May 14, 2008
LM4755
Stereo 11W Audio Power Amplifier with Mute
General Description
The LM4755 is a stereo audio amplifier capable of delivering
11W per channel of continuous average output power to a
4Ω load or 7W per channel into 8Ω using a single 24V supply
at 10% THD+N. The internal mute circuit and pre-set gain re-
sistors provide for a very economical design solution.
Output power specifications at both 20V and 24V supplies
and low external component count offer high value to con-
sumer electronic manufacturers for stereo TV and compact
stereo applications. The LM4755 is specifically designed for
single supply operation.
Key Specifications
■Output power at 10% THD with 1kHz into 4Ω at
VCC = 24V: 11W (typ)
■Output power at 10% THD with 1kHz into 8Ω at
VCC = 24V: 7W (typ)
■Closed loop gain: 34dB (typ)
■PO at 10% THD+N @ 1kHz into 4Ω single-ended TO-263
package at VCC=12V: 2.5W (typ)
■PO at 10% THD+N @ 1kHz into 8Ω bridged TO-263
package at VCC=12V: 5W (typ)
Features
■Drives 4Ω and 8Ω loads
■Integrated mute function
■Internal Gain Resistors
■Minimal external components needed
■Single supply operation
■Internal current limiting and thermal protection
■Compact 9-lead TO-220 package
■Wide supply range 9V - 40V
Applications
■Stereos TVs
■Compact stereos
■Mini component stereos
Typical Application
10005901
FIGURE 1. Typical Audio Amplifier Application Circuit
© 2008 National Semiconductor Corporation 100059 www.national.com
LM4755 Stereo 11W Audio Power Amplifier with Mute
Connection Diagrams
Plastic Package
10005902
Package Description
Top View
Order Number LM4755T
Package Number TA09A
10005936
Top View
Order Number LM4755TS
Package Number TS9A
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LM4755
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 40V
Input Voltage ±0.7V
Input Voltage at Output Pins (Note 8) GND -0.4V
Output Current Internally Limited
Power Dissipation (Note 3) 62.5W
ESD Susceptibility (Note 4) 2 kV
Junction Temperature 150°C
Soldering Information
T Package (10 seconds) 250°C
Storage Temperature −40°C to 150°C
Operating Ratings
Temperature Range
TMIN ≤ TA ≤ TMAX −40°C ≤ TA ≤ +85°C
Supply Voltage 9V to 32V
θJC 2°C/W
θJA 76°C/W
Electrical Characteristics
The following specifications apply to each channel with VCC = 24V, TA = 25°C unless otherwise specified.
Symbol Parameter Conditions
LM4755 Units
(Limits)
Typical
(Note 5)
Limit
ITOTAL Total Quiescent Power
Supply Current
Mute Off 10 15 mA(max)
7 mA(min)
Mute On 7 mA
POOutput Power (Continuous
Average per Channel)
f = 1 kHz, THD+N = 10%, RL = 8Ω 7 W
f = 1 kHz, THD+N = 10%, RL = 4Ω 11 10 W(min)
VS = 20V, RL = 8Ω 4 W
VS = 20V, RL = 4Ω 7 W
f = 1 kHz, THD+N = 10%, RL = 4Ω
VS = 12V, TO-263 Pkg. 2.5 W
THD Total Harmonic Distortion f = 1 kHz, PO = 1 W/ch, RL = 8Ω 0.08 %
VOSW Output Swing PO = 10W, RL = 8Ω 15 V
PO = 10W, RL = 4Ω 14 V
XTALK Channel Separation See Apps. Circuit 55 dB
f = 1 kHz, VO = 4 Vrms
PSRR Power Supply Rejection Ratio See Apps. Circuit 50 dB
f = 120 Hz, VO = 1 mVrms
VODV Differential DC Output Offset
Voltage
VIN = 0V 0.09 0.4 V(max)
SR Slew Rate 2 V/µs
RIN Input Impedance 83 kΩ
PBW Power Bandwidth 3 dB BW at PO = 2.5W, RL = 8Ω 65 kHz
AVCL Closed Loop Gain (Internally Set) RL = 8Ω 34 33 dB(min)
35 dB(max)
εIN Noise IHF-A Weighting Filter, RL = 8Ω
Output Referred
0.2 mVrms
IOOutput Short Circuit Limit VIN = 0.5V, RL = 2Ω 2 A(min)
Mute Pin
VIL
Mute Low Input Voltage Not in Mute Mode 0.8 V(max)
VIH Mute High Input Voltage In Mute Mode 2.0 2.5 V(min)
AMMute Attenuation VMUTE = 5.0V 80 dB
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LM4755
Note 1: All voltages are measured with respect to the GND pin (5), unless otherwse 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: For operating at case temperatures above 25°C, the device must be derated based on a 150°C maximum junction temperature and a thermal resistance
of θJC = 2°C/W (junction to case). Refer to the section Determining the Maximum Power Dissipation in the Application Information section for more information.
Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 5: Typicals are measured at 25°C and represent the parametric norm.
Note 6: Limits are guaranteed that all parts are tested in production to meet the stated values.
Note 7: The TO-263 Package is not recommended for VS > 16V due to impractical heatsinking limitations.
Note 8: The outputs of the LM4755 cannot be driven externally in any mode with a voltage lower than -0.4V below GND or permanent damage to the LM4755
will result.
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LM4755
Equivalent Schematic
10005903
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LM4755
Test Circuit
10005904
FIGURE 2. Test Circuit
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LM4755
System Application Circuit
10005905
FIGURE 3. Circuit for External Components Description
External Components Description
Components Function Description
1, 2 CSProvides power supply filtering and bypassing.
3, 4 RSN Works with CSN to stabilize the output stage from high frequency oscillations.
5, 6 CSN Works with RSN to stabilize the output stage from high frequency oscillations.
7 CbProvides filtering for the internally generated half-supply bias generator.
8, 9 CiInput AC coupling capacitor which blocks DC voltage at the amplifier's input terminals. Also creates a high pass
filter with fc=1/(2 • π • Rin • Cin).
10, 11 CoOutput AC coupling capacitor which blocks DC voltage at the amplifier's output terminal. Creates a high pass
filter with fc=1/(2 • π • Rout • Cout).
12, 13 RiVoltage control - limits the voltage level allowed to the amplifier's input terminals.
14 RmWorks with Cm to provide mute function timing.
15 CmWorks with Rm to provide mute function timing.
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LM4755
THD+N vs Output Power
10005915
THD+N vs Output Power
10005916
THD+N vs Output Power
10005917
THD+N vs Output Power
10005909
THD+N vs Output Power
10005910
THD+N vs Output Power
10005911
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LM4755
THD+N vs Output Power
10005938
THD+N vs Output Power
10005939
THD+N vs Output Power
10005940
THD+N vs Output Power
10005941
THD+N vs Output Power
10005942
THD+N vs Output Power
10005943
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LM4755
THD+N vs Output Power
10005944
THD+N vs Output Power
10005945
THD+N vs Output Power
10005946
THD+N vs Output Power
10005947
THD+N vs Output Power
10005948
THD+N vs Output Power
10005949
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LM4755
Output Power vs Supply Voltage
10005918
Output Power vs Supply Voltage
10005919
Frequency Response
10005920
THD+N vs Frequency
10005921
THD+N vs Frequency
10005922
Frequency Response
10005923
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LM4755
Channel Separation
10005924
PSRR vs Frequency
10005925
Supply Current vs Supply Voltage
10005926
Power Derating Curve
10005927
Power Dissipation vs Output Power
10005928
Power Dissipation vs Output Power
10005929
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LM4755
Power Dissipation vs Output Power
10005960
Power Dissipation vs Output Power
10005961
Application Information
The LM4755 contains circuitry to pull down the bias line in-
ternally, effectively shutting down the input stage. An external
R-C should be used to adjust the timing of the pull-down. If
the bias line is pulled down too quickly, currents induced in
the internal bias resistors will cause a momentary DC voltage
to appear across the inputs of each amplifier's internal differ-
ential pair, resulting in an output DC shift towards Vsupply.
