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LM4858
Mono 1.5 W / Stereo 300mW Power Amplifier
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FEATURES
APPLICATIONS
•
•
•
•
•
•
•
•
•
•
•
1
2
Mono 1.5W BTL or stereo 300mW output
Logic controlled headphone sense
“Click and pop” suppression circuitry
No bootstrap capacitors required
Thermal shutdown protection
Unity-gain stable
Available in space-saving VSSOP and WSON
packaging
KEY SPECIFICATIONS
•
•
•
•
Output Power at 1% THD+N, 1kHz:
– LM4858LD 3Ω BTL: 1.9 W (typ)
– LM4858LD 4Ω BTL: 1.7 W (typ)
– LM4858MM 4Ω BTL: 1.5 W (typ)
– LM4858MM,LD 8Ω BTL: 1.1 W (typ)
– LM4858MM,LD 8Ω SE: 300 mW (typ)
– LM4858MM,LD 32Ω SE: 95 mW (typ)
THD+N at 1kHz, 95mW into 32Ω SE: 1% (typ)
Single Supply Operation: 2.4 to 5.5V
Shutdown Current: 18μA (typ)
Portable computers
Desktop computers
PDA's
Handheld games
DESCRIPTION
The LM4858 is an audio power amplifier capable of
delivering 1.5W (typ) of continuous average power
into a mono 4Ω bridged-tied load (BTL) with 1%
THD+N or 95mW per channel of continuous average
power into stereo 32Ω single-ended (SE) loads with
1% THD+N, using a 5V power supply.
The LM4858 can automatically switch between mono
BTL and stereo SE modes utilizing a headphone
sense pin. It is ideal for any system that provides
both a monaural speaker output and a stereo line or
headphone output
Boomer audio power amplifiers were designed
specifically to provide high quality output power with a
minimal amount of external components. Since the
LM4858 does not require bootstrap capacitors or
snubber networks, it is optimally suited for low-power
portable systems.
The LM4858 features an externally controlled,
micropower consumption shutdown mode and
thermal shutdown protection. The unity-gain stable
LM4858's gain is set by external gain-setting resistors
Connection Diagram
Figure 1. Top View
10 Lead VSSOP
See Package Number DGS
Figure 2. Top View
14 Lead WSON
See Package Number NHE0014A
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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OBSOLETE
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Typical Application
Figure 3. Typical Audio Amplifier Application Circuit
2
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
Supply Voltage
6.0V
−65°C to +150°C
Storage Temperature
ESD Susceptibility (3)
ESD Machine model
3.5kV
(4)
250V
Junction Temperature (TJ)
Solder Information Small Outline Package
150°C
Vapor Phase (60 sec.)
215°C
Infrared (15 sec.)
Thermal Resistance
220°C
θJA (typ)—DGS
194°C/W
θJC (typ)—DGS
θJA (typ)—NHE0014A
52°C/W
(5)
θJC (typ)—NHE0014A
(1)
(2)
(3)
(4)
(5)
56°C/W
4.3°C/W
Absolute Maximum Rating indicate limits beyond which damage to the device may occur.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
Human body model, 100pF discharged through a 1.5kΩ resistor.
Machine Model ESD test is covered by specification EIAJ IC-121-1981. A 200pF cap is charged to the specified voltage, then
discharged directly into the IC with no external series resistor (resistance of discharge path must be under 50Ω).
The given θJA is for an LM4858LD with the Exposed-DAP soldered to an exposed 1in2 area of 1oz printed circuit board copper.
Operating Ratings (1) (2)
Temperature Range
−40°C ≤ to 85°C
Supply Voltage VDD
2.4V ≤ VDD ≤ 5.5V
(1)
(2)
Absolute Maximum Rating indicate limits beyond which damage to the device may occur.
Operating Ratings indicate conditions for which the device is functional, but do not ensure specific performance limits. For ensured
specifications and test conditions, see the Electrical Characteristics. The ensured specifications apply only for the test conditions listed.
