LM7171 datasheet(14/20 Pages) NSC

14 / 20 page

Termination (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 stability 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 LM7171, a 50

Ω isolation resistor is

recommended for initial evaluation.

Figure 6 shows the

LM7171 driving a 150 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

is the power dissipation in a device

J(max)

is the maximum junction temperature

is the ambient temperature

is the thermal resistance of a particular package

For example, for the LM7171 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.

The total power dissipated in a device can be calculated as:

D = PQ +PL

Q is the quiescent power dissipated in a device with no load

connected at the output. P

L is the power dissipated in the de-

vice with a load connected at the output; it is not the power

dissipated by the load.

Furthermore,

supply current x total supply voltage with no load

output current x (voltage difference between

supply voltage and output voltage of the same

side of supply voltage)

DS012385-17

FIGURE 3. Properly Terminated Signal

DS012385-18

FIGURE 4. Improperly Terminated Signal

DS012385-12

FIGURE 5. Isolation Resistor Used

to Drive Capacitive Load

DS012385-13

FIGURE 6. The LM7171 Driving a 150 pF Load

with a 50

Ω Isolation Resistor

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