An R-C timing circuit should be used to limit the pull-down
time such that output “pops” and signal feedthroughs will be
minimized. The pull-down timing is a function of a number of
factors, including the internal mute circuitry, the voltage used
to activate the mute, the bias capacitor, the half-supply volt-
age, and internal resistances used in the half-supply genera-
tor. Table 1 shows a list of recommended values for the
external R-C.
TABLE 1. Recommended Values for Mute Circuit
VMUTE VCC Rm Cm
5V 12V 18 kΩ10 µF
5V 15V 18 kΩ10 µF
5V 20V 12 kΩ10 µF
5V 24V 12 kΩ10 µF
5V 28V 8.2 kΩ10 µF
5V 30V 8.2 kΩ10 µF
CAPACITOR SELECTION AND FREQUENCY RESPONSE
With the LM4755, as in all single supply amplifiers, AC cou-
pling capacitors are used to isolate the DC voltage present at
the inputs (pins 3, 7) and outputs (pins 1, 8). As mentioned
earlier in the External Components section these capacitors
create high-pass filters with their corresponding input/output
impedances. The Typical Application Circuit shown in Fig-
ure 1 shows input and output capacitors of 0.1 µF and 1,000
µF respectively. At the input, with an 83 kΩ typical input re-
sistance, the result is a high pass 3 dB point occurring at 19
Hz. There is another high pass filter at 39.8 Hz created with
the output load resistance of 4Ω. Careful selection of these
components is necessary to ensure that the desired frequen-
cy response is obtained. The Frequency Response curves in
the Typical Performance Characteristics section show how
different output coupling capacitors affect the low frequency
roll-off.
OPERATING IN BRIDGE-MODE
Though designed for use as a single-ended amplifier, the
LM4755 can be used to drive a load differentially (bridge-
mode). Due to the low pin count of the package, only the non-
inverting inputs are available. An inverted signal must be
provided to one of the inputs. This can easily be done with the
use of an inexpensive op-amp configured as a standard in-
verting amplifier. An LF353 is a good low-cost choice. Care
must be taken, however, for a bridge-mode amplifier must
theoretically dissipate four times the power of a single-ended
type. The load seen by each amplifier is effectively half that
of the actual load being used, thus an amplifier designed to
drive a 4Ω load in single-ended mode should drive an 8Ω load
when operating in bridge-mode.
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LM4755
10005930
FIGURE 4. Bridge-Mode Application
10005931 10005937
FIGURE 5. THD+N vs POUT for Bridge-Mode Application
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LM4755
PREVENTING OSCILLATIONS
With the integration of the feedback and bias resistors on-
chip, the LM4755 fits into a very compact package. However,
due to the close proximity of the non-inverting input pins to
the corresponding output pins, the inputs should be AC ter-
minated at all times. If the inputs are left floating, the amplifier
will have a positive feedback path through high impedance
coupling, resulting in a high frequency oscillation. In most ap-
plications, this termination is typically provided by the previ-
ous stage's source impedance. If the application will require
an external signal, the inputs should be terminated to ground
with a resistance of 50 kΩ or less on the AC side of the input
coupling capacitors.
