LM4835
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LM4835
Stereo 2W Audio Power Amplifiers
with DC Volume Control and Selectable Gain
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FEATURES
DESCRIPTION
•
•
•
•
The LM4835 is a monolithic integrated circuit that
provides DC volume control, and stereo bridged
audio power amplifiers capable of producing 2W into
4Ω with less than 1.0% THD or 2.2W into 3Ω with
less than 1.0% THD (see Notes below).
1
23
•
•
•
PC98 Compliant
DC Volume Control Interface
System Beep Detect
Stereo Switchable Bridged/Single-Ended
Power Amplifiers
Selectable Internal/External Gain and Bass
Boost Configurable
“Click and Pop” Suppression Circuitry
Thermal Shutdown Protection Circuitry
APPLICATIONS
•
•
•
Portable and Desktop Computers
Multimedia Monitors
Portable Radios, PDAs, and Portable TVs
KEY SPECIFICATIONS
•
•
•
•
•
PO at 1% THD+N into 3Ω (LM4835LQ,
LM4835MTE) 2.2 W (typ)
PO at 1% THD+N into 4Ω (LM4835LQ,
LM4835MTE) 2.0 W (typ)
PO at 1% THD+N into 8Ω (LM4835) 1.1 W (typ)
1.0 % (typ)
Single-Ended Mode - THD+N at 85mW into 32Ω
Shutdown Current 0.7 µA (typ)
Boomer™ audio integrated circuits were designed
specifically to provide high quality audio while
requiring a minimum amount of external components.
The LM4835 incorporates a DC volume control,
stereo bridged audio power amplifiers and a
selectable gain or bass boost, making it optimally
suited for multimedia monitors, portable radios,
desktop, and portable computer applications.
The LM4835 features an externally controlled, lowpower consumption shutdown mode, and both a
power amplifier and headphone mute for maximum
system flexibility and performance.
NOTE: When properly mounted to the circuit board,
the LM4835LQ and LM4835MTE will deliver 2W into
4Ω. The LM4835MT will deliver 1.1W into 8Ω. See
the Application Information section LM4835LQ and
for LM4835MTE usage information.
NOTE: An LM4835LQ and LM4835MTE that have
been properly mounted to the circuit board and
forced-air cooled will deliver 2.2W into 3Ω.
1
2
3
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.
Boomer is a trademark of Texas Instruments.
All other 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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LM4835
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CONNECTION DIAGRAMS
Figure 1. 28-Pin WQFN - Top View
See NJB0028A Package
Figure 2. 28-Pin TSSOP - Top View
See PW Package
BLOCK DIAGRAM
Figure 3. LM4835 Block Diagram
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
Storage Temperature
-65°C to +150°C
−0.3V to VDD
+0.3V
Input Voltage
Power Dissipation
(3)
Internally limited
ESD Susceptibility
(4)
2000V
ESD Susceptibility
(5)
200V
Junction Temperature
150°C
Soldering Information
Small Outline
Package
Vapor Phase (60 sec.)
215°C
Infrared (15 sec.)
220°C
θJC (typ)—LQA028AA
θJA (typ)—LQA028AA
3.0°C/W
(6)
42°C/W
θJC (typ)—MTC28
20°C/W
θJA (typ)—MTC28
80°C/W
θJC (typ)—MXA28A
2°C/W
θJA (typ)—MXA28A
(7)
41°C/W
θJA (typ)—MXA28A
(8)
54°C/W
θJA (typ)—MXA28A
(9)
59°C/W
θJA (typ)—MXA28A
(10)
93°C/W
(1)
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
(3) The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θ JA, and the ambient temperature
TA. The maximum allowable power dissipation is PDMAX = (TJMAX − TA )/θJA. For the LM4835LQ and LM4835MT, TJMAX = 150°C, and
the typical junction-to-ambient thermal resistance, when board mounted, is 80°C/W for the MTC28 package and 42°C/W for the
LM4835LQ package.
(4) Human body model, 100pF discharged through a 1.5kΩ resistor.
(5) Machine Model, 220pF–240pF discharged through all pins.
(6) The given θJA is for an LM4835 packaged in an LQA24A with the exposed-DAP soldered to an exposed 2in 2 area of 1oz printed circuit
board copper.
(7) The θJA given is for an MXA28A package whose exposed-DAP is soldered to a 2in2 piece of 1 ounce printed circuit board copper on a
bottom side layer through 21 8mil vias.
(8) The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 2in 2 piece of 1 ounce printed circuit board
copper.
(9) The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 1in 2 piece of 1 ounce printed circuit board
copper.
