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February 7, 2008
LME49600
High Performance, High Fidelity, High Current Audio Buffer
General Description
The LME49600 is a high performance, low distortion high fidelity 250mA audio buffer. The LME49600 is designed for a
wide range of applications. It can be used inside the feedback
loop of op amps.
The LME49600 offers a pin-selectable bandwidth: a low current, 110MHz bandwidth mode that consumes 7.3mA and a
wide 180MHz bandwidth mode that consumes 13.2mA. In
both modes the LME49600 has a nominal 2000V/μs slew
rate. Bandwidth is easily adjusted by either leaving the BW
pin unconnected or connecting a resistor between the BW pin
and the VEE pin.
The LME49600 is fully protected through internal current limit
and thermal shutdown.
Key Specifications
■ Low THD+N
(V
OUT
= 3V
RMS
, f = 1kHz, Figure 2)
0.00003% (typ)
■ Slew Rate 2000V/μs (typ)
■ High Output Current 250mA (typ)
■ Bandwidth
BW pin floating 110MHz (typ)
BW connected to V
EE
180MHz (typ)
■ Supply Voltage Range ±2.25V ≤ V
S
≤ ±18V
Features
■
Pin-selectable bandwidth and quiescent current
■
Pure fidelity. Pure performance
■
Short circuit protection
■
Thermal shutdown
■
TO–263 surface-mount package
Applications
■
Headphone amplifier output drive stage
■
Line drivers
■
Low power audio amplifiers
■
High-current operational amplifier output stage
■
ATE Pin Driver Buffer
■
Power supply regulator
Functional Block Diagram
30029805
FIGURE 1. Simplified Circuit Diagram (Note: I1 and I2 are mirrored from I)
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 300298 www.national.com
LME49600 High Performance, High Fidelity, High Current Audio Buffer
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Connection Diagrams
300298a0
Top View
Order Number LME49600TS
See NS Package Number TS5B
30029832
Top View
U — Wafer fabrication code
Z — Assembly plant
XY — 2 Digit date code
TT — Lot traceability
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LME49600
Page 3
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage ±20V
ESD Ratings(Note 4) 2000V
ESD Ratings (Note 5) 200V
Storage Temperature −40°C to +150°C
Junction Temperature 150°C
Thermal Resistance
θ
JC
4°C/W
θ
JA
65°C/W
θJA (Note 3)
20°C/W
Soldering Information
TO-263 Package (10 seconds) 260°C
Operating Ratings (Notes 1, 2)
Temperature Range
T
MIN
≤ TA ≤ T
MAX
−40°C ≤ TA ≤ 85°C
Supply Voltage ±2.25V to ±18V
System Electrical Characteristics for LME49600 The following specifications apply for V
S
= ±15V,
fIN = 1kHz, unless otherwise specified. Typicals and limits apply for TA = 25°C.
Symbol Parameter Conditions
LME49600
Units
(Limits)
Typical Limit
(Note 6) (Note 7)
I
Q
Total Quiescent Current
I
OUT
= 0
BW pin: No connect
BW pin: Connected to VEE pin
7.3
13.2
10.5
18
mA (max)
mA (max)
THD+N
Total Harmonic Distortion + Noise
(Note 8)
AV = 1, V
OUT
= 3V
RMS
, R
L
= 32Ω, BW = 80kHz,
closed loop see Figure 2.