Some performance characteristics may degrade when the device is not operated under the listed test conditions.
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Electrical Characteristics (1) (2)
The following specifications apply for VDD= 5.0V, TA= 25°C unless otherwise specified.
Symbol
Parameter
Conditions
LM4858
Typical
VDD
(3)
Limit
Supply Voltage
IDD
Supply Current
ISD
Shutdown Current
VOS
PO
(4) (5)
Units
(Limits)
2.4
V (min)
5.5
V (max)
BTL Mode; VIN = 0V; IO = 0A
2.4
7.0
mA
SE Mode; VIN = 0V; IO = 0A
2.4
7.0
mA
SD Mode; VSHUTDOWN = VDD
18
Output Offset Voltage
BTL Mode; AV = 2
BTL OUT+ to BTL OUT−
5.0
40
mV
Output Power
BTL Mode; RL = 3Ω
THD+N = 1%; LM4858LD
1.9
W
BTL Mode; RL = 4Ω
THD+N = 1%; LM4858LD
1.7
W
BTL Mode; RL = 4Ω
THD+N = 1%; LM4858MM
1.5
W
BTL Mode; RL = 8Ω
THD+N = 1%; LM4858MM, LD
1.1
W
SE Mode; RL = 8Ω
THD+N = 1%; LM4858MM, LD
300
mW
SE Mode; RL = 32Ω
THD+N = 1%; LM4858MM, LD
95
mW
µA
VIH
Input Voltage High
SHUTDOWN, HP-IN
2.0
V (min)
VIL
Input Voltage Low
SHUTDOWN, HP-IN
0.8
V (max)
Crosstalk Channel Seperation
(1)
(2)
(3)
(4)
(5)
SE Mode, RL = 32Ω; f = 1kHz
73
dB
Absolute Maximum Rating indicate limits beyond which damage to the device may occur.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typical specifications are specified at +25°C and represent the most likely parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
Limits are ensured to Texas Instruments' AOQL ( Average Outgoing Quality Level ).
Electrical Characteristics (1) (2)
The following specifications apply for VDD= 3.3V, TA= 25°C unless otherwise specified.
Symbol
Parameter
Conditions
LM4858
Typical (3)
IDD
Supply Current
ISD
Shutdown Current
VOS
PO
Limit (4) (5)
Units
(Limits)
BTL Mode; VIN = 0V; IO = 0A
2.0
mA
SE Mode; VIN = 0V; IO = 0A
2.0
mA
SD Mode; VSHUTDOWN = VDD
12
Output Offset Voltage
BTL Mode; AV = 2
BTL OUT+ to BTL OUT−
5.0
Output Power
BTL Mode; RL = 8Ω
THD+N = 1%
440
mW
SE Mode; RL = 32Ω
THD+N = 1%
40
mW
µA
40
mV
VIH
Input Voltage High
SHUTDOWN, HP-IN
2.0
V (min)
VIL
Input Voltage Low
SHUTDOWN, HP-IN
0.8
V (max)
(1)
(2)
(3)
(4)
(5)
4
Absolute Maximum Rating indicate limits beyond which damage to the device may occur.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typical specifications are specified at +25°C and represent the most likely parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
Limits are ensured to Texas Instruments' AOQL ( Average Outgoing Quality Level ).
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Electrical Characteristics (1) (2)
The following specifications apply for VDD= 2.7V, TA= 25°C unless otherwise specified.