(10) The θJA given is for an MXA28A package whose exposed-DAP is not soldered to any copper.
OPERATING RATINGS
Temperature Range
TMIN ≤ TA ≤TMAX
−40°C ≤TA ≤ 85°C
2.7V≤ VDD ≤ 5.5V
Supply Voltage
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ELECTRICAL CHARACTERISTICS FOR ENTIRE IC (1) (2)
The following specifications apply for VDD = 5V unless otherwise noted. Limits apply for TA = 25°C.
Symbol
VDD
Parameter
LM4835
Conditions
Typical
(3)
Limit (4)
Supply Voltage
Units
(Limits)
2.7
V (min)
5.5
V (max)
30
mA (max)
2.0
μA (max)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
15
ISD
Shutdown Current
Vpin 2 = VDD
0.7
VIH
Headphone Sense High Input Voltage
4
V (min)
VIL
Headphone Sense Low Input Voltage
0.8
V (max)
(1)
(2)
(3)
(4)
All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical
application as shown in Figure 3.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
ELECTRICAL CHARACTERISTICS FOR VOLUME ATTENUATORS (1) (2)
The following specifications apply for VDD = 5V. Limits apply for TA = 25°C.
Symbol
CRANGE
AM
(1)
(2)
(3)
(4)
4
Parameter
Attenuator Range
Mute Attenuation
Conditions
Gain with Vpin 7 = 5V
LM4835
Typical (3)
Limit (4)
Units
(Limits)
0
±0.5
dB (max)
Attenuation with Vpin 7 = 0V
-81
-80
dB (min)
Vpin 5 = 5V, Bridged Mode
-88
-80
dB (min)
Vpin 5 = 5V, Single-Ended Mode
-88
-80
dB (min)
All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical
application as shown in Figure 3.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
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ELECTRICAL CHARACTERISTICS FOR SINGLE-ENDED MODE OPERATION (1) (2)
The following specifications apply for VDD = 5V. Limits apply for TA = 25°C.
Symbol
PO
Parameter
Output Power
LM4835
Conditions
Typical
(3)
Limit (4)
Units
(Limits)
THD = 1.0%; f = 1kHz; RL = 32Ω
85
mW
THD = 10%; f = 1 kHz; RL = 32Ω
95
mW
0.065
%
THD+N
Total Harmonic Distortion+Noise
VOUT = 1VRMS, f=1kHz, RL = 10kΩ,
AVD = 1
PSRR
Power Supply Rejection Ratio
CB = 1.0 μF, f =120 Hz,
VRIPPLE = 200 mVrms
58
dB
SNR
Signal to Noise Ratio
POUT =75 mW, R L = 32Ω, A-Wtd Filter
102
dB
Xtalk
Channel Separation
f=1kHz, CB = 1.0 μF
65
dB
(1)
All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical
application as shown in Figure 3.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
(2)
(3)
(4)
ELECTRICAL CHARACTERISTICS FOR BRIDGED MODE OPERATION (1) (2)
The following specifications apply for VDD = 5V, unless otherwise noted. Limits apply for TA = 25°C.
Symbol
VOS
Parameter
Output Offset Voltage
PO
Output Power
LM4835
Conditions
VIN = 0V
THD + N = 1.0%; f=1kHz; RL = 3Ω
(5) (6)
THD + N = 1.0%; f=1kHz; RL = 4Ω (7)
(6)
Typical (3)
Limit (4)
Units
(Limits)
5
30
mV (max)
2.2
W
2
W
THD = 0.5% (max);f = 1 kHz;
RL = 8Ω
1.1
1.0
W (min)
THD+N = 10%;f = 1 kHz; RL = 8Ω
1.5
W
PO = 1W, 20 Hz< f < 20 kHz,
RL = 8Ω, AVD = 2
0.3
%
THD+N
Total Harmonic Distortion+Noise
PO = 340 mW, RL = 32Ω
1.0
%
PSRR
Power Supply Rejection Ratio
CB = 1.0 µF, f = 120 Hz,
VRIPPLE = 200 mVrms; RL = 8Ω
74
dB
SNR
Signal to Noise Ratio
VDD = 5V, POUT = 1.1W,
RL = 8Ω, A-Wtd Filter
93
dB
Xtalk
Channel Separation
f=1kHz, CB = 1.0 μF
70
dB
(1)
(2)
(3)
(4)
(5)
(6)
(7)
All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical
application as shown in Figure 3.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric norm.
Datasheet min/max specification limits are ensured by design, test, or statistical analysis.
