LM6172 datasheet(13/16 Pages) NSC

13 / 16 page

Power Supply Bypassing (Continued)

Termination

In high frequency applications, reflections occur if signals

are not properly terminated.

Figure 6 shows a properly termi-

nated signal while

Figure 7 shows an improperly terminated

signal.

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.

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 LM6172 in a SO-8 package, the maxi-

mum power dissipation at 25˚C ambient temperature is

780 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 (95˚C/W) than

that of 8-pin SO (160˚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 sup-

ply voltage and output voltage of the same supply)

For example, the total power dissipated by the LM6172 with

S = ±15V and both channels swinging output voltage of

10V into 1 k

Ω is

Q +PL

2[(2.3 mA)(30V)] + 2[(10 mA)(15V − 10V)]

138 mW + 100 mW

238 mW

DS012581-47

FIGURE 5. Power Supply Bypassing

DS012581-53

FIGURE 6. Properly Terminated Signal

DS012581-54

FIGURE 7. Improperly Terminated Signal

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