f = 1kHz
f = 20kHz
0.000035
0.0005
%
%
SR Slew Rate
30 ≤ BW ≤ 180MHz
V
OUT
= 20V
P-P
, RL = 100Ω
2000
V/μs
BW
Bandwidth
AV = –3dB
BW pin: No Connect
RL = 100Ω
RL = 1kΩ
100
110
MHz
MHz
AV = –3dB
BW pin: Connected to VEE pin
RL = 100Ω
RL = 1kΩ
160
180
MHz
MHz
Voltage Noise Density
f = 10kHz
BW pin: No Connect
3.0
nV/√Hz
f = 10kHz
BW pin: Connected to V
EE
pin
2.6
nV/√Hz
t
s
Settling Time
ΔV = 10V, RL = 100Ω
1% Accuracy
BW pin: No connect
BW pin: Connected to VEE pin
200
60
ns
ns
A
V
Voltage Gain
V
OUT
= ±10V
RL = 67Ω
RL = 100Ω
RL = 1kΩ
0.93
0.95
0.99
0.90
0.92
0.98
V/V (min)
V/V (min)
V/V (min)
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LME49600
Page 4
Symbol Parameter Conditions
LME49600
Units
(Limits)
Typical Limit
(Note 6) (Note 7)
V
OUT
Voltage Output
Positive
I
OUT
= 10mA
I
OUT
= 100mA
I
OUT
= 150mA
VCC –1.4
VCC –2.0
VCC –2.3
VCC –1.6
VCC –2.1
VCC –2.7
V (min)
V (min)
V (min)
Negative
I
OUT
= –10mA
I
OUT
= –100mA
I
OUT
= –150mA
VEE +1.5
VEE +3.1
VEE +3.5
VEE +1.6
VEE +2.4
VEE +3.2
V (min)
V (min)
V (min)
I
OUT
Output Current
±250 mA
I
OUT-SC
Short Circuit Output Current
BW pin: No Connect
BW pin: Connected to VEE pin
±490
±490
±550
mA (max)
mA (max)
I
B
Input Bias Current
VIN = 0V
BW pin: No Connect
BW pin: Connected to VEE pin
±1.0
±3.0
±2.5
±5.0
μA (max)
μA (max)
Z
IN
Input Impedance
RL = 100Ω
BW pin: No Connect
BW pin: Connected to VEE pin
7.5
5.5
MΩ
MΩ
V
OS
Offset Voltage ±17 ±60 mV (max)
VOS/°C
Offset Voltage vs Temperature
40°C ≤ TA ≤ +125°C
±100
μV/°C
Note 1: All voltages are measured with respect to ground, unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions
which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters
where no limit is given, however, the typical value is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
JMAX
, θJA, and the ambient temperature TA. The maximum
allowable power dissipation is P
DMAX
= (T
JMAX–TA
)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LME49600, typical
application (shown in Figure 2) with V
SUPPLY
= 30V, RL = 32Ω, the total power dissipation is 1.9W. θJA = 20°C/W for the TO–263 package mounted to 16in2 1oz
copper surface heat sink area.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 5: Machine Model, 220pF – 240pF discharged through all pins.
Note 6: Typical specifications are specified at 25°C and represent the parametric norm.
Note 7: Tested limits are guaranteed to National's AOQL (Average Outgoing Quality Level).
Note 8: This is the distortion of the LME49600 operating in a closed loop configuration with an LME49710. When operating in an operational amplifier's feedback
loop, the amplifier’s open loop gain dominates, linearizing the system and determining the overall system distortion.
Note 9: The TSB package is non-isolated package. The package's metal back and any heat sink to which it is mounted are connected to the same potential as
the -VEE pin.
