values of R1 and R2

will slow the circuit down while decreasing overshoot. In-

creasing the value of L1 will speed up the circuit as well as

increase overshoot. It is very important to use inductors with

very high self-resonant frequencies, preferably above 300

MHz. Ferrite core inductors from J.W. Miller Magnetics (part

# 78FR_ _k) were used for optimizing the performance of the

device in the NSC application board. The values shown in

Figure 14 and Figure 15 can be used as a good starting point

for the evaluation of the LM2429. Using a variable resistor

for R1 will simplify finding the value needed for optimum

performance in a given application. Once the optimum value

is determined, the variable resistor can be replaced with a

fixed value.

20073110

FIGURE 13. One Channel of the LM2429 with the

Recommended Application Circuit

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