LM4912MMBD - Texas Instruments

June 17, 2008

LM4912

Stereo 40mW Low Noise Headphone Amplifier

General Description

The LM4912 is an stereo audio power amplifier capable of

delivering 40mW per channel of continuous average power

into a 16Ω load or 25mW per channel into a 32Ω load at 1%

THD form a 3V power supply.

Boomer audio power amplifiers were designed specifically to

provide high quality output power with a minimal amount of

external components. Since the LM4912 does not require

bootstrap capacitors or snubber networks, it is optimally suit-

ed for low-power portable systems.

The LM4912 features a low-power consumption shutdown

mode and a power mute mode that allows for faster turn on

time with less than 1mV voltage change at outputs on release.

Additionally, the LM4912 features an internal thermal shut-

down protection mechanism.

The LM4912 is unity gain stable and may be configured with

external gain-setting resistors.

Key Specifications

■PSRR at 217 Hz and 1kHz 65dB (typ)

■Output Power at 1kHz with VDD = 2.4V, 1% THD

+N into a 16Ω load

25mW

(typ)

■Output Power at 1kHz with VDD = 3V, 1% THD

+N into a 16Ω load

40mW

(typ)

■Shutdown Current 1.0µA (max)

■Output Voltage change on release from Shutdown

VDD = 2.4V, RL = 16Ω

1mV

(max)

■Output Noise, 20Hz to 20kHz, A-weighted 10µV (typ)

Features

■External gain-setting capability

■Available in space-saving MSOP package

■Ultra low current shutdown mode

■Mute mode allows fast turn-on (10ms) with less than 1mV

change on outputs

■2.0V - 5.5V operation

■Ultra low noise

■Operation at low supply voltages

Applications

■Portable CD players

■PDAs

■Portable electronics devices

Typical Application

20048180

FIGURE 1. Typical Capacitive Coupled Output Configuration Circuit

Boomer® is a registered trademark of National Semiconductor Corporation.

LM4912 Stereo 40mW Low Noise Headphone Amplifier

Connection Diagrams

MSOP Package

20048179

Top View

Order Number LM4912MM

See NS Package Number MUB10A

MSOP Package

200481a9

Top View

G-Boomer Family

A3 - LM4912MM

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LM4912

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 6.0V

Storage Temperature −65°C to +150°C

Input Voltage -0.3V to VDD + 0.3V

Power Dissipation (Note 3) Internally Limited

ESD Susceptibility (Note 4) 2000V

ESD Susceptibility (Note 5) 250V

Junction Temperature 150°C

Thermal Resistance

θJC (MSOP) 56°C/W

θJA (MSOP) 190°C/W

Operating Ratings

Temperature Range

TMIN ≤ TA ≤ TMAX −40°C ≤ T A ≤ 85°C

Supply Voltage (VDD) 2.0V ≤ VDD ≤ 5.5V

Electrical Characteristics VDD = 5.0V (Notes 1, 2)

The following specifications apply for VDD = 5.0V, RL = 16Ω, CO = 100µ, and CB = 4.7µF unless otherwise specified. Limits apply

to TA = 25°C.

Symbol Parameter Conditions LM4912 Units

(Limits)

Typ

(Note 6)

Limit

(Note 7)

IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 2 5 mA (max)

ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA (max)

IMMute Current VMUTE = VDD 2 5 mA (max)

VSDIH Shutdown Voltage Input High 1.8 V

VSDIL Shutdown Voltage Input Low 0.4 V

VMIH Mute Voltage Input High 1.8 V

VMIL Mute Voltage Input Low 0.4 V

POOutput Power

THD = 1%; f = 1kHz

mWR = 16Ω145

R = 32Ω80

VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV

PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB

AMMute Attenuation f = 1kHz 85 dB

Electrical Characteristics VDD = 3.0V (Notes 1, 2)

The following specifications apply for VDD = 3.0V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply

to TA = 25°C.

Symbol Parameter Conditions LM4912 Units

(Limits)

Typ

(Note 6)

Limit

(Note 7)

IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 1.5 3 mA (max)

ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA (max)

IMMute Current VMUTE = VDD 1.5 3 mA (max)

POOutput Power

THD = 1%; f = 1kHz

mWR = 16Ω40

R = 32Ω25

VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV

PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB

AMMute Attenuation f = 1kHz 80 dB

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LM4912

Electrical Characteristics VDD = 2.4V (Notes 1, 2)

The following specifications apply for VDD = 2.4V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply

to TA = 25°C.