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LME49600
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Typical Performance Characteristics
Gain vs Frequency vs Quiescent Current
30029899
Phase vs Frequency vs Quiescent Current
30029881
Gain vs Frequency vs Power Supply Voltage
Wide BW Mode
30029898
Phase vs Frequency vs Supply Voltage
Wide BW Mode
30029880
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LME49600
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Gain vs Frequency vs Power Supply Voltage
Low IQ Mode
30029897
Phase vs Frequency vs Supply Voltage
Low IQ Mode
30029879
Gain vs Frequency vs R
LOAD
Wide BW Mode
30029896
Phase vs Frequency vs R
LOAD
Wide BW Mode
30029878
Gain vs Frequency vs R
LOAD
Low IQ Mode
30029895
Phase vs Frequency vs R
LOAD
Low IQ Mode
30029877
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LME49600
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Gain vs Frequency vs C
LOAD
Wide BW Mode
30029894
Phase vs Frequency vs C
LOAD
Wide BW Mode
30029875
Gain vs Frequency vs C
LOAD
Low IQ Mode
30029893
Phase vs Frequency vs C
LOAD
Low IQ Mode
30029876
±PSRR vs Frequency
VS = ±15V, Wide BW Mode
30029890
±PSRR vs Frequency
VS = ±15V, Low IQ Mode
30029889
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LME49600
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±PSRR vs Frequency
VS = ±15V, Wide BW Mode
30029892
±PSRR vs Frequency
VS = ±15V, Low IQ Mode
30029891
Quiescent Current vs Bandwidth Control Resistance
30029888
THD+N vs Output Voltage
VS = ±15V, RL = 32Ω, f = 1kHz
Both channels driven
300298j4
High BW Noise Curve
30029845
Low BW Noise Curve
30029846
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LME49600
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Typical Application Diagram
300298j5
FIGURE 2. High Performance, High Fidelity LME49600 Audio Buffer Application
DISTORTION MEASUREMENTS
The vanishingly low residual distortion produced by
LME49710/LME49600 is below the capabilities of all commercially available equipment. This makes distortion measurements just slightly more difficult than simply connecting
a distortion meter to the amplifier’s inputs and outputs. The
solution, however, is quite simple: an additional resistor.
Adding this resistor extends the resolution of the distortion
measurement equipment.
The LME49710/LME49600’s low residual distortion is an input referred internal error. As shown in Figure 3, adding the
10Ω resistor connected between the amplifier’s inverting and
non-inverting inputs changes the amplifier’s noise gain. The
result is that the error signal (distortion) is amplified by a factor
of 101. Although the amplifier’s closed-loop gain is unaltered,
the feedback available to correct distortion errors is reduced
by 101, which means that measurement resolution increases
by 101. To ensure minimum effects on distortion measurements, keep the value of R1 low as shown in Figure 3.
This technique is verified by duplicating the measurements
with high closed loop gain and/or making the measurements
at high frequencies. Doing so produces distortion components that are within the measurement equipment’s capabilities. This datasheet’s THD+N and IMD values were generated using the above described circuit connected to an Audio
Precision System Two Cascade.
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LME49600
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30029843
FIGURE 3. THD+N Distortion Test Circuit
Application Information
HIGH PERFORMANCE, HIGH FIDELITY HEADPHONE
AMPLIFIER
The LME49600 is the ideal solution for high output, high performance high fidelity head phone amplifiers. When placed in
the feedback loop of the LME49710, LME49720 or
LME49740 High Performance, High Fidelity audio operational
amplifier, the LME49600 is able to drive 32Ω headphones to
a dissipation of greater than 500mW at 0.00003% THD+N
while operating on ±15V power supply voltages. The circuit
schematic for a typical headphone amplifier is shown in Figure 4.
Operation
The following describes the circuit operation for the headphone amplifier’s Left Channel. The Right Channel operates
identically.
The audio input signal is applied to the input jack (HP31 or
J1/J2) and dc-coupled to the volume control, VR1. The output
signal from VR1’s wiper is applied to the non-inverting input
of U2-A, an LME49720 High Performance, High Fidelity audio
operational amplifier. U2-A’s AC signal gain is set by resistors
R2, R4, and R6. To allow for a DC-coupled signal path and to
ensure minimal output DC voltage regardless of the closedloop gain, the other half of the U2 is configured as a DC servo.
By constantly monitoring U2-A’s output, the servo creates a
voltage that compensates for any DC voltage that may be
present at the output. A correction voltage is generated and
applied to the feedback node at U2-A, pin 2. The servo ensures that the gain at DC is unity. Based on the values shown
in Figure 4, the RC combination formed by R11 and C7 sets
the servo’s high-pass cutoff at 0.16Hz. This is over two
decades below 20Hz, minimizing both amplitude and phase
perturbations in the audio frequency band’s lowest frequencies.