Symbol
Parameter
Conditions
LM4858
Typical
IDD
Supply Current
(3)
Limit
(4) (5)
Units
(Limits)
BTL Mode; VIN = 0V; IO = 0A
1.8
mA
SE Mode; VIN = 0V; IO = 0A
1.8
mA
µA
ISD
Shutdown Current
SD Mode; VSHUTDOWN = VDD
10
VOS
Output Offset Voltage
BTL Mode; AV = 2
BTL OUT+ to BTL OUT−
5.0
PO
Output Power
BTL Mode; RL = 8Ω
THD+N = 1%
300
mW
SE Mode; RL = 32Ω
THD+N = 1%
25
mW
40
mV
VIH
Shutdown Input Voltage High
SHUTDOWN, HP-IN
2.0
V (min)
VIL
Shutdown Input Voltage Low
SHUTDOWN, HP-IN
0.8
V (max)
(1)
(2)
(3)
(4)
(5)
Absolute Maximum Rating indicate limits beyond which damage to the device may occur.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typical specifications are specified at +25°C and represent the most likely parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
Limits are ensured to Texas Instruments' AOQL ( Average Outgoing Quality Level ).
External Components Description
See Figure 3.
Components
Functional Description
1.
Ri
Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter
with Ci at fc = 1/(2πRiCi).
2.
Ci
Input coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a highpass filter with Ri
at fc = 1/(2πRiCi). Refer to the section, PROPER SELECTION OF EXTERNAL COMPONENTS, for an explanation of how
to determine the value of Ci.
3.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
4.
Cs
Supply bypass capacitor which provides power supply filtering. Refer to the POWER SUPPLY BYPASSING section for
information concerning proper placement and selection of the supply bypass capacitor.
5.
CB
Bypass pin capacitor which provides half-supply filtering. Refer to the section, PROPER SELECTION OF EXTERNAL
COMPONENTS, for information concerning proper placement and selection of CB.
6.
CO
Output coupling capacitor which blocks the DC voltage at the amplifier's output. Forms a high pass filter with the singleended load RL at fO = 1/(2π RLCO).
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Typical Performance Characteristics
LD Specific Characteristics
6
LM4858LD
THD+N vs Frequency
LM4858LD
THD+N vs Frequency
Figure 4.
Figure 5.
LM4858LD
THD+N vs Output Power
LM4858LD
THD+N vs Output Power
Figure 6.
Figure 7.
LM4858LD
Power Dissipation vs Output Power
LM4858LD
Power Derating Curve
Figure 8.
Figure 9.
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Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
Figure 10.
Figure 11.
THD+N vs Frequency
THD+N vs Frequency
Figure 12.
Figure 13.
THD+N vs Frequency
THD+N vs Frequency
Figure 14.
Figure 15.
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Typical Performance Characteristics (continued)
8
THD+N vs Frequency
THD+N vs Output Power
Figure 16.
Figure 17.
THD+N vs Output Power
THD+N vs Output Power
Figure 18.
Figure 19.
THD+N vs Output Power
THD+N vs Output Power
Figure 20.
Figure 21.
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Typical Performance Characteristics (continued)
THD+N vs Output Power
THD+N vs Output Power
Figure 22.
Figure 23.
THD+N vs Output Power
Output Power vs Load Resistance
Figure 24.
Figure 25.
Output Power vs Load Resistance
Output Power vs Supply Voltage
Figure 26.
Figure 27.
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Typical Performance Characteristics (continued)
10
Output Power vs Supply Voltage
Dropout Voltage vs Supply Voltage
Figure 28.
Figure 29.
Power Dissipation vs Output Power
Power Dissipation vs Output Power
Figure 30.
Figure 31.
Power Derating Curve
Channel Separation
Figure 32.
Figure 33.
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Typical Performance Characteristics (continued)
Noise Floor
Open Loop Frequency Response
Figure 34.
Figure 35.
Supply Current vs Supply Voltage
Power Supply Rejection Ratio
Figure 36.
Figure 37.
Power Supply Rejection Ratio
Figure 38.
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APPLICATION INFORMATION
BRIDGED AND SINGLE-ENDED OPERATION
As shown in Figure 3, the LM4858 contains three operational amplifiers (A1-A3). These amplifiers can be
configured for SE or BTL modes.
