P

= (V x I

) - Power in Load

= (24 x 21)mW - 40mW

= 464mW

D(TOTAL)

S

avg

LM7372

SNOS926F – MAY 1999 – REVISED SEPTEMBER 2014

www.ti.com

Application Details (continued)

The average current for a load power of 20 mW is 14.1 mA (=

√(20mW/100)). Neglecting the AB bias current,

this appears as a full-wave rectified current waveform in the supply current with a peak value of 19.9mA. The

peak to average ratio for a waveform of this shape is 1.57, so the total average load current is 12.7 mA (= 19.9

mA/1.57). Adding this to the quiescent current, and subtracting the power dissipated in the load (20 mV x 2 = 40

mW) gives the same package power dissipation level calculated above (= (12.7 + 13) mA x 24 V –40 mV = 576

mW). Nevertheless, when the supply current peak swing is measured, it is found to be significantly lower

because the AB bias current is contributing to the load current. The supply current has a peak swing of only 14

mA (compared to 19.9 mA) superimposed on the quiescent current, with a total average value of only 21 mA.

Therefore, the total package power dissipation in this application is:

(5)

This level of power dissipation would not take the junction temperature in the 8-Pin SO PowerPAD package over

the absolute maximum rating at elevated ambient temperatures (barely), but there is no margin to allow for

component tolerances or signal variances.

To develop 20 mW in a 100

of signal swing does not require a high supply voltage but the application uses a 24V supply. This is because the

modulation technique uses a large number of tones to transmit the data. While the average power level is held to

20 mW, at any time the phase and amplitude of individual tones will be such as to generate a combined signal

with a higher peak value than 2 V. For DMT this crest factor is taken to be around 5.33 so each amplifier has to

be able to handle a peak voltage swing of:

(6)

If other factors, such as transformer loss or even higher peak to average ratios are allowed for, this means the

The required signal swing can be reduced by using a step-up transformer to drive the line. For example a 1:2

ratio will reduce the peak swing requirement by half, and this would allow the supply to be reduced by a

corresponding amount. This is not recommended for the LM7372 in this particular application for two reasons.

First, although the quiescent power contribution to the overall dissipation is reduced by about 150 mW, the

internal power dissipation to drive the load remains the same, since the load for each amplifier is now 25

Ω

instead of 100

Ω. Secondly, this is a transceiver application where downstream signals are simultaneously

appearing at the transformer secondary. The down stream signals appear differentially across the back

termination resistors and are now stepped down by the transformer turns ratio with a consequent loss in receiver

sensitivity compared to using a 1:1 transformer. Any trade-off to reduce the supply voltage by an increase in

turns ratio should bear these factors in mind, as well as the increased signal current levels required with lower

impedance loads.

At an elevated ambient temperature of 85°C and with an average power dissipation of 464mW, a package

thermal resistance between 60°C/W and 80°C/W will be needed to keep the maximum junction temperature in

the range 110°C to 120°C. The SO PowerPAD package would be the package of choice here with ample board

copper area to aid in heat dissipation (see Table 2).

For most standard surface mount packages (8-Pin SOIC, 14-Pin SOIC, 16-Pin SOIC, and so forth), the only

means of heat removal from the die is through the bond wires to external copper connecting to the leads. Usually

it will be difficult to reduce the thermal resistance of these packages below 100°C/W by these methods and

several manufacturers, including Texas Instruments, offer package modifications to enhance the thermal

characteristics.

16

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