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LME49600
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30029858
FIGURE 4. LME49600 delivers high output current for this high performance headphone amplifier
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LME49600
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AUDIO BUFFERS
Audio buffers or unity-gain followers, have large current gain
and a voltage gain of one. Audio buffers serve many applications that require high input impedance, low output
impedance and high output current. They also offer constant
gain over a very wide bandwidth.
Buffers serve several useful functions, either in stand-alone
applications or in tandem with operational amplifiers. In standalone applications, their high input impedance and low output
impedance isolates a high impedance source from a low
impedance load.
SUPPLY BYPASSING
The LME49600 will place great demands on the power supply
voltage source when operating in applications that require
fast slewing and driving heavy loads. These conditions can
create high amplitude transient currents. A power supply’s
limited bandwidth can reduce the supply’s ability to supply the
needed current demands during these high slew rate conditions. This inability to supply the current demand is further
exacerbated by PCB trace or interconnecting wire inductance. The transient current flowing through the inductance
can produce voltage transients.
For example, the LME49600’s output voltage can slew at a
typical ±2000V/μs. When driving a 100Ω load, the di/dt current
demand is 20 A/μs. This current flowing through an inductance of 50nH (approximately 1.5” of 22 gage wire) will produce a 1V transient. In these and similar situations, place the
parallel combination of a solid 5μF to 10μF tantalum capacitor
and a ceramic 0.1μF capacitor as close as possible to the
device supply pins.
Ceramic capacitors with values in the range of 10μF to
100μF, ceramic capacitor have very lower ESR (typically less
than 10mΩ) and low ESL when compared to the same valued
tantalum capacitor. The ceramic capacitors, therefore, have
superior AC performance for bypassing high frequency noise.
In less demanding applications that have lighter loads or lower slew rates, the supply bypassing is not as critical. Capacitor
values in the range of 0.01μF to 0.1μF are adequate.
SIMPLIFIED LME49600 CIRCUIT DIAGRAM
The LME49600’s simplified circuit diagram is shown in Figures 1 and 5. The diagram shows the LME49600’s complementary emitter follower design, bias circuit and bandwidth
adjustment node.
30029805
FIGURE 5. Simplified Circuit Diagram
Figure 6 shows the LME49600 connected as an open-loop
buffer. The source impedance and optional input resistor,
RS, can alter the frequency response. As previously stated,
the power supplies should be bypassed with capacitors connected close to the LME49600’s power supply pins. Capacitor
values as low as 0.01μF to 0.1μF will ensure stable operation
in lightly loaded applications, but high output current and fast
output slewing can demand large current transients from the
power supplies. Place a recommended parallel combination
of a solid tantalum capacitor in the 5μF to 10μF range and a
ceramic 0.1μF capacitor as close as possible to the device
supply pins.
30029860
FIGURE 6. Buffer Connections
OUTPUT CURRENT
The LME49600 can continuously source or sink 250mA. Internal circuitry limits the short circuit output current to approximately ±450mA. For many applications that fully utilize the
LME49600’s current source and sink capabilities, thermal dissipation may be the factor that limits the continuous output
current.
The maximum output voltage swing magnitude varies with
junction temperature and output current. Using sufficient PCB
copper area as a heat sink when the metal tab of the
LME49600’s surface mount TO–263 package is soldered directly to the circuit board reduces thermal impedance. This in
turn reduces junction temperature. The PCB copper area
should be in the range of 3in2 (12.9cm2) to 6in2 (38.7cm2).
THERMAL PROTECTION
LME49600 power dissipated will cause the buffer’s junction
temperature to rise. A thermal protection circuit in the
LME49600 will disable the output when the junction temperature exceeds 150°C. When the thermal protection is activated, the output stage is disabled, allowing the device to cool.
The output circuitry is enabled when the junction temperature
drops below 150°C.
The TO–263 package has excellent thermal characteristics.