In the SE mode, the LM4858 operates as a high current output dual op amp. A1 and A3 are independent
amplifiers with an externally configured gain of AV = - RF/RI. The outputs of A1 and A3 are used to drive an
external set of headphones plugged into the headphone jack. Amplifier A2 is shut down to a high output
impedance state in SE mode. This prevents any current flow into the mono bridge-tied load, thereby muting it.
In BTL mode, A3 is shut down to a high impedance state. The audio signal from the RIGHT IN pin is directed to
the inverting input of A1. As a result, the LEFT IN and RIGHT IN audio signals, VINL and VINR, are summed
together at the input of A1. A2 is then activated with a closed-loop gain of AV = -1 fixed by two internal 20kΩ
resistors. The outputs of A1 and A2 are then used to drive the mono bridged-tied load.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4858's exposed-DAP (die attach paddle) package (LD) provides a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the
surrounding PCB copper traces, ground plane, and surrounding air. The result is a low voltage audio power
amplifier that produces 1.7W at ≤ 1% THD+N with a 4Ω load. This high power is achieved through careful
consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4858's
high power performance and activate unwanted, though necessary, thermal shutdown protection.
The LD package must have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad is
connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink, and
radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner
layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper
heat sink area with 4(2x2) vias. The via diameter should be 0.012in-0.013in with a 1.27mm pitch. Ensure efficient
thermal conductivity by plating through the vias.
Best thermal performance is achieved with the largest practical heat sink area. If the heatsink and amplifier share
the same PCB layer, a nominal 2.5in2 area is necessary for 5V operation with a 4Ω load. Heatsink areas not
placed on the same PCB layer as the LM4858 should be 5in2 (min) for the same supply voltage and load
resistance. The last two area recommendations apply for 25°C ambient temperature. Increase the area to
compensate for ambient temperatures above 25°C. The LM4858's power de-rating curve in the Typical
Performance Characteristics shows the maximum power dissipation versus temperature. An example PCB layout
for the LD package is shown in the Demonstration Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mounting an LD (WSON) package is available from Texas
Instruments' Package Engineering Group under application note AN1187 (SNOA401).
BRIDGE CONFIGURATION EXPLANATION
When the LM4858 is in BTL mode, the output of amplifier A1 serves as the input to amplifier A2, which results in
both amplifiers producing signals identical in magnitude, but out of phase by 180°. Consequently, the differential
gain for the mono channel is:
AVD = VOUT / (VINL + VINR) = 2 × (RF / RI)
(1)
Driving a load differentially through the BTL OUT- and BTL OUT+ outputs is an amplifier configuration commonly
referred to as "bridged mode". Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration. It drives a load
differentially, which doubles output swing for a specified supply voltage. This produces four times the output
power as that produced by a single-ended amplifier under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closed-loop
gain without causing excessive output signal clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN
section.
12
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A bridge configuration, such as the one used in LM4858, also creates a second advantage over single-ended
amplifiers. Since the differential outputs, BTL OUT- and BTL OUT+, are biased at half-supply, no net DC voltage
exists across the load. This eliminates the need for the output coupling capacitor that a single supply, singleended amplifier configuration requires. Eliminating an output coupling capacitor in a single-ended configuration
forces the half-supply bias voltage across the load. This increases internal IC power dissipation and may cause
permanent loudspeaker damage.
POWER DISSIPATION
Whether the power amplifier is bridged or single-ended, power dissipation is a major concern when designing the
amplifier. Equation 2 states the maximum power dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified load.
PDMAX = (VDD)2 / (2π2 RL): Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase
in internal power dissipation. Equation 3 states the maximum power dissipation point for a bridge amplifier
operating at the same given conditions.