UNDERVOLTAGE SHUTDOWN
If the power supply voltage drops below the minimum oper-
ating supply voltage, the internal under-voltage detection cir-
cuitry pulls down the half-supply bias line, shutting down the
preamp section of the LM4755. Due to the wide operating
supply range of the LM4755, the threshold is set to just under
9V. There may be certain applications where a higher thresh-
old voltage is desired. One example is a design requiring a
high operating supply voltage, with large supply and bias ca-
pacitors, and there is little or no other circuitry connected to
the main power supply rail. In this circuit, when the power is
disconnected, the supply and bias capacitors will discharge
at a slower rate, possibly resulting in audible output distortion
as the decaying voltage begins to clip the output signal. An
external circuit may be used to sense for the desired thresh-
old, and pull the bias line (pin 6) to ground to disable the input
preamp. Figure 6 shows an example of such a circuit. When
the voltage across the zener diode drops below its threshold,
current flow into the base of Q1 is interrupted. Q2 then turns
on, discharging the bias capacitor. This discharge rate is gov-
erned by several factors, including the bias capacitor value,
the bias voltage, and the resistor at the emitter of Q2. An
equation for approximating the value of the emitter discharge
resistor, R, is given below:
R = (0.7v) / (Cb • (VCC/2) / 0.1s)
Note that this is only a linearized approximation based on a
discharge time of 0.1s. The circuit should be evaluated and
adjusted for each application.
As mentioned earlier in the Built-in Mute Circuit section,
when using an external circuit to pull down the bias line, the
rate of discharge will have an effect on the turn-off induced
distortions. Please refer to the Built-in Mute Circuit section
for more information.
10005932
FIGURE 6. External Undervoltage Pull-Down
THERMAL CONSIDERATIONS
Heat Sinking
Proper heatsinking is necessary to ensure that the amplifier
will function correctly under all operating conditions. A
heatsink that is too small will cause the die to heat excessively
and will result in a degraded output signal as the thermal pro-
tection circuitry begins to operate.
The choice of a heatsink for a given application is dictated by
several factors: the maximum power the IC needs to dissi-
pate, the worst-case ambient temperature of the circuit, the
junction-to-case thermal resistance, and the maximum junc-
tion temperature of the IC. The heat flow approximation equa-
tion used in determining the correct heatsink maximum
thermal resistance is given below:
TJ–TA = PDMAX • (θJC + θCS + θSA)
where:
PDMAX = maximum power dissipation of the IC
TJ(°C) = junction temperature of the IC
TA(°C) = ambient temperature
θJC(°C/W) = junction-to-case thermal resistance of the IC
θCS(°C/W) = case-to-heatsink thermal resistance (typically
0.2 to 0.5 °C/W)
θSA(°C/W) = thermal resistance of heatsink
When determining the proper heatsink, the above equation
should be re-written as:
θSA ≤ [(TJ–TA) / PDMAX] - θJC–θCS
TO-263 HEATSINKING
Surface mount applications will be limited by the thermal dis-
sipation properties of printed circuit board area. The TO-263
package is not recommended for surface mount applications
with VS > 16V due to limited printed circuit board area. There
are TO-263 package enhancements, such as clip-on
heatsinks and heatsinks with adhesives, that can be used to
improve performance.
Standard FR-4 single-sided copper clad will have an approx-
imate Thermal resistance (θSA) ranging from:
1.5 × 1.5 in. sq. 20–27°C/W (TA=28°C, Sine wave
testing, 1 oz. Copper)
2 × 2 in. sq. 16–23°C/W
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LM4755
The above values for θSA vary widely due to dimensional pro-
portions (i.e. variations in width and length will vary θSA).
For audio applications, where peak power levels are short in
duration, this part will perform satisfactory with less heatsink-
ing/copper clad area. As with any high power design proper
bench testing should be undertaken to assure the design can
dissipate the required power. Proper bench testing requires
attention to worst case ambient temperature and air flow. At
high power dissipation levels the part will show a tendency to
increase saturation voltages, thus limiting the undistorted
power levels.
