LM6171 datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

To minimize reflection, coaxial cable with matching charac-

teristic impedance to the signal source should be used. The

other end of the cable should be terminated with the same

value terminator or resistor. For the commonly used cables,

RG59 has 75

Ω characteristic impedance, and RG58 has

Ω characteristic impedance.

Driving Capacitive Loads

Amplifiers driving capacitive loads can oscillate or have ring-

ing at the output. To eliminate oscillation or reduce ringing,

an isolation resistor can be placed as shown below in

Figure

5. The combination of the isolation resistor and the load ca-

pacitor forms a pole to increase stablility by adding more

phase margin to the overall system. The desired perfor-

mance depends on the value of the isolation resistor; the big-

ger the isolation resistor, the more damped the pulse re-

sponse becomes. For LM6171, a 50

Ω isolation resistor is

recommended for initial evaluation.

Figure 6 shows the

LM6171 driving a 200 pF load with the 50

Ω isolation resistor.

Power Dissipation

The maximum power allowed to dissipate in a device is de-

fined as:

D = (TJ(max) −TA)/θJA

Where P

D is the power dissipation in a device

J(max) is the maximum junction temperature

A is the ambient temperature

JA is the thermal resistance of a particular package

For example, for the LM6171 in a SO-8 package, the maxi-

mum power dissipation at 25˚C ambient temperature is

730 mW.

Thermal resistance,

JA, depends on parameters such as

die size, package size and package material. The smaller

the die size and package, the higher

JA becomes. The 8-pin

DIP package has a lower thermal resistance (108˚C/W) than

that of 8-pin SO (172˚C/W). Therefore, for higher dissipation

capability, use an 8-pin DIP package.

DS012336-15

FIGURE 4. Improperly Terminated Signal

DS012336-13

FIGURE 5. Isolation Resistor Used

to Drive Capacitive Load

DS012336-16

FIGURE 6. The LM6171 Driving a 200 pF Load

with a 50

Ω Isolation Resistor

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