Symbol Parameter Conditions LM4912 Units

(Limits)

Typ

(Note 6)

Limit

(Note 7)

IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 1.5 3 mA (max)

ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA (max)

IMMute Current VMUTE = VDD 1.5 3 mA (max)

POOutput Power

THD = 1%; f = 1kHz

mWR = 16Ω25

R = 32Ω12

VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV

PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB

TWU Wake Up Time from Shutdown 2 s

VOSD Output Voltage Change on

Release from Shutdown 1 mV (max)

TUM Time to Un-Mute 0.01 0.02 s (max)

AMMute Attenuation f = 1kHz 80 db

Note 1: All voltages are measured with respect to the GND pin unless otherwise specified.

Note 2: : Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device

is functional but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions

which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters

where no limit is given, however, the typical value is a good indication of device performance.

Note 3: : The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The

maximum allowable power dissipation is PDMAX = (TJMAX - TA)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4912, see

power derating currents for more information.

Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.

Note 5: Machine Model, 220pF-240pF discharged through all pins.

Note 6: Typicals are measured at 25°C and represent the parametric norm.

Note 7: Limits are guaranteed to National's AOQL (Average Outgoing Quality Level).

Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.

Note 9: 10Ω Terminated input.

External Components Description

See (Figure 1)

Components Functional Description

1. RIInverting 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. CIInput coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a high-pass

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. RfFeedback resistance which sets the closed-loop gain in conjunction with Ri.

4. CSSupply 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. CBBypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of Proper

Components, for information concerning proper placement and selection of CB

6. CoOutput coupling capacitor which blocks the DC voltage at the amplifier's output. Forms a high pass filter with RL at

fo = 1/(2πRLCo)

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LM4912

Typical Performance Characteristics

THD+N vs Frequency

20048184

THD+N vs Frequency

20048185

THD+N vs Frequency

20048186

THD+N vs Frequency

20048187

THD+N vs Frequency

20048188

THD+N vs Frequency

20048189

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LM4912

THD+N vs Output Power

20048190

THD+N vs Output Power

20048191

THD+N vs Output Power

20048108

THD+N vs Output Power

20048107

THD+N vs Output Power

20048110

THD+N vs Output Power

20048109

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LM4912

Output Resistance vs Load Resistance

20048113

Output Power vs Supply Voltage

20048112

Output Power vs Supply Voltage

20048111

Output Power vs Load Resistance

20048196

Power Dissipation vs. Output Power

20048198

Power Dissipation vs. Output Power

20048199

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LM4912

Power Dissipation vs Output Power

200481a0

Channel Separation

200481a1

Channel Separation

200481a2

Channel Separation

20048117

Channel Separation

20048119

Channel Separation

20048118

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LM4912

Channel Separation

20048116

Mute Attenuation

200481b4

Power Supply Rejection Ratio

200481a3

Power Supply Rejection Ratio

200481a4

Power Supply Rejection Ratio

20048120

Power Supply Rejection Ratio

20048123

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LM4912

Power Supply Rejection Ratio

20048121

Power Supply Rejection Ratio

20048122

Frequency Response vs

Input Capacitor Size

20048127

Open Loop Frequency Response

200481a7

Supply Voltage vs

Supply Current

20048124

Clipping Voltage vs

Supply Voltage

20048125

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LM4912

Noise Floor

200481a8

Shutdown Hysteresis Voltage, Vdd=5V

200481b1

Shutdown Hysteresis Voltage, Vdd=3V

200481b2

Power Derating Curve

200481b3

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LM4912

Application Information

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4912 has three operational am-

plifiers internally. Two of the amplifier's have externally con-

figurable gain while the other amplifier is internally fixed at the

bias point acting as a unity-gain buffer. The closed-loop gain

of the two configurable amplifiers is set by selecting the ratio

of Rf to Ri. Consequently, the gain for each channel of the IC

AVD = -(Rf / Ri)

By driving the loads through outputs VoA and VoB with VoC

acting as a buffered bias voltage the LM4912 does not require

output coupling capacitors. The classical single-ended am-

plifier configuration where one side of the load is connected

to ground requires large, expensive output coupling capaci-

tors.

A configuration, such as the one used in the LM4912, has a

major advantage over single supply, single-ended amplifiers.

Since the outputs VoA, VoB, and VoC are all biased at 1/2

VDD, no net DC voltage exists across each load. This elimi-

nates the need for output coupling capacitors which are re-

quired in a single-supply, single-ended amplifier configura-

tion. Without output coupling capacitors in a typical single-

supply, single-ended amplifier, the bias voltage is placed

across the load resulting in both increased internal IC power

dissipation and possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a suc-

cessful design. When operating in capacitor-coupled mode,

Equation 1 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) (1)

Since the LM4912 has two operational amplifiers in one pack-

age, the maximum internal power dissipation point is twice

that of the number which results from Equation 1. From Equa-

tion 1, assuming a 3V power supply and a 32Ω load, the

maximum power dissipation point is 14mW per amplifier.