DETERMINING MAXIMUM POWER DISSIPATION
For a single-ended class AB power amplifier, the theoretical
maximum power dissipation point is a function of the supply
voltage, VS, and the load resistance, RL and is given by the
following equation:
(single channel)
PDMAX (W) = [VS2 / (2 • π2 • RL)]
The above equation is for a single channel class-AB power
amplifier. For dual amplifiers such as the LM4755, the equa-
tion for calculating the total maximum power dissipated is:
(dual channel)
PDMAX (W) = 2 • [VS2 / (2 • π2 • RL)]
or
VS2 / (π2 • RL)
(Bridged Outputs)
PDMAX (W) = 4[VS2 / (2π2 • RL)]
HEATSINK DESIGN EXAMPLE
Determine the system parameters:
VS = 24V Operating Supply Voltage
RL = 4Ω Minimum Load Impedance
TA = 55°C Worst Case Ambient Temperature
Device parameters from the datasheet:
TJ = 150°C Maximum Junction Temperature
θJC = 2°C/W Junction-to-Case Thermal Resistance
Calculations:
2 • PDMAX = 2 • [VS2 / 2 • π2 • RL)] = (24V)2 / (2 • π2 • 4Ω) =
14.6W
θSA ≤ [(TJ-TA) / PDMAX] - θJC–θCS = [ (150°C - 55°C) / 14.6W]
- 2°C/W–0.2°C/W = 4.3°C/W
Conclusion: Choose a heatsink with θSA ≤ 4.3°C/W.
TO-263 HEATSINK DESIGN EXAMPLES
Example 1: (Stereo Single-Ended Output)
Given: TA=30°C
TJ=150°C
RL=4Ω
VS=12V
θJC=2°C/W
PDMAX from PD vs PO Graph:
PDMAX ≈ 3.7W
Calculating PDMAX:
PDMAX = VCC2/(π2RL) = (12V)2/π2(4Ω)) = 3.65W
Calculating Heatsink Thermal Resistance:
θSA < TJ − TA / PDMAX − θJC − θCS
θSA < 120°C/3.7W − 2.0°C/W − 0.2°C/W = 30.2°C/W
Therefore the recommendation is to use 1.5 × 1.5 square inch
of single-sided copper clad.
Example 2: (Stereo Single-Ended Output)
Given: TA=50°C
TJ=150°C
RL=4Ω
VS=12V
θJC=2°C/W
PDMAX from PD vs PO Graph:
PDMAX ≈ 3.7W
Calculating PDMAX:
PDMAX = VCC2/(π2RL)= (12V) 2/(π2(4Ω)) = 3.65W
Calculating Heatsink Thermal Resistance:
θSA < [(TJ − TA) / PDMAX] − θJC − θCS
θSA < 100°C/3.7W − 2.0°C/W − 0.2°C/W = 24.8°C/W
Therefore the recommendation is to use 2.0 × 2.0 square inch
of single-sided copper clad.
Example 3: (Bridged Output)
Given: TA=50°C
TJ=150°C
RL=8Ω
VS=12V
θJC=2°C/W
Calculating PDMAX:
PDMAX = 4[VCC2/(2π2RL)] = 4(12V)2/(2π2(8Ω)) = 3.65W
Calculating Heatsink Thermal Resistance:
θSA < [(TJ − TA) / PDMAX] − θJC − θCS
θSA < 100°C / 3.7W − 2.0°C/W − 0.2°C/W = 24.8°C/W
Therefore the recommendation is to use 2.0 × 2.0 square inch
of single-sided copper clad.
LAYOUT AND GROUND RETURNS
Proper PC board layout is essential for good circuit perfor-
mance. When laying out a PC board for an audio power
amplifier, particular attention must be paid to the routing of the
output signal ground returns relative to the input signal and
bias capacitor grounds. To prevent any ground loops, the
ground returns for the output signals should be routed sepa-
rately and brought together at the supply ground. The input
signal grounds and the bias capacitor ground line should also
be routed separately. The 0.1 µF high frequency supply by-
pass capacitor should be placed as close as possible to the
IC.
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LM4755
PC BOARD LAYOUT-COMPOSITE
10005933
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LM4755
PC BOARD LAYOUT-SILK SCREEN
10005934
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LM4755
PC BOARD LAYOUT-SOLDER SIDE
10005935
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LM4755
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4755T
NS Package Number TA9A
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LM4755
Order Number LM4755TS
NS Package Number TS9A
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LM4755
Notes
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LM4755
Notes
LM4755 Stereo 11W Audio Power Amplifier with Mute
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