To minimize thermal impedance, its exposed die attach paddle should be soldered to a circuit board copper area for good
heat dissipation. Figure 7 shows typical thermal resistance
from junction to ambient as a function of the copper area. The
TO–263’s exposed die attach paddle is electrically connected
to the VEE power supply pin.
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LME49600
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LOAD IMPEDANCE
The LME49600 is stable under any capacitive load when driven by a source that has an impedance of 50Ω or less. When
driving capacitive loads, any overshoot that is present on the
output signal can be reduced by shunting the load capacitance with a resistor.
OVERVOLTAGE PROTECTION
If the input-to-output differential voltage exceeds the
LME49600’s Absolute Maximum Rating of 3V, the internal
diode clamps shown in Figures 1 and 5 conduct, diverting
current around the compound emitter followers of Q1/Q5 (D1
– D7 for positive input), or around Q2/Q6 (D8 – D14 for negative inputs). Without this clamp, the input transistors Q1/Q2
and Q5/Q6 will zener and damage the buffer.
To ensure that the current flow through the diodes is held to
a save level, the internal 200Ω resistor in series with the input
limits the current through these clamps. If the additional current that flows during this situation can damage the source
that drives the LME49600’s input, add an external resistor in
series with the input (see Figure 6).
BANDWITH CONTROL PIN
The LME49600’s –3dB bandwidth is approximately 110MHz
in the low quiescent-current mode (7.3mA typical). Select this
mode by leaving the BW pin unconnected.
Connect the BW pin to the V
EE
pin to extend the LME49600’s
bandwidth to a nominal value of 180MHz. In this mode, the
quiescent current increases to approximately 13.2mA. Bandwidths between these two limits are easily selected by connecting a series resistor between the BW pin and V
EE
.
Regardless of the connection to the LME49600’s BW pin, the
rated output current and slew rate remain constant. With the
power supply voltage held constant, the wide-bandwidth
mode’s increased quiescent current causes a corresponding
increase in quiescent power dissipation. For all values of the
BW pin voltage, the quiescent power dissipation is equal to
the total supply voltage times the quiescent current (IQ *
(VCC + |V
EE
|)).
BOOSTING OP AMP OUTPUT CURRENT
When placed in the feedback loop, the LME49600 will increase an operational amplifier’s output current. The operational amplifier’s open loop gain will correct any LME49600
errors while operating inside the feedback loop.
To ensure that the operational amplifier and buffer system are
closed loop stable, the phase shift must be low. For a system
gain of one, the LME49600 must contribute less than 20° at
the operational amplifier’s unity-gain frequency. Various operating conditions may change or increase the total system
phase shift. These phase shift changes may affect the operational amplifier's stability.
Unity gain stability is preserved when the LME49600 is placed
in the feedback loop of most general-purpose or precision op
amps. When the LME46900 is driving high value capacitive
loads, the BW pin should be connected to the VEE pin for wide
bandwidth and stable operation. The wide bandwidth mode is
also suggested for high speed or fast-settling operational amplifiers. This preserves their stability and the ability to faithfully
amplify high frequency, fast-changing signals. Stability is ensured when pulsed signals exhibit no oscillations and ringing
is minimized while driving the intended load and operating in
the worst-case conditions that perturb the LME49600’s phase
response.
HIGH FREQUENCY APPLICATIONS
The LME49600’s wide bandwidth and very high slew rate
make it ideal for a variety of high-frequency open-loop applications such as an ADC input driver, 75Ω stepped volume
attenuator driver, and other low impedance loads. Circuit
board layout and bypassing techniques affect high frequency,
fast signal dynamic performance when the LME49600 operates open-loop.
A ground plane type circuit board layout is best for very high
frequency performance results. Bypass the power supply pins
(VCC and VEE) with 0.1μF ceramic chip capacitors in parallel
with solid tantalum 10μF capacitors placed as close as possible to the respective pins.
