Datasheet

P = (V x I ) - Power in Load
= (24 x 21)mW - 40mW
= 464mW
D(TOTAL) S avg
LM7372
www.ti.com SNOS926E –MAY 1999–REVISED MARCH 2013
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 modulation. 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 frequency of 135kHz. At first sight,
these relatively low transmission 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 spacing 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 generated 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 amplifier 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 100Ω this is only 20mW (= 1mW x
10
(13/10)
). 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
L
for each amplifier is also
100Ω, and each amplifier is required to deliver 20mW to this load.
Since V
L
2
/2RL = 20mW then V
L
= 2V(peak). (3)
Using Equation 2 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
D(TOTAL)
= 312mW + (2 x 132.8mW) = 578mW (4)
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 increases from zero. In fact with load
currents in excess of 3.3mA, all the bias current is flowing in the load, consequently reducing the quiescent
component of power dissipation. 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 average power
dissipation levels.
The average current for a load power of 20mW is 14.1mA (= √(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 12.7mA (= 19.9mA/1.57).
Adding this to the quiescent current, and subtracting the power dissipated in the load (20mV x 2 = 40mW) gives
the same package power dissipation level calculated above (= (12.7 + 13) mA x 24V –40mV = 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 14mA (compared
to 19.9mA) superimposed on the quiescent current, with a total average value of only 21mA. 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 20mW in a 100Ω requires each amplifier to deliver a peak voltage of only 2V, or 4V(
P-P
). This level 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
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
Lpeak
= 1.4 x 5.33 = 7.5V or 15V(
P-P
) (6)
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(
P-P
).
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