Theory of Operation

The LM2412 is a high voltage monolithic three channel CRT

driver suitable for very high resolution display applications,

up to 1600 x 1200 at 85 Hz refresh rate. The LM2412 oper-

ates using 80V and 12V power supplies. The part is housed

in the industry standard 11-lead TO-220 molded plastic

power package.

The simplified circuit diagram of one channel of the LM2412

is shown in

Figure 1. A PNP emitter follower, Q5, provides in-

put buffering. This minimizes the current loading of the video

pre-amp. R9 is used to turn on Q5 when there is no input.

With Q5 turned on, Q1 will be almost completely off, minimiz-

ing the current flow through Q1 and Q2. This will drive the

output stage near the V

tion with no inputs. R6 is a pull-up resistor for Q5 and also

limits the current flow through Q5. R3 and R2 are used to set

the current flow through Q1 and Q2. The ratio of R1 to R2 is

used to set the gain of the LM2412. R1, R2 and R3 are all re-

lated when calculating the output voltage of the CRT driver.

R

in a cascode configuration. Q1 is a low voltage and very fast

transistor. Q2 is a higher voltage transistor. The cascode

configuration gives the equivalent of a very fast and high

voltage transistor. The two output transistors, Q3 and Q4,

form a class B amplifier output stage. R4 and R5 are used to

limit the current through the output stage and set the output

impedance of the LM2412. Q6, along with R7 and R8 set the

bias current through Q3 and Q4 when there is no change in

the signal level. This bias current minimizes the crossover

distortion of the output stage. With this bias current the out-

put stage now becomes a class AB amplifier with a cross-

over distortion much lower than a class B amplifier.

Figure 2 shows a typical test circuit for evaluation of the

LM2412. Due to the very wide bandwidth of the LM2412, it is

highly recommended that the stand alone board suplied by

NSC be used for the evaluation of the CRT driver’s perfor-

mance. The 50

Ω resistor is used to duplicate the required

series resistor in the actual application. This resistor would

be part of the arc-over protection circuit. The input signal

from the generator is AC coupled to the input of the CRT

driver.

Application Hints

INTRODUCTION

National Semiconductor (NSC) is committed to providing ap-

plication information that assists our customers in obtaining

the best performance possible from our products. The follow-

ing information is provided in order to support this commit-

ment. The reader should be aware that the optimization of

performance was done using a specific printed circuit board

designed at NSC. Variations in performance can be realized

due to physical changes in the printed circuit board and the

application. Therefore, the designer should know that com-

ponent value changes may be required in order to optimize

performance in a given application. The values shown in this

document can be used as a starting point for evaluation pur-

poses. When working with high bandwidth circuits, good lay-

out practices are also critical to achieving maximum perfor-

mance.

POWER SUPPLY BYPASS

Since the LM2412 is a very high 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 0.1 µF capacitor should be

connected from the supply pin, V

the supply and ground pins as is practical. Additionally, a

10 µF to 100 µF electrolytic capacitor should be connected

from the supply pin to ground. The electrolytic capacitor

should also be placed reasonably close to the LM2412’s

supply and ground pins. A 0.1 µF capacitor should be con-

nected from the bias pin, V

tical to the part.

ARC PROTECTION

During normal CRT operation, internal arcing may occasion-

ally occur. Spark gaps, in the range of 200V, connected from

the CRT cathodes to CRT ground will limit the maximum volt-

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

LM2412. This fast, high voltage, high energy pulse can dam-

age the LM2412 output stage. The application circuit shown

in

Figure 9 is designed to help clamp the voltage at the out-

put of the LM2412 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

equivalent diodes are recommended. D1 and D2 should

have short, low impedance connections to V

respectively. The cathode of D1 should be located very close

to a separately decoupled bypass capacitor. The ground

connection of the diode and the decoupling capacitor should

be very close to the LM2412 ground. This will significantly re-

duce the high frequency voltage transients that the LM2412

would be subjected to during an arc-over condition. Resistor

R2 limits the arc-over current that is seen by the diodes while

R1 limits the current into the LM2412 as well as the voltage

type resistor. Inductor L1 is critical to reduce the initial high

frequency voltage levels that the LM2412 would be sub-

jected to during an arc-over. Having large value resistors for

R1 and R2 would be desirable, but this has the effect of in-

creasing rise and fall times. The inductor will not only help

protect the device but it will also help optimize 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 9. The values of L1 and R1 may need to be

adjusted for a particular application. The recommended mini-

mum value for R1 is 75

Ω, with L1 = .049 µH.

OPTIMIZING TRANSIENT RESPONSE

Referring to

Figure 9, there are three components (R1, R2

and L1) that can be adjusted to optimize the transient re-

sponse of the application circuit. Increasing the values of R1

and R2 will slow the circuit down while decreasing over-

shoot. Increasing the value of L1 will speed up the circuit as

well as increase overshoot. It is very important to use induc-

tors with very high self-resonant frequencies, preferably

DS101298-10

FIGURE 9. One Channel of the LM2412 with the

Recommended Arc Protection Circuit.

www.national.com

4