PDMAX = 4 × (VDD)2 / (2π2 RL): Bridge Mode
(3)
The LM4858 is designed to drive either two single-ended loads simultaneously or one mono bridged-tied load. In
SE mode, the maximum internal power dissipation is 2 times that of Equation 2. In BTL mode, the maximum
internal power dissipation is the result of Equation 3. Even with this substantial increase in power dissipation, the
LM4858 does not require heatsinking. The power dissipation from Equation 3 must not be greater than the power
dissipation predicted by Equation 4:
PDMAX = (TJMAX - TA) / θJA
(4)
For the package DGS, θJA = 194°C/W. TJMAX = 150°C for the LM4858. Depending on the ambient temperature,
TA, of the surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by the
IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage must be
decreased, the load impedance increased, or the ambient temperature reduced. For the typical application of a
5V power supply, and an 8Ω bridged load, the maximum ambient temperature possible without violating the
maximum junction temperature is approximately 27°C for package DGS. This assumes the device operates at
maximum power dissipation and uses surface mount packaging. Internal power dissipation is a function of output
power. If typical operation is not around the maximum power dissipation point, operation at higher ambient
temperatures is possible. Refer to the Typical Performance Characteristics curves for power dissipation
information for different output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as
possible. The value of the pin bypass capacitor, CB, directly affects the LM4858's half-supply voltage stability and
PSRR. The stability and supply rejection increase as the bypass capacitor's value increases Typical applications
employ a 5V regulator with a 10µF and a 0.1µF bypass capacitors which aid in supply filtering. This does not
eliminate the need for bypassing the supply nodes of the LM4858. The selection of bypass capacitors, especially
CB, is thus dependent upon desired PSRR requirements, click and pop performance, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4858 features amplifier bias circuitry shutdown.
This shutdown function is activated by applying a logic high to the SHUTDOWN pin. The trigger point is 2.0V
minimum for a logic high level, and 0.8V maximum for a logic low level. It is best to switch between ground and
the supply, VDD, to ensure correct shutdown operation. By switching the SHUTDOWN pin to VDD, the LM4858
supply current draw will be minimized in idle mode. Whereas the device will be disabled with shutdown voltages
less than VDD, the idle current may be greater than the typical value of 18µA. In either case, the SHUTDOWN pin
should be tied to a fixed voltage to avoid unwanted state changes.
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In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry. This
provides a quick, smooth shutdown transition. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the SHUTDOWN pin is connected to
ground and enables the amplifier. If the switch is open, the external pull-up resistor, RPU2 will disable the
LM4858. This scheme ensures that the SHUTDOWN pin will not float, thus preventing unwanted state changes.
HP-IN FUNCTION
The LM4858 features a headphone control pin, HP-IN, that enables the switching between BTL and SE modes.
A logic-low to HP-IN activates the BTL mode, while a logic-high activates the SE mode. The trigger point is 2.0V
minimum for a logic high level and 0.8V maximum for a logic low level. A microcontroller or microprocessor
output should be used to control the headphone sense circuitry. This provides a quick, smooth shutdown
transition. Another solution is to use a single-pole, single-throw switch in conjunction with an external pull-up
resistor. When the switch is closed, the HP-IN pin is connected to the ground and activates BTL mode. If the
switch is open, the external pull-up resistor, RPU1, will activate SE mode. This scheme ensures that the HP-IN pin
will not float, thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers is critical for optimum
device and system performance. While the LM4858 is tolerant to a variety of external component combinations,
consideration must be given to the external component values that maximize overall system quality.
The LM4858's unity-gain stability allows a designer to maximize system performance. The LM4858's gain should
be set no higher than necessary for any given application. A low gain configuration maximizes signal-to-noise
performance and minimizes THD+N. However, a low gain configuration also requires large input signals to obtain
a given output power. Input signals equal to or greater than 1VRMS are available from sources such as audio
codecs. Please refer to the section, AUDIO POWER AMPLIFIER DESIGN, for a more complete explanation of
proper gain selection.
