IMPORTANT INFORMATION

The LM2453 performance is targeted for the high-resolution

market (up to 135 MHz pixel clock frequencies). It is the first

device in a new product family that is being developed by

National Semiconductor. Since this device is significantly dif-

ferent from all our previous CRT drivers, it is very important

to read all the application information supplied in this docu-

ment. Should you have additional questions, please contact

your local FAE or sales office.

Please refer to the NSC neck board schematic and layout

that are shown in

Figures 17, 18, 19 during the following dis-

cussion.

POWER SUPPLY BYPASS

Since the LM2453 is a high bandwidth amplifier, proper

power supply bypassing is critical for optimum performance.

Improper power supply bypassing can result in large over-

shoot, ringing or oscillation.

nected from the V

supply pin and heatsink (device tab) ground point as pos-

sible. Additionally, a 47 µF electrolytic capacitor (C39) should

be connected through a ferrite bead (FB2 in the schematic)

to the supply pin and ground. A Fair-Rite 2743003112-TR

(100

nected from the V

to the device with as short as possible connections. Addition-

ally, a 47 µF electrolytic capacitor (C21) should be con-

nected through a ferrite bead (FB1) to the supply pin and

ground. A Fair-Rite 2743003112-TR (100

equivalent should be used.

REFERENCE VOLTAGE

The LM2453 receives a reference voltage from the LM1253A

preamp. It is critical that a 0.1 µF bypass cap be added from

the V

close as possible to the video input side of the device (see

C30 in

Figure 17 ).A75

Ω resistor should also be used be-

tween the V

LM1253A. V

REF

should also be bypassed close to the

preamp.

THE VIDEO APPLICATION CIRCUIT

Figure 14 shows the recommended circuit topology for the

video amplifiers in the LM2453. This circuit is designed to

yield the best transient response and reduce radiated emis-

sions while also protecting the amplifier from damage that

can be caused by CRT Arcs. The schematic in

Figures 17,

18 shows the values that yielded best performance in the

NSC reference design board. The following sections discuss

arc protection and transient response in detail. Refer to

Fig-

ures 17, 18, 19 for the full application schematic and PCB

layout.

ARC Protection

During normal CRT operation, internal arcing may occasion-

ally occur. At the beginning of an arc event there is some

∼ 100 MHz). Spark gaps, in

the range of 200V, 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 LM2453. They are also

ineffective at limiting the peak voltage of the initial high fre-

quency burst. This fast, high voltage, high energy pulse can

damage the LM2453 video output stages. The application

circuit shown in

Figure 14 is designed to help clamp the volt-

age at the output of the LM2453 to a safe level. The clamp

diodes should have a fast transient response, high peak cur-

rent rating, low series impedance and low shunt capaci-

tance. FDH400 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 14). The ground connection of the diode

and the decoupling capacitor should be very close to the

LM2453 ground and should be of as low impedance as pos-

sible. This will significantly reduce the high frequency voltage

transients that the LM2453 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

LM2453 as well as the voltage stress at the video outputs of

the device. R2 should be a 1/2W solid carbon type resistor.

R1 can be a 1/4W metal or carbon film type resistor. Inductor

L1 is critical to reduce the initial high frequency voltage lev-

els that the LM2453 would be subjected to. Having large

value resistors for R1 and R2 would be desirable, but this

has the effect of increasing rise and fall times. The inductor

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

mize rise and fall times as well as minimize EMI. For proper

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

tection components shown in

Figure 14. Omitting any of

these components could cause serious reliability problems in

production.

Optimizing Transient Response

Referring to

Figure 14, 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

above 300 MHz. Ferrite core inductors from J.W. Miller Mag-

netics (part # 78FXXXM, where XXX specifies the value)

were used for optimizing the performance of the device in

the NSC application board. The values shown in

Figure 17

exhibited the best overall performance in a 17 inch monitor in

our lab. These can be used as a good starting point for the

evaluation of the LM2453 in a new monitor application. Us-

ing a variable resistor for R1 in series with a fixed value in-

ductor is a great way to help dial in the values needed for op-

timum performance in a given application. Once the

optimum values are determined the variable resistors can be

replaced with fixed values and corresponding inductor can

be installed. In addition to the output circuit, a series inductor

is used between the LM1253A video outputs and the

LM2453 inputs.

DS101302-14

FIGURE 14. One Video Channel of the LM2453 with the

Recommended Application Circuit

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