When driving 3Ω loads and operating on a 5V supply, the LM4835MTE must be mounted to the circuit board and forced-air cooled.
When driving a 3Ω or 4Ω loads and operating on a 5V supply, the LM4835LQ must be mounted to the circuit board that has a minimum
of 2.5in 2 of exposed, uninterrupted copper area connected to the WQFN package's exposed DAP.
When driving 4Ω loads and operating on a 5V supply, the LM4835MTE must be mounted to the circuit board.
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TYPICAL APPLICATION
Figure 4. Typical Application Circuit
TRUTH TABLE FOR LOGIC INPUTS (1)
(1)
6
Mute
Mode
HP Sense
DC Vol. Control
Bridged Output
Single-Ended Output
0
0
0
Fixed Level
Vol. Fixed
_
0
0
1
Fixed Level
Muted
Vol. Fixed
0
1
0
Adjustable
Vol. Changes
_
0
1
1
Adjustable
Muted
Vol. Changes
1
X
X
_
Muted
Muted
If system beep is detected on the Beep In pin (pin 11), the system beep will be passed through the bridged amplifier regardless of the
logic of the Mute and HP sense pins.
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TYPICAL PERFORMANCE CHARACTERISTICS
MTE SPECIFIC CHARACTERISTICS
LM4835MTE THD+N vs Output Power
LM4835MTE THD+N vs Frequency
Figure 5.
Figure 6.
LM4835MTE THD+N vs Output Power
LM4835MTE THD+N vs Frequency
Figure 7.
Figure 8.
LM4835MTE Power Dissipation vs Output Power
LM4835MTE Power Derating Curve
Figure 9.
These curves show the thermal dissipation ability of the LM4835MTE
at different ambient temperatures given these conditions:
500LFPM + 2in2: The part is soldered to a 2in2, 1 oz. copper plane
with 500 linear feet per minute of forced-air flow across it.
2in2on bottom: The part is soldered to a 2in2, 1oz. copper plane that
is on the bottom side of the PC board through 21 8 mil vias.
2in2: The part is soldered to a 2in2, 1oz. copper plane.
1in2: The part is soldered to a 1in2, 1oz. copper plane.
Not Attached: The part is not soldered down and is not forced-air
cooled.
Figure 10.
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TYPICAL PERFORMANCE CHARACTERISTICS
MTE SPECIFIC CHARACTERISTICS (continued)
LM4835LQ Power Derating Curve
Figure 11.
8
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TYPICAL PERFORMANCE CHARACTERISTICS
NON-MTE SPECIFIC CHARACTERISTICS
THD+N vs Frequency
THD+N vs Frequency
Figure 12.
Figure 13.
THD+N vs Frequency
THD+N vs Frequency
Figure 14.
Figure 15.
THD+N vs Frequency
THD+N vs Frequency
Figure 16.
Figure 17.
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TYPICAL PERFORMANCE CHARACTERISTICS
NON-MTE SPECIFIC CHARACTERISTICS (continued)
10
THD+N vs Frequency
THD+N vs Frequency
Figure 18.
Figure 19.
THD+N vs Frequency
THD+N vs Frequency
Figure 20.
Figure 21.
THD+N vs Frequency
THD+N vs Output Power
Figure 22.
Figure 23.
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TYPICAL PERFORMANCE CHARACTERISTICS
NON-MTE SPECIFIC CHARACTERISTICS (continued)
THD+N vs Output Power
THD+N vs Output Power
Figure 24.
Figure 25.
THD+N vs Output Power
THD+N vs Output Power
Figure 26.
Figure 27.
THD+N vs Output Power
THD+N vs Output Power
Figure 28.
Figure 29.
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TYPICAL PERFORMANCE CHARACTERISTICS
NON-MTE SPECIFIC CHARACTERISTICS (continued)
12
THD+N vs Output Power
THD+N vs Output Power
Figure 30.
Figure 31.
THD+N vs Output Power
THD+N vs Output Power
Figure 32.
Figure 33.
THD+N vs Output Voltage Docking Station Pins
THD+N vs Output Voltage Docking Station Pins
Figure 34.
Figure 35.
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TYPICAL PERFORMANCE CHARACTERISTICS
Output Power vs Load Resistance
Output Power vs Load Resistance
Figure 36.
Figure 37.
Output Power vs Load Resistance
Power Supply Rejection Ratio
Figure 38.
Figure 39.
Dropout Voltage
Output Power vs Load Resistance
Figure 40.
Figure 41.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
14
Noise Floor
Noise Floor
Figure 42.
Figure 43.
Volume Control Characteristics
Power Dissipation vs Output Power
Figure 44.