Selecting Input and Output Capacitor Values
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 3. The input coupling capacitor CI
and resistor RI form a first order high pass filter that limits low frequency response. CI's value should be based on
the desired frequency response weighed against the following: Large value input and output capacitors are both
expensive and space consuming for portable designs. Clearly a certain sized capacitor is needed to couple in
low frequencies without severe attenuation. But in many cases the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals below 150Hz. Thus, large value input and output
capacitors may not increase system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W / 8Ω Bridged Audio Amplifier
Given:
• Power Output: 1WRMS
• Load Impedance 8Ω
• Input Level: 1VRMS
• Input Impedance: 20kΩ
• Bandwidth: 100Hz - 20kHz ± 0.25dB
A designer must first determine the minimum supply voltage needed to obtain the specified output power. By
extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics
section, the supply rail can be easily found. A second way to determine the minimum supply rail is to calculate
the required VOPEAK using Equation 5 and add the dropout voltage. This results in Equation 6, where VODTOP and
VODBOT are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance
Characteristics section.
(5)
14
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VDD ≥ (VOPEAK + (VODTOP + VODBOT))
(6)
Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 4.7V. But since 5V is
a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4858 to reproduce peaks in excess of 1W without producing audible distortion.
However, the designer must make sure that the chosen power supply voltage and output load does not violate
the conditions explained in the POWER DISSIPATION section.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation 7.
(7)
(8)
RF / RI = AVD / 2
From Equation 6, the minimum AVD is 2.83; use AVD = 3.
The desired input impedance was 20kΩ, and with an AVD of 3, using Equation 8 results in an allocation of RI =
20kΩ and RF = 30kΩ.
The final design step is to set the amplifier's −3dB frequency bandwidth. To achieve the desired ± 0.25dB pass
band magnitude variation limit, the low frequency response must extend to at least one−fifth the lower bandwidth
limit and the high frequency response must extend o at least five times the upper bandwidth limit. The variation
for both response limits is 0.17dB, well within the ± 0.25dB desired limit. This results in:
fL = 100Hz / 5 = 20Hz
fH = 20kHz x 5 = 100kHz
(9)
(10)
As stated in the External Components Description section, RI in conjunction with CI create a highpass filter. Find
the coupling capacitor's value using Equation 9.
CI ≥ 1 / (2πRIfL)
CI ≥ 1 / ( 2π × 20kΩ × 20Hz) = 0.397µF
(11)
(12)
Use a 0.39µF capacitor, the closest standard value.
The high frequency pole is determined by the product of the desired high frequency pole, fH, and the differential
gain, AVD. With AVD = 3 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4858
GBWP of 10MHz. This difference indicates that a designer can still use the LM4858 at higher differential gains
without bandwidth limitations.
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω
LOADS
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance. As load
impedance decreases, load dissipation becomes increasingly dependant on the interconnect (PCB trace and
wire) resistance between the amplifier output pins and the load's connections. Residual trace resistance causes
a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1Ω
trace resistance reduces the output power dissipated by a 4Ω load from 2.0W to 1.95W. This problem of
decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load
dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide
as possible.
Poor power supply regulation adversely affects maximum output power. A poorly regulated supply's output
voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output
signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the
same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps
maintain full output voltage swing.
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Copyright © 2004, Texas Instruments Incorporated
Product Folder Links: LM4858
15
OBSOLETE
LM4858
SNOSA59B – MAY 2004 – REVISED MAY 2004
www.ti.com
Demonstration Board Layout
Figure 39. Recommended MM PC Board Layout:
Component-Side SilkScreen
Figure 40. Recommended MM PC Board Layout:
Component-Side Layout
16
Submit Documentation Feedback
Copyright © 2004, Texas Instruments Incorporated
Product Folder Links: LM4858
OBSOLETE
LM4858
www.ti.com
SNOSA59B – MAY 2004 – REVISED MAY 2004
Figure 41. Recommended MM PC Board Layout:
Bottom-Side Layout
Submit Documentation Feedback
Copyright © 2004, Texas Instruments Incorporated
Product Folder Links: LM4858
17
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