IMPORTANT INFORMATION

The LM2457 performance is targeted for monitor applica-

tions with pixel clock frequencies up to 95MHz. It is a part of

the AC2DC new product family that is being developed by

National Semiconductor. Since this device is significantly

different from all our previous CRT drivers, it is very impor-

tant to read all the application information supplied in this

document. 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

discussion.

POWER SUPPLY BYPASS

Since the LM2457 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. Addi-

tionally, 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 LM2457 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 18 ). A 300

Ω resistor should also be used

between 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 LM2457. 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

Figures 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 LM2457. They are also

ineffective at limiting the peak voltage of the initial high

frequency burst. This fast, high voltage, high energy pulse

can damage the LM2457 video output stages. The applica-

tion circuit shown in

Figure 14 is designed to help clamp the

voltage at the output of the LM2457 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 recom-

mended. D1 and D2 should have short, low impedance

connections to V

D1 should be located very close to a separately decoupled

bypass capacitor (C3 in

Figure 14). The ground connection

of the diode and the decoupling capacitor should be very

close to the LM2457 ground and should be of as low imped-

ance as possible. This will significantly reduce the high

frequency voltage transients that the LM2457 would be sub-

jected to during an arcover condition. Resistor R2 limits the

arcover current that is seen by the diodes while R1 limits the

current into the LM2457 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 levels that the LM2457 would be sub-

jected 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 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 14. Omit-

ting 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

Magnetics (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 18 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 LM2457 in a new monitor

application. Using a variable resistor for R1 in series with a

fixed value inductor is a great way to help dial in the values

needed for optimum performance in a given application.

Once the optimum values are determined the variable resis-

tors 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 LM2457 inputs.

DS101379-14

FIGURE 14. One Video Channel of the LM2457 with the

Recommended Application Circuit

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