Figure 45.
Power Dissipation vs Output Power
External Gain/ Bass Boost Characteristics
Figure 46.
Figure 47.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Power Derating Curve
Crosstalk
Figure 48.
Figure 49.
Crosstalk
Output Power vs Supply voltage
Figure 50.
Figure 51.
Output Power vs Supply Voltage
Supply Current vs Supply Voltage
Figure 52.
Figure 53.
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APPLICATION INFORMATION
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS
The LM4835's exposed-DAP (die attach paddle) packages (MTE and LQ) provide 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, finally, surrounding air. The result is a low voltage
audio power amplifier that produces 2.1W at ≤ 1% THD with a 4Ω load. This high power is achieved through
careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the
LM4835's high power performance and activate unwanted, though necessary, thermal shutdown protection.
The MTE and LQ packages must have their DAPs 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 and 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 32(4x8) (MTE) or 6(3x2) (LQ) vias. The via diameter should be 0.012in–0.013in with a
1.27mm pitch. Ensure efficient thermal conductivity by plating-through and solder-filling the vias.
Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and
amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4Ω load.
Heatsink areas not placed on the same PCB layer as the LM4835 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. In systems using cooling fans, the LM4835MTE
can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in2 exposed
copper or 5.0in2 inner layer copper plane heatsink, the LM4835MTE can continuously drive a 3Ω load to full
power. The LM4835LQ achieves the same output power level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below 150°C to prevent activating the LM4835's thermal
shutdown protection. The LM4835's power de-rating curve in the Typical Performance Characteristics shows the
maximum power dissipation versus temperature. Example PCB layouts for the exposed-DAP TSSOP and LQ
packages are shown in the Demonstration Board Layout section. Further detailed and specific information
concerning PCB layout, fabrication, and mounting an LQ (WQFN) package is available in TI's SNOA401.
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 dependent 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 2.1W to 2.0W. 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.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 4, the LM4835 consists of two pairs of operational amplifiers, forming a two-channel (channel
A and channel B) stereo amplifier. (Though the following discusses channel A, it applies equally to channel B.)
External resistors Rf and Ri set the closed-loop gain of Amp1A, whereas two internal 20kΩ resistors set Amp2A's
gain at −1. The LM4835 drives a load, such as a speaker, connected between the two amplifier outputs, −OUTA
and +OUTA.
16
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Figure 4 shows that Amp1A's output serves as Amp2A's input. This results in both amplifiers producing signals
identical in magnitude, but 180° out of phase. Taking advantage of this phase difference, a load is placed
between −OUTA and +OUTA and driven differentially (commonly referred to as “bridge mode”). This results in a
differential gain of
AVD = 2 * (Rf / Ri)
(1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage over the singleended configuration: its differential output doubles the voltage swing across the load. This produces four times
the output power when compared to a single-ended amplifier under the same conditions. This increase in
attainable output power assumes that the amplifier is not current limited or that the output signal is not clipped.
To ensure minimum output signal clipping when choosing an amplifier's closed-loop gain, refer to the AUDIO
POWER AMPLIFIER DESIGN section.
Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by
biasing channel A's and channel B's outputs at half-supply. This eliminates the coupling capacitor that single
supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration
forces a single-supply amplifier's half-supply bias voltage across the load. This increases internal IC power
dissipation and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation 2
states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and
driving a specified output load.
PDMAX = (VDD)2 / (2π2RL)
Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher
internal power dissipation for the same conditions.
The LM4835 has two operational amplifiers per channel. The maximum internal power dissipation per channel
operating in the bridge mode is four times that of a single-ended amplifier. From Equation 3, assuming a 5V
power supply and a 4Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo
operation.
PDMAX = 4 * (VDD)2 / (2π2RL)
Bridge Mode
(3)
The LM4835's power dissipation is twice that given by Equation 2 or Equation 3 when operating in the singleended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation 3 must
not exceed the power dissipation given by Equation 4:
PDMAX′ = (TJMAX − TA) / θJA
(4)
The LM4835's TJMAX = 150°C. In the LQ package soldered to a DAP pad that expands to a copper area of 5in2
on a PCB, the LM4835's θJA is 20°C/W. In the MTE package soldered to a DAP pad that expands to a copper
area of 2in2 on a PCB, the LM4835's θJA is 41°C/W. At any given ambient temperature TA, use Equation 4 to find
the maximum internal power dissipation supported by the IC packaging. Rearranging Equation 4 and substituting
PDMAX for PDMAX′ results in Equation 5. This equation gives the maximum ambient temperature that still allows
maximum stereo power dissipation without violating the LM4835's maximum junction temperature.
