LM7372MR/NOPB National Semiconductor, LM7372MR/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM7372MR/NOPB
• Description: IC OPAMP DUAL HS HI OUTP 8-PSOP
• Manufacturer: National Semiconductor
• Series: VIP™ IIIr

Specifications of LM7372MR/NOPB

• Amplifier Type: Voltage Feedback
• Number Of Circuits: 2
• Slew Rate: 3000 V/µs
• Gain Bandwidth Product: 120MHz
• -3db Bandwidth: 220MHz
• Current - Input Bias: 2.7µA
• Voltage - Input Offset: 2000µV
• Current - Supply: 13mA
• Current - Output / Channel: 260mA
• Voltage - Supply, Single/dual (±): 9 V ~ 36 V, ±4.5 V ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-PSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Output Type: -
• Other names: *LM7372MR; *LM7372MR/NOPB; LM7372MR

the signal voltage across the load increases with load current,
the voltage across the output transistor (which is the differ-
ence voltage between the supply voltage and the instanta-
neous voltage across the load) will decrease and a point will
be reached where the dissipation in the transistor will begin
to decrease again. If the signal is driven into a square wave,
ideally the transistor dissipation will fall all the way back to
zero.
For each amplifier then, with an effective load each of R
a sine wave source, integration over the half cycle with a sup-
ply voltage V
dissipation
Where V
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 adja-
cent 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 fre-
quencies 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 chan-
nels 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
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
amplifier is also 100Ω, and each amplifier is required to deliver
20mW to this load.
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 am-
plifier dissipation gives the total package internal power dis-
sipation as
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 divert-
ed 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 as-
sumes a sine wave signal waveform when the actual wave-
form is composed of many tones of different phases and
P
Since V
P
D
D(TOTAL)
S
= V
is the supply voltage and V
S
S
and a load voltage V
V
L
2
L
/2RL = 20mW then V
= 312mW + (2 x 132.8mW) = 578mW
/
L
π
.
R
L
- V
L
2
/2R
L
..........(1)
L
yields the average power
L
is the peak signal swing
L
= 2V(peak).
(13/10)
). Because the
L
for each
L
and
11
amplitudes which may demonstrate lower average power dis-
sipation levels.
The average current for a load power of 20mW is 14.1mA
(=
pears 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 dis-
sipation 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 to-
tal average value of only 21mA. Therefore the total package
power dissipation in this application is
This level of power dissipation would not take the junction
temperature in the 8-Pin SOIC 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(
does not require a high supply voltage but the application us-
es 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
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 re-
duce 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 inter-
nal power dissipation to drive the load remains the same,
since the load for each amplifier is now 25Ω instead of
100Ω. Furthermore, this is a transceiver application where
downstream signals are simultaneously appearing at the
transformer secondary. The down stream signals appear dif-
ferentially across the back termination resistors and are now
stepped down by the transformer turns ratio with a conse-
quent 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 av-
erage power dissipation of 464mW, a package thermal resis-
tance 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 package would be the package of choice
√
(20mW/100)). Neglecting the AB bias current, this ap-
P
= (24 x 21)mW - 40mW
= 464mW
V
D(TOTAL)
Lpeak
= 1.4 x 5.33 = 7.5V or 15V(
= (V
S
x I
avg
) - Power in Load
P-P
). This level of signal swing
P-P
).
P-P
)
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