Typical Performance

pF, V

Circuit - Figure 4 unless otherwise specified) (Continued)

Theory of Operation

The LM2476 is a high voltage monolithic three channel CRT

driver and triple bias clamp suitable for low-cost color moni-

tor applications. The LM2476 operates with 85V and 8V

power supplies. The part is housed in the 19-lead TO-247

molded plastic power package. The pinout and internal con-

nection diagram is shown in Figure 1.

The CRT Driver circuit diagram is shown in Figure 1. The

PNP emitter follower, Q5, provides input buffering. Q1 and

Q2 form a fixed gain cascode amplifier with resistors R1 and

R2 setting the gain at −27. Emitter followers Q3 and Q4

isolate the high output impedance of the cascode stage from

the capacitance of the CRT cathode which decreases the

sensitivity of the device to load capacitance. Q6 provides

biasing to the output emitter follower stage to reduce cross-

over distortion at low signal levels. The typical driver DC

transfer function is shown in Figure 5.

The Bias Clamp circuit diagram is shown in Figure 3. The

clamp circuit amplifies the DC inputs, V

fixed gain of –16.5. Each clamp output, V

pull-up resistor to V

tion is shown in Figure 6.

Application Hints

INTRODUCTION

National Semiconductor (NSC) is committed to provide ap-

plication information that assists our customers in obtaining

the best performance possible from our products. The fol-

lowing information is provided in order to support this com-

mitment. 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

purposes. When working with high bandwidth circuits, good

layout practices are also critical to achieving maximum per-

formance.

IMPORTANT INFORMATION

The LM2476 performance is targeted for the VGA (640 x

480) to XGA (1024 x 768, 85Hz) resolution market. The

application circuits shown in this document to optimize per-

formance and to protect against damage from CRT arcover

are designed specifically for the LM2476. If another member

of the NSC CRT Driver or Bias Clamp family is used, please

refer to its data sheet.

POWER SUPPLY BYPASS

Since the LM2476 contains wide bandwidth video amplifiers,

proper power supply bypassing is critical for optimum per-

formance. Improper power supply bypassing can result in

large overshoot, ringing or oscillation. 0.1 µF capacitors

should be connected from the supply pins, V

ground, as close to the LM2476 as is practical. Additionally,

a 47 µF or larger electrolytic capacitor should be connected

from both supply pins to ground reasonably close to the

LM2476. For optimal supply bypassing, the bypass capaci-

tors should have the shortest connections between the sup-

ply and ground pins of the LM2476.

ARC PROTECTION

During normal CRT operation, internal arcing may occasion-

ally occur. A spark gap SG1 – in the range of 200V –

connected from each of the CRT cathodes to CRT ground

will limit the maximum voltage, but to a value that is much

higher than allowable on the LM2476. This fast, high voltage,

high energy pulse can damage the LM2476 driver and/or

clamp output stages. The application circuit shown in Figure

12 is designed to help clamp the voltage at the outputs of the

LM2476 to a safe level. The arc protection clamp diodes, D1

and D2, should have a fast transient response, high peak

current rating, low series impedance and low shunt capaci-

tance. FDH400 or equivalent diodes are recommended. Do

not use 1N4148 diodes for the clamp diodes. D1 and D2

should have short, low impedance connections to V

ground respectively. The cathode of D1 should be located

very close to a separately decoupled bypass capacitor (C1 in

Figure 12). The ground connection of D2 and the decoupling

capacitor should be closest to the CRT ground. This will

significantly reduce the high frequency voltage transients

that the LM2476 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 LM2476 as

well as the voltage stress at the outputs of the device. R2

metal or carbon film type resistor. Having large value resis-

tors 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

LM2476 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. Current-limiting resistor

R3 and bypass capacitor C2 should be placed very close to

the clamp output pins to protect the LM2476 against damage

during an arcover condition. The ground connection of C2

should have a short return path to CRT ground to shunt

arcover currents away from the LM2476. For proper arc

protection, it is important to not omit any of the arc protection

components shown in Figure 12.

20121907

FIGURE 11. Power Dissipation vs Frequency

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