TA = TJMAX – 2*PDMAX θJA
(5)
For a typical application with a 5V power supply and an 4Ω load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 99°C
for the LQ package and 45°C for the MTE package.
TJMAX = PDMAX θJA + TA
(6)
Equation 6 gives the maximum junction temperature TJMAX. If the result violates the LM4835's 150°C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further
allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface mount part operating around the maximum power
dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are
allowed as output power or duty cycle decreases.
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If the result of Equation 3 is greater than that of Equation 4, then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to
reduce θJA. The heat sink can be created using additional copper area around the package, with connections to
the ground pin(s), supply pin and amplifier output pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation. When adding a heat sink, the θJA is the sum of θJC, θCS,
and θSA. (θJC is the junction-to-case thermal impedance, θCS is the case-to-sink thermal impedance, and θSA is
the sink-to-ambient thermal impedance.) Refer to the Typical Performance Characteristics curves for power
dissipation information at lower 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. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to
stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response.
However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected
between the LM4835's supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so
may cause oscillation. Keep the length of leads and traces that connect capacitors between the LM4835's power
supply pin and ground as short as possible. Connecting a 1µF capacitor, CB, between the BYPASS pin and
ground improves the internal bias voltage's stability and improves the amplifier's PSRR. The PSRR
improvements increase as the bypass pin capacitor value increases. Too large, however, increases turn-on time
and can compromise amplifier's click and pop performance. The selection of bypass capacitor values, especially
CB, depends on desired PSRR requirements, click and pop performance (as explained in the section,
SELECTING PROPER EXTERNAL COMPONENTS), system cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4835's performance requires properly selecting external components. Though the LM4835
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4835 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more
than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-tonoise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources
such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the AUDIO POWER AMPLIFIER
DESIGN section for more information on selecting the proper gain.
INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires high value input coupling capacitor (0.33µF in Figure 4). A high
value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases,
however, the speakers used in portable systems, whether internal or external, have little ability to reproduce
signals below 150 Hz. Applications using speakers with this limited frequency response reap little improvement
by using large input capacitor.
Besides effecting system cost and size, the input coupling capacitor has an affect on the LM4835's click and pop
performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input
capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input
capacitor's size. Higher value capacitors need more time to reach a quiescent DC voltage (usually VDD/2) when
charged with a fixed current. The amplifier's output charges the input capacitor through the feedback resistor, Rf.
Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the
desired −3dB frequency.
A shown in Figure 4, the input resistor (20kΩ) and the input capacitor produce a −3dB high pass filter cutoff
frequency that is found using Equation 7.
(7)
As an example when using a speaker with a low frequency limit of 150Hz, the input coupling capacitor, using
Equation 7, is 0.063µF. The 0.33µF input coupling capacitor shown in Figure 4 allows the LM4835 to drive high
efficiency, full range speaker whose response extends below 30Hz.
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OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4835 contains circuitry that minimizes turn-on and shutdown transients or “clicks and pop”. For this
discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated.
While the power supply is ramping to its final value, the LM4835's internal amplifiers are configured as unity gain
buffers. An internal current source changes the voltage of the BYPASS pin in a controlled, linear manner. Ideally,
the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains
unity until the voltage on the bypass pin reaches 1/2 VDD. As soon as the voltage on the bypass pin is stable, the
device becomes fully operational. Although the BYPASS pin current cannot be modified, changing the size of CB
alters the device's turn-on time and the magnitude of “clicks and pops”. Increasing the value of CB reduces the
magnitude of turn-on pops. However, this presents a tradeoff: as the size of CB increases, the turn-on time
increases. There is a linear relationship between the size of CB and the turn-on time. Here are some typical turnon times for various values of CB:
CB
TON
0.01µF
2ms
0.1µF
20ms
0.22µF
44ms
0.47µF
94ms
1.0µF
200ms
In order eliminate “clicks and pops”, all capacitors must be discharged before turn-on. Rapidly switching VDD may
not allow the capacitors to fully discharge, which may cause “clicks and pops”. In a single-ended configuration,
the output is coupled to the load by COUT. This capacitor usually has a high value. COUT discharges through
internal 20kΩ resistors. Depending on the size of COUT, the discharge time constant can be relatively large. To
reduce transients in single-ended mode, an external 1kΩ–5kΩ resistor can be placed in parallel with the internal
20kΩ resistor. The tradeoff for using this resistor is increased quiescent current.