Source resistance can affect high-frequency peaking and
step response overshoot and ringing. Depending on the signal source, source impedance and layout, best nominal response may require an additional resistance of 25Ω to
200Ω in series with the input. Response with some loads (especially capacitive) can be improved with an output series
resistor in the range of 10Ω to 150Ω.
THERMAL MANAGEMENT
Heatsinking
For some applications, the LME49600 may require a heat
sink. The use of a heat sink is dependent on the maximum
LME49600 power dissipation and a given application’s maximum ambient temperature. In the TO-263 package, heat
sinking the LME49600 is easily accomplished by soldering
the package’s tab to a copper plane on the PCB. (Note: The
tab on the LME49600’s TO-263 package is electrically connected to VEE.)
Through the mechanisms of convection, heat conducts from
the LME49600 in all directions. A large percentage moves to
the surrounding air, some is absorbed by the circuit board
material and some is absorbed by the copper traces connected to the package’s pins. From the PCB material and the
copper, it then moves to the air. Natural convection depends
on the amount of surface area that contacts the air.
If a heat conductive copper plane has perfect thermal conduction (heat spreading) through the plane’s total area, the
temperature rise is inversely proportional to the total exposed
area. PCB copper planes are, in that sense, an aid to convection. These planes, however, are not thick enough to
ensure perfect heat conduction. Therefore, eventually a point
of diminishing returns is reached where increasing copper
area offers no additional heat conduction to the surrounding
air. This is apparent in Figure 7 as the thermal resistance
reaches an asymptote above a copper area of 8in2). As can
be seen, increasing the copper area produces decreasing
improvements in thermal resistance. This occurs, roughly, at
4in2 of 1 oz copper board. Some improvement continues until
about 16in2. Boards using 2 oz copper boards will have decrease thermal resistance providing a better heat sink compared to 1 oz. copper. Beyond 1oz or 2oz copper plane areas,
external heat sinks are required. Ultimately, the 1oz copper
area attains a nominal value of 20°C/W junction to ambient
thermal resistance (θJA) under zero air flow.
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LME49600
Page 14
30029861
FIGURE 7. Thermal Resistance for 5 lead TO–263
Package Mounted on 1oz. Copper
A copper plane may be placed directly beneath the tab. Additionally, a matching plane can be placed on the opposite
side. If a plane is placed on the side opposite of the
LME49600, connect it to the plane to which the buffer’s metal
tab is soldered with a matrix of thermal vias per JEDEC Standard JESD51-5.
Determining Copper Area
Find the required copper heat sink area using the following
guidelines:
1. Determine the value of the circuit’s power dissipation, PD.
2. Specify a maximum operating ambient temperature, T
A
(MAX).
(Note that the die temperature, TJ, will be higher than
TA by an amount that is dependent on the thermal resistance
from junction to ambient, θJA). Therefore, TA must be specified such that TJ does not exceed the absolute maximum die
temperature of 150°C.
3. Specify a maximum allowable junction temperature, T
J
(MAX)
, This is the LME49600’s die temperature when the buffer
is drawing maximum current (quiescent and load). It is prudent to design for a maximum continuous junction temperature of 100°C to 130°C. Ensure, however, that the junction
temperature never exceeds the 150°C absolute maximum
rating for the part.
4. Calculate the value of junction to ambient thermal resistance, θ
JA
5. θJA as a function of copper area in square inches is shown
in Figure 7. Choose a copper area that will guarantee the
specified T
J(MAX)
for the calculated θJA. The maximum value
of junction to ambient thermal resistance, θJA, is defined as:
θJA= (T
J(MAX)
- T
A(MAX)
)/ P
D(MAX)
(°C/W) (1)
where:
T
J(MAX)
= the maximum recommended junction temperature
T
A(MAX)
= the maximum ambient temperature in the
LME49600’s environment
P
D(MAX)
= the maximum recommended power dissipation
Note: The allowable thermal resistance is determined by the
maximum allowable temperature increase:
T
RISE
= T
J(MAX)
- T
A(MAX)
Thus, if ambient temperature extremes force T
RISE
to exceed
the design maximum, the part must be de-rated by either decreasing PD to a safe level, reducing θJA further or, if available,
using a larger copper area.
