Typical Performance

Pulse Response

TLH11442 – 5

Test Circuit

Figure 1 shows a typical test circuit for evaluation of the

LM2419 The input signal is AC coupled into the input of

LM2419 and is referenced to 39V DC using an external

39V DC bias through a 390X resistor The test circuit is

designed to allow testing of the LM2419 in a 50X environ-

ment such as a 50X oscilloscope or network analyzer The

4950X resistor in series with the output of the LM2419

forms a 1001 voltage divider when connected to a 50X

oscilloscope or network analyzer

Theory of Operation

The LM2419 is a high voltage triple CRT driver suitable for

SVGA IBM 8514 and 1024 x 768 display resolution moni-

tors The device is packaged in the industry standard

11 lead TO-220 molded power package The heat sink is

electrically isolated and may be grounded for ease of manu-

facturing and RFlEMl shielding

The schematic diagram of LM2419 is shown in

Figure 2 Q1

and R2 provide a conversion of the input voltage to current

while Q2 acts as a common base amplifier to drive the load

resistor R1 Resistor R4 along with R2 sets up the DC bias

at the base of Q1 Emitter followers Q3 and Q4 isolate R1

from the capacitive load at the output thus making the rise

and fall times relatively insensitive to the load capacitance

limited by the time constant due to R1 and the capacitances

associated with D1 Q2 Q3 and Q4 Diode D1 is used to

provide some bias voltage for Q3 and Q4 so as to reduce

small signal cross over distortion Resistor R3 is used to

prevent Q2 from oscillating at high frequencies

TLH11442 – 7

FIGURE 2 Schematic Diagram of

One Section of LM2419

Application Hints

POWER SUPPLY BYPASS

Since the LM2419 is a wide-bandwidth amplifier with greater

than 10000 V ms slew rate proper power supply bypassing

is critical for optimum performance Improper power supply

bypassing can result in large overshoot ringing and oscilla-

tion A 001 mF ceramic capacitor should be connected as

close to the supply pin as is practical (preferably less than

from the supply pin) The lead length of the 001 mF

ceramic capacitor should be as small as is practical In addi-

tion 10 mF – 100 mF electrolytic capacitor should be con-

nected from the supply pin to ground The electrolytic ca-

pacitor should be placed reasonably close to the LM2419’s

supply pin

ARC PROTECTION

The LM2419 must be protected from arcing within the CRT

To limit the arcover voltage a 200V spark gap is recom-

mended at the cathode Clamp diodes D1 and D2 (as

shown in

Figure 3 ) are used to clamp the voltage at the

output of LM2419 to a safe level The clamp diodes used

should have high current rating low series impedance and

low shunt capacitance Resistor R2 in

Figure 3 limits the

arcover current while R1 limits the current into LM2419 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 desir-

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

3