Figure 54. Resistor for Varying Output Loads
DOCKING STATION INTERFACE
Applications such as notebook computers can take advantage of a docking station to connect to external devices
such as monitors or audio/visual equipment that sends or receives line level signals. The LM4835 has two
outputs, Pin 9 and Pin 13, which connect to outputs of the internal input amplifiers that drive the volume control
inputs. These input amplifiers can drive loads of >1kΩ (such as powered speakers) with a rail-to-rail signal. Since
the output signal present on the RIGHT DOCK and LEFT DOCK pins is biased to VDD/2, coupling capacitors
should be connected in series with the load. Typical values for the coupling capacitors are 0.33µF to 1.0µF. If
polarized coupling capacitors are used, connect their "+" terminals to the respective output pin.
Since the DOCK outputs precede the internal volume control, the signal amplitude will be equal to the input
signal's magnitude and cannot be adjusted. However, the input amplifier's closed-loop gain can be adjusted
using external resistors. These resistors are shown in Figure 4 as 20kΩ devices that set each input amplifier's
gain to -1. Use Equation 8 to determine the input and feedback resistor values for a desired gain.
- Av = RF / Ri
(8)
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Adjusting the input amplifier's gain sets the minimum gain for that channel. The DOCK outputs adds circuit and
functional flexibility because their use supercedes using the inverting outputs of each bridged output amplifier as
line-level outputs.
BEEP DETECT FUNCTION
Computers and notebooks produce a system “beep“ signal that drives a small speaker. The speaker's auditory
output signifies that the system requires user attention or input. To accommodate this system alert signal, the
LM4835's pin 11 is a mono input that accepts the beep signal. Internal level detection circuitry at this input
monitors the beep signal's magnitude. When a signal level greater than VDD/2 is detected on pin 11, the bridge
output amplifiers are enabled. The beep signal is amplified and applied to the load connected to the output
amplifiers. A valid beep signal will be applied to the load even when MUTE is active. Use the input resistors
connected between the BEEP IN pin and the stereo input pins to accommodate different beep signal amplitudes.
These resistors are shown as 200kΩ devices in Figure 4. Use higher value resistors to reduce the gain applied to
the beep signal. The resistors must be used to pass the beep signal to the stereo inputs. The BEEP IN pin is
used only to detect the beep signal's magnitude: it does not pass the signal to the output amplifiers. The
LM4835's shutdown mode must be deactivated before a system alert signal is applied to BEEP IN pin.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the LM4835's shutdown function. Activate micro-power
shutdown by applying VDD to the SHUTDOWN pin. When active, the LM4835's micro-power shutdown feature
turns off the amplifier's bias circuitry, reducing the supply current. The logic threshold is typically VDD/2. The low
0.7µA typical shutdown current is achieved by applying a voltage that is as near as VDD as possible to the
SHUTDOWN pin. A voltage that is less than VDD may increase the shutdown current. Table 1 shows the logic
signal levels that activate and deactivate micro-power shutdown and headphone amplifier operation.
There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw
switch, a microprocessor, or a microcontroller. When using a switch, connect an external 10kΩ pull-up resistor
between the SHUTDOWN pin and VDD. Connect the switch between the SHUTDOWN pin and ground. Select
normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to VDD
through the pull-up resistor, activating micro-power shutdown. The switch and resistor ensure that the
SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or a
microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN
pin with active circuitry eliminates the pull up resistor.
Table 1. LOGIC LEVEL TRUTH TABLE FOR SHUTDOWN, HP-IN, AND MUX OPERATION
SHUTDOWN
PIN
HP-IN PIN
MUX CHANNEL
SELECT PIN
OPERATIONAL MODE
(MUX INPUT CHANNEL #)
Logic Low
Logic Low
Logic Low
Bridged Amplifiers (1)
Logic Low
Logic Low
Logic High
Bridged Amplifiers (2)
Logic Low
Logic High
Logic Low
Single-Ended Amplifiers (1)
Logic Low
Logic High
Logic High
Single-Ended Amplifiers (2)
Logic High
X
X
Micro-Power Shutdown
MODE FUNCTION
The LM4835's MODE function has two states controlled by the voltage applied to the MODE pin (pin 4). Mode 0,
selected by applying 0V to the MODE pin, forces the LM4835 to effectively function as a "line-out," unity-gain
amplifier. Mode 1, which uses the internal DC controlled volume control, is selected by applying VDD to the
MODE pin. This mode sets the amplifier's gain according to the DC voltage applied to the DC VOL CONTROL
pin. Prevent unanticipated gain behavior by connecting the MODE pin to VDD or ground. Do not let pin 4 float.