Procedure
1. First determine the maximum power dissipated by the
LME49600, P
D(MAX)
. For the simple case of the buffer driving
a resistive load, and assuming equal supplies, P
D(MAX)
is giv-
en by:
P
DMAX(AC)
= (IS x VS) + (VS)2 / (2π2RL) (Watts) (2)
P
DMAX(DC)
= (IS x VS) + (VS)2 / RL (Watts) (3)
where:
VS = |VEE| + V
CC
(V)
IS =quiescent supply current (A)
Equation (2) is for sinusoidal output voltages and (3) is for DC
output voltages
2. Determine the maximum allowable die temperature rise,
T
RISE(MAX)
= T
J(MAX)
- T
A(MAX)
(°C)
3. Using the calculated value of T
RISE(MAX)
and P
D(MAX)
, find
the required value of junction to ambient thermal resistance
combining equation 1 and equation 4 to derive equation 5:
θJA = T
RISE(MAX)
/ P
D(MAX)
(4)
4. Finally, choose the minimum value of copper area from
Figure 7 based on the value for θJA.
Example
Assume the following conditions: VS = |VEE| + VCC = 30V, R
L
= 32Ω, IS = 15mA, sinusoidal output voltage, T
J(MAX)
= 125°
C, T
A(MAX)
= 85°C.
Applying Equation (2):
P
DMAX
= (IS x VS) + (VS)2 / 2π2R
L
= (15mA)(30V) + 900V2 / 142Ω
= 1.86W
Applying Equation (4):
T
RISE(MAX)
= 125°C – 85°C
= 40°C
Applying Equation (5):
θJA = 40°C/1.86W
= 21.5°C/W
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LME49600
Page 15
Examining the Copper Area vs. θJA plot indicates that a thermal resistance of 50°C/W is possible with a 12in2 plane of one
layer of 1oz copper. Other solutions include using two layers
of 1oz copper or the use of 2oz copper. Higher dissipation
may require forced air flow. As a safety margin, an extra 15%
heat sinking capability is recommended.
When amplifying AC signals, wave shapes and the nature of
the load (reactive, non-reactive) also influence dissipation.
Peak dissipation can be several times the average with reactive loads. It is particularly important to determine dissipation
when driving large load capacitance.
The LME49600’s dissipation in DC circuit applications is easily computed using Equation (3). After the value of dissipation
is determined, the heat sink copper area calculation is the
same as for AC signals.
SLEW RATE
A buffer’s voltage slew rate is its output signal’s rate of change
with respect to an input signal’s step changes. For resistive
loads, slew rate is limited by internal circuit capacitance and
operating current (in general, the higher the operating current
for a given internal capacitance, the faster the slew rate).
However, when driving capacitive loads, the slew rate may be
limited by the available peak output current according to the
following expression.
dv/dt = IPK / C
L
(5)
Output voltages with high slew rates will require large output
load currents. For example if the part is required to slew at
1000V/μs with a load capacitance of 1nF, the current demanded from the LME49600 is 1A. Therefore, fast slew rate
is incompatible with a capacitive load of this value. Also, if
CL is in parallel with the load, the peak current available to the
load decreases as CL increases.
15 www.national.com
LME49600
Page 16
30029844
FIGURE 8. High Speed Positive and Negative Regulator
www.national.com 16
LME49600
Page 17
Revision History
Rev Date Description
1.0 01/15/08 Initial release.
1.01 01/16/08 Edited specification table.
1.02 02/07/08 Edited applications information.
17 www.national.com
LME49600
Page 18
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LME49600TS
See NS Package TS5B
www.national.com 18
LME49600
Page 19
Notes
19 www.national.com
LME49600
Page 20
Notes
LME49600 High Performance, High Fidelity, High Current Audio Buffer
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