TILT AND OVERSHOOT COMPENSATlON

When a low frequency square is displayed on a monitor

screen some tilt may appear on the video signal due to the

large power and thermal dissipation changes in the input

transistors This problem is illustrated in

Figure 3

TLH11967 – 7

FIGURE 3 Tilt on a Low Frequency

Signal and Its Effects

The tilt can be compensated by adding an external RC feed-

back network as shown in

Figure 4 The RC feedback helps

by reducing the gain of the amplifier during the edge tran-

TLH11967 – 8

FIGURE 4 RC Feedback Network for Tilt Compensation

To find the value of resistor R the following formula can be

used

x%

where x% is the percentage value of DV to the peak-to-

The value of capacitor C is determined by

For optimum results in a specific application the values for

R and C may need to be tested and adjusted in the given

application board

PROTECTlNG AMPLIFIER OUTPUT

FROM TUBE ARCING

During normal CRT operation internal arcing may occasion-

ally occur Spark gap protectors will limit the maximum volt-

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

LM2427 This fast high voltage high energy pulse can dam-

age the LM2427 output stage The addition of two current

limiting resistors of 50X to 100X total and clamping diodes

D1 and D2 will provide protection but will slow down the

response The diodes should have a fast transient re-

sponse high peak current rating low series impedance and

low shunt capacitance FDH400 or equivalent diodes are

recommended Adding a series peaking inductor of 100 nH

to 150 nH will restore the bandwidth and provide additional

protection (See

Figure 5 )

The value of the inductor can be calculated from

24

quency output resistance of the amplifier generally 160X

TLH11967 – 9

FIGURE 5 One Section of the LM2427

with Tilt Compensation Arc Protection

SHORT CIRCUIT PROTECTION

WARNING

To provide maximum output speed the LM2427 does not

have short circuit protection Shorting the output can de-

stroy the device

SUPPLY BYPASSlNG

Although the LM2427 has internal supply bypassing some

values of supply line inductance can cause ringing in the

supply lines If this occurs an additional bypass capacitor or

a low-pass filter should be placed as close as possible to

CAPACITlVE LOADS

The LM2427 is designed to drive capacitive loads however

the very high output slew rate of about 13700 V ms can

result in charging currents of over 200 mA into a 20 pF load

These very high currents can damage the output transistors

HEAT SlNKlNG

Power consumption by the LM2427 will depend on the sup-

ply voltage used the output loading the peak-to-peak out-

put swing and the operating frequency Since the LM2427

will dissipate up to 14W an external heatsink is always re-

quired The maximum allowed case temperature is 90 C To

calculate maximum heatsink thermal resistance use the fol-

lowing formula

14

4