MUTE FUNCTION
The LM4835 mutes the amplifier and DOCK outputs when VDD is applied to pin 5, the MUTE pin. Even while
muted, the LM4835 will amplify a system alert (beep) signal whose magnitude satisfies the BEEP DETECT
circuitry. Applying 0V to the MUTE pin returns the LM4835 to normal, unmated operation. Prevent unanticipated
mute behavior by connecting the MUTE pin to VDD or ground. Do not let pin 5 float.
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Figure 55. Headphone Sensing Circuit
HP-IN FUNCTION
Applying a voltage between 4V and VDD to the LM4835's HP-IN headphone control pin turns off Amp2A and
Amp2B, muting a bridged-connected load. Quiescent current consumption is reduced when the IC is in this
single-ended mode.
Figure 55 shows the implementation of the LM4835's headphone control function. With no headphones
connected to the headphone jack, the R1-R2 voltage divider sets the voltage applied to the HP-IN pin (pin 16) at
approximately 50mV. This 50mV enables Amp1B and Amp2B, placing the LM4835 in bridged mode operation.
The output coupling capacitor blocks the amplifier's half supply DC voltage, protecting the headphones.
The HP-IN threshold is set at 4V. While the LM4835 operates in bridged mode, the DC potential across the load
is essentially 0V. Therefore, even in an ideal situation, the output swing cannot cause a false single-ended
trigger. Connecting headphones to the headphone jack disconnects the headphone jack contact pin from −OUTA
and allows R1 to pull the HP Sense pin up to VDD. This enables the headphone function, turns off Amp2A and
Amp2B, and mutes the bridged speaker. The amplifier then drives the headphones, whose impedance is in
parallel with resistor R2 and R3. These resistors have negligible effect on the LM4835's output drive capability
since the typical impedance of headphones is 32Ω.
Figure 55 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a
three-wire plug. The plug's tip and ring should each carry one of the two stereo output signals, whereas the
sleeve should carry the ground return. A headphone jack with one control pin contact is sufficient to drive the HPIN pin when connecting headphones.
A microprocessor or a switch can replace the headphone jack contact pin. When a microprocessor or switch
applies a voltage greater than 4V to the HP-IN pin, a bridge-connected speaker is muted and Amp1A and
Amp2A drive a pair of headphones.
GAIN SELECT FUNCTION (Bass Boost)
The LM4835 features selectable gain, using either internal and external feedback resistors. Either set of
feedback resistors set the gain of the output amplifiers. The voltage applied to pin 3 (GAIN SELECT pin) controls
which gain is selected. Applying VDD to the GAIN SELECT pin selects the external gain mode. Applying 0V to the
GAIN SELECT pin selects the internally set unity gain.
In some cases a designer may want to improve the low frequency response of the bridged amplifier or
incorporate a bass boost feature. This bass boost can be useful in systems where speakers are housed in small
enclosures. A resistor, RLFE, and a capacitor, CLFE, in parallel, can be placed in series with the feedback resistor
of the bridged amplifier as seen in Figure 56.
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At low, frequencies CLFE is a virtual open circuit and at high frequencies, its nearly zero ohm impedance shorts RLFE.
The result is increased bridge-amplifier gain at low frequencies. The combination of RLFE and CLFE form with a -3dB
corner frequency at
Figure 56. Low Frequency Enhancement
fC = 1 / (2πRLFEC LFE)
(9)
The bridged-amplifier low frequency differential gain is:
AVD = 2(RF + RLFE) / Ri
(10)
Using the component values shown in Figure 4 (RF = 20kΩ, RLFE = 20kΩ, and CLFE = 0.068µF), a first-order, 3dB pole is created at 120Hz. Assuming R i = 20kΩ, the low frequency differential gain is 4. The input (Ci) and
output (CO) capacitor values must be selected for a low frequency response that covers the range of frequencies
affected by the desired bass-boost operation.
DC VOLUME CONTROL
The LM4835 has an internal stereo volume control whose setting is a function of the DC voltage applied to the
DC VOL CONTROL pin. The volume control's voltage input range is 0V to VDD. The volume range is from 0dB
(DC control voltage = 80% VDD) to -80dB (DC control voltage = 0V). The volume remains at 0dB for DC control
voltages greater than 80% VDD. When the MODE input is 0V, the LM4835 operates at unity gain, bypassing the
volume control. A graph showing a typical volume response versus DC control voltage is shown in the Typical
Performance Characteristics section.
