For each amplifier then, with an effective load each of R

a sine wave source, integration over the half cycle with a

supply voltage V

power dissipation

P

Where V

swing across the load R

For the package, the power dissipation will be doubled since

there are two amplifiers in the package, each contributing

half the swing across the load.

The circuit in

Figure 1 is using the LM7372 as the upstream

driver in an ADSL application with Discrete MultiTone modu-

lation. With DMT the upstream signal is spread into 32

adjacent channels each 4kHz wide. For transmission over

POTS, the regular telephone service, this upstream signal

from the CPE (Customer Premise Equipment) occupies a

frequency band from around 20kHz up to a maximum fre-

quency of 135kHz. At first sight, these relatively low trans-

mission frequencies certainly do not seem to require the use

of very high speed amplifiers with GBW products in the

range of hundreds of megahertz. However, the close spac-

ing of multiple channels places stringent requirements on the

linearity of the amplifier, since non-linearities in the presence

of multiple tones will cause harmonic products to be gener-

ated that can easily interfere with the higher frequency down

stream signals also present on the line. The need to deliver

3rd Harmonic distortion terms lower than −75dBc is the

reason for the LM7372 quiescent current levels. Each am-

plifier is running over 3mA in the output stage alone in order

to minimize crossover distortion.

xDSL signal levels are adjusted to provide a given power

level on the line, and in the case of ADSL this is an average

power of 13dBm. For a line with a characteristic impedance

of 100

Ω this is only 20mW. Because the transformer shown

in

Figure

1

is

part

of

a

transceiver

circuit,

two

back-termination resistors are connected in series with each

amplifier output. Therefore the equivalent R

fier is also 100

Ω, and each amplifier is required to deliver

20mW to this load.

Since V

L

Using Equation (1) with this value for signal swing and a 24V

supply, the internal power dissipation per amplifier is

132.8mW. Adding the quiescent power dissipation to the

amplifier dissipation gives the total package internal power

dissipation as

P

This result is actually quite pessimistic because it assumes

that the dissipation as a result of load current is simply added

to the dissipation as a result of quiescent current. This is not

correct since the AB bias current in the output stage is

diverted to load current as the signal swing amplitude in-

creases from zero. In fact with load currents in excess of

3.3mA, all the bias current is flowing in the load, conse-

quently reducing the quiescent component of power dissipa-

tion. Also, it assumes a sine wave signal waveform when the

actual waveform is composed of many tones of different

phases and amplitudes which may demonstrate lower aver-

age power dissipation levels.

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

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

12.7mA. Adding this to the quiescent current, and subtract-

ing the power dissipated in the load gives the same package

power dissipation level calculated above. 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 14mA (compared to 19.9mA) superim-

posed on the quiescent current, with a total average value of

only 21mA. Therefore the total package power dissipation in

this application is

P

= (24 x 21)mW - 40mW

= 464mW

This level of power dissipation would not take the junction

temperature in the SO-8 package over the absolute maxi-

mum rating at elevated ambient temperatures (barely), but

there is no margin to allow for component tolerances or

signal variances.

To develop 20mW in a 100

Ω requires each amplifier to

deliver a peak voltage of only 2V, or 4V(

signal swing does not require a high supply voltage but the

application uses a 24V supply. This is because the modula-

tion technique uses a large number of tones to transmit the

data. While the average power level is held to 20mW, 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 2V. 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

V

If other factors, such as transformer loss or even higher peak

to average ratios are allowed for, this means the amplifiers

must each swing between 16 to 18V(

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. Although the quiescent power

contribution to the overall dissipation is reduced by about

150mW, the internal power dissipation to drive the load

remains the same, since the load for each amplifier is now

25

Ω instead of 100Ω. Furthermore, 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 re-

sistors 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 PSOP or LLP 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, SO-8, SO-14,

SO-16 etc, 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 resis-

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