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PDF L7372 Data sheet ( Hoja de datos )
| Número de pieza | L7372 | |
| Descripción | High Speed/ High Output Current/ Dual Operational Amplifier | |
| Fabricantes | National Semiconductor | |
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February 2002
LM7372
High Speed, High Output Current, Dual Operational
Amplifier
General Description
The LM7372 is a high speed dual voltage feedback amplifier
that has the slewing characteristic of current feedback am-
plifiers; yet it can be used in all traditional voltage feedback
amplifier configurations.
The LM7372 is stable for gains as low as +2 or −1. It
provides a very high slew rate at 3000V/µs and a wide gain
bandwidth product of 120MHz, while consuming only
6.5mA/per amplifier of supply current. It is ideal for video and
high speed signal processing applications such as xDSL and
pulse amplifiers. With 150mA output current, the LM7372
can be used for video distribution, as a transformer driver or
as a laser diode driver.
Operation on ±15V power supplies allows for large signal
swings and provides greater dynamic range and
signal-to-noise ratio. The LM7372 offers high SFDR and low
THD, ideal for ADC/DAC systems. In addition, the LM7372 is
specified for ±5V operation for portable applications.
The LM7372 is built on National’s Advance VIP™ III (Verti-
cally integrated PNP) complementary bipolar process.
Features
n −80dBc highest harmonic distortion @1MHz, 2VPP
n Very high slew rate: 3000V/µs
n Wide gain bandwidth product: 120MHz
n −3dB frequency @ AV = +2: 200MHz
n Low supply current: 13mA (both amplifiers)
n High open loop gain: 85dB
n High output current: 150mA
n Differential gain and phase: 0.01%, 0.02˚
Applications
n HDSL and ADSL Drivers
n Multimedia broadcast systems
n Professional video cameras
n CATV/Fiber optics signal processing
n Pulse amplifiers and peak detectors
n HDTV amplifiers
Typical Application
FIGURE 1. Single Supply Application (SOIC-16)
20004903
© 2002 National Semiconductor Corporation DS200049
www.national.com
1 page  
±5V DC Electrical Characteristics (Continued)
Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature
extremes.
Symbol
Parameter
PSRR
Power Supply Rejection Ratio
VCM Input Common-Mode Voltage Range
AV Large Signal Voltage Gain (Note 7)
Conditions
VS = ±15V to ±5V
CMRR > 60dB
RL = 1kΩ
Min
(Note 6)
75
70
70
65
Typ
(Note 5)
90
±3
78
Max
(Note 6)
Units
dB
V
dB
RL = 100Ω
64 72
60
dB
VO Output Swing
RL = 1kΩ
3.2 3.4
3.0
V
−3.2
−3.0
−3.4
V
IOUT = − 80mA
2.5 2.8
2.2
V
IOUT = 80mA
−2.5
−2.2
−2.7
V
ISC Output Short Circuit Current
Sourcing
Sinking
150 mA
150 mA
IS Supply Current (both Amps)
12.4 16 mA
18
±5V AC Electrical Characteristics
Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature
extremes.
Symbol
Parameter
Conditions
Min Typ Max Units
(Note 6) (Note 5) (Note 6)
SR Slew Rate (Note 8)
Unity Bandwidth Product
AV = +2, VIN 3VP-P
700 V/µs
100 MHz
−3dB Frequency
φm Phase Margin
tS Settling Time (0.1%)
tP Propagation Delay
AV = +2
AV = −1, VO = ±1V, RL =
500Ω
AV = +2, VIN = ±1V, RL =
500Ω
125 MHz
70 deg
70 ns
7 ns
AD Differential Gain (Note 9)
φD Differential Phase (Note 9)
hd2 Second Harmonic Distortion
FIN = 1MHz, AV = +2
hd3 Third Harmonic Distortion
FIN = 1MHz, AV = +2
en Input-Referred Voltage Noise
VOUT = 2VP-P, RL = 100Ω
VOUT = 2VP-P, RL = 100Ω
f = 10kHz
0.02 %
0.03 deg
−84 dBc
−94 dBc
14 nV/
in Input-Referred Current Noise
f = 10kHz
1.8 pA/
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics.
Note 2: For testing purposes, ESD was applied using human body model, 1.5kΩ in series with 100pF. Machine model, 0Ω in series with 200pF.
Note 3: Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the
maximum allowed junction temperature of 150˚C.
5 www.national.com
5 Page 
Application Notes (Continued)
For each amplifier then, with an effective load each of RL and
a sine wave source, integration over the half cycle with a
supply voltage VS and a load voltage VL yields the average
power dissipation
PD = VSVL/πRL - VL2/2RL..........(1)
Where VS is the supply voltage and VL is the peak signal
swing across the load RL.
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 100Ω 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 RL for each ampli-
fier is also 100Ω, and each amplifier is required to deliver
20mW to this load.
Since VL2/2RL = 20mW then VL = 2V(peak).
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
PD(Total) = 312mW + (2 x 132.8mW) = 578mW
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
PD(Total) = (VS x Iavg) - Power in Load
= (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 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 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
VLpeak = 1.4 x 5.33 = 7.5V or 15V(P-P)
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).
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
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 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-
11 www.national.com
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| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet L7372.PDF ] | |
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