Like all volume controls, the LM4835's internal volume control is set while listening to an amplified signal that is
applied to an external speaker. The actual voltage applied to the DC VOL CONTROL pin is a result of the
volume a listener desires. As such, the volume control is designed for use in a feedback system that includes
human ears and preferences. This feedback system operates quite well without the need for accurate gain. The
user simply sets the volume to the desired level as determined by their ear, without regard to the actual DC
voltage that produces the volume. Therefore, the accuracy of the volume control is not critical, as long as the
volume changes monotonically, matches well between stereo channels, and the step size is small enough to
reach a desired volume that is not too loud or too soft. Since gain accuracy is not critical, there will be volume
variation from part-to-part even with the same applied DC control voltage. The gain of a given LM4835 can be
set with a fixed external voltage, but another LM4835 may require a different control voltage to achieve the same
gain. The typical part-to-part variation can be as large as 8dB for the same control voltage.
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AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Power Output: 1 WRMS
Load Impedance: 8Ω
Input Level: 1 VRMS
Input Impedance: 20 kΩ
Bandwidth: 100 Hz−20 kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage necessary to obtain the specified output power.
One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical
Performance Characteristics section. Another way, using Equation 10, is to calculate the peak output voltage
necessary to achieve the desired output power for a given load impedance. To account for the amplifier's dropout
voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the Typical Performance
Characteristics curves, must be added to the result obtained by Equation 11. The result is Equation 12.
(11)
(12)
VDD ≥ (VOUTPEAK+ (VODTOP + VODBOT))
The Output Power vs Supply Voltage graph for an 8Ω load indicates a minimum supply voltage of 4.6V. This is
easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom,
allowing the LM4835 to produce peak output power in excess of 1W without clipping or other audible distortion.
The choice of supply voltage must also not create a situation that violates of maximum power dissipation as
explained above in the POWER DISSIPATION section.
After satisfying the LM4835's power dissipation requirements, the minimum differential gain needed to achieve
1W dissipation in an 8Ω load is found using Equation 13.
(13)
Thus, a minimum gain of 2.83 allows the LM4835's to reach full output swing and maintain low noise and THD+N
performance. For this example, let AVD = 3.
The amplifier's overall gain is set using the input (Ri) and feedback (Ri) resistors. With the desired input
impedance set at 20kΩ, the feedback resistor is found using Equation 14.
Rf / Ri = AVD / 2
(14)
The value of Rf is 30kΩ.
The last step in this design example is setting 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 to at least five times the upper bandwidth
limit. The gain variation for both response limits is 0.17dB, well within the ±0.25dB desired limit. The results are
an
fL = 100Hz / 5 = 20Hz
(15)
and an
fH = 20kHz x 5 = 100kHz
(16)
As mentioned in the Selecting Proper External Components section, Ri and Ci create a highpass filter that sets
the amplifier's lower bandpass frequency limit. Find the input coupling capacitor's value using Equation 17.
Ci≥ 1 / (2πRifL)
(17)
The result is
1 / (2π*20kΩ*20Hz) = 0.397μF
(18)
Use a 0.39μF capacitor, the closest standard value.
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The product of the desired high frequency cutoff (100kHz in this example) and the differential gain AVD,
determines the upper passband response limit. With AVD = 3 and fH = 100kHz, the closed-loop gain bandwidth
product (GBWP) is 300kHz. This is less than the LM4835's 3.5MHz GBWP. With this margin, the amplifier can
be used in designs that require more differential gain while avoiding performance, restricting bandwidth
limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figure 57 through Figure 61 show the recommended four-layer PC board layout that is optimized for the 24-pin
LQ-packaged LM4835 and associated external components. This circuit is designed for use with an external 5V
supply and 4Ω speakers.
This circuit board is easy to use. Apply 5V and ground to the board's VDD and GND pads, respectively. Connect
4Ω speakers between the board's −OUTA and +OUTA and OUTB and +OUTB pads.
Figure 57. Recommended LQ PC Board Layout: Component-Side Silkscreen
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Figure 58. Recommended LQ PC Board Layout: Component-Side Layout
Figure 59. Recommended LQ PC Board Layout: Upper Inner-Layer Layout
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Figure 60. Recommended LQ PC Board Layout: Lower Inner-Layer Layout
Figure 61. Recommended LQ PC Board Layout: Bottom-Side Layout
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LM4835 MDC MWC
STEREO 2W AUDIO POWER AMPLIFIER WITH DC VOLUME CONTROL AND SELECTABLE
GAIN
Figure 62. Die Layout (A - Step)
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REVISION HISTORY
Changes from Revision E (May 2013) to Revision F
•
28
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 27
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM4835MTX/NOPB
ACTIVE
TSSOP
PW
28
2500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 85
LM4835MT
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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