Thus the maximum package dissipation point is 28mW.

The maximum power dissipation point obtained from Equa-

tion 1 must not be greater than the power dissipation that

results from Equation 2:

PDMAX = (TJMAX - TA) / θJA (2)

For package MUB10A, θJA = 190°C/W. TJMAX = 150°C for the

LM4912. Depending on the ambient temperature, TA, of the

system surroundings, Equation 2 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be decreased,

the load impedance increased or TA reduced. For the typical

application of a 3V power supply, with an 32Ω load, the max-

imum ambient temperature possible without violating the

maximum junction temperature is approximately 144°C pro-

vided that device operation is around the maximum power

dissipation point. Thus, for typical applications, power dissi-

pation is not an issue. Power dissipation is a function of output

power and thus, if typical operation is not around the maxi-

mum power dissipation point, the ambient temperature may

be increased accordingly. Refer to the Typical Performance

Characteristics curves for power dissipation information for

lower output powers.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be as

close to the device as possible.

Typical applications employ a 3V regulator with 10mF tanta-

lum or electrolytic capacitor and a ceramic bypass capacitor

which aid in supply stability. This does not eliminate the need

for bypassing the supply nodes of the LM4912. A bypass ca-

pacitor value in the range of 0.1µF to 1µF is recommended

for CS.

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4912's shutdown function. Activate micro-power shutdown

by applying a logic-low voltage to the SHUTDOWN pin. When

active, the LM4912's micro-power shutdown feature turns off

the amplifier's bias circuitry, reducing the supply current. The

trigger point varies depending on supply voltage and is shown

in the Shutdown Hysteresis Voltage graphs in the Typical

Performance Characteristics section. The low 0.1µA(typ)

shutdown current is achieved by applying a voltage that is as

near as ground as possible to the SHUTDOWN pin. A voltage

that is higher than ground may increase the shutdown current.

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a mi-

croprocessor, or a microcontroller. When using a switch,

connect an external 100kΩ pull-up resistor between the

SHUTDOWN pin and VDD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by opening the switch. Closing the switch connects the

SHUTDOWN pin to ground, activating micro-power shut-

down.

The switch and resistor guarantee that the SHUTDOWN pin

will not float. This prevents unwanted state changes. In a sys-

tem with a microprocessor or 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.

Shutdown enable/disable times are controlled by a combina-

tion of CB and VDD. Larger values of CB results in longer turn

on/off times from Shutdown. Smaller Vdd values also in-

crease turn on/off time for a given value of CB. Longer shut-

down times also improve the LM4912's resistance to click and

pop upon entering or returning from shutdown. For a 2.4V

supply and CB = 4.7µF, the LM4912 requires about 2 seconds

to enter or return from shutdown. This longer shutdown time

enables the LM4912 to have virtually zero pop and click tran-

sients upon entering or release from shutdown.

Smaller values of CB will decrease turn-on time, but at the cost

of increased pop and click and reduced PSRR. Since shut-

down enable/disable times increase dramatically as supply

voltage gets below 2.2V, this reduced turn-on time may be

desirable if extreme low supply voltage levels are used as this

would offset increases in turn-on time caused by the lower

supply voltage.

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LM4912

MUTE

When in C-CUPL mode, the LM4912 also features a mute

function that is independent of load impedance and enables

extremely fast turn-on/turn-off with a minimum of output pop

and click. The mute function leaves the outputs at their bias

level, thus resulting in higher power consumption than shut-

down mode, but also provides much faster turn on/off times.

Mute mode is enabled by providing a logic high signal on the

MUTE pin in the opposite manner as the shutdown function

described above. Threshold voltages and activation tech-

niques match those given for the shutdown function as well.

Additionally, Mute should not be enabled during shutdown or

while entering or returning from shutdown. This is not a valid

operation condition and may result in much higher pop and

click values.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications using

integrated power amplifiers is critical to optimize device and

system performance. While the LM4912 is tolerant of external

component combinations, consideration to component values

must be used to maximize overall system quality.

The LM4912 is unity-gain stable which gives the designer

maximum system flexibility. The LM4912 should be used in

low gain configurations to minimize THD+N values, and max-

imize the signal to noise ratio. Low gain configurations require

large input signals to obtain a given output power. Input sig-

nals equal to or greater than 1Vrms are available from sources

such as audio codecs. Very large values should not be used

for the gain-setting resistors. Values for Ri and Rf should be

less than 1MΩ. Please refer to the section, Audio Power

Amplifier Design, for a more complete explanation of proper

gain selection

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components shown

in Figures 2 and 3. The input coupling capacitor, Ci, forms a

first order high pass filter which limits low frequency response.

This value should be chosen based on needed frequency re-

sponse and turn-on time.

SELECTION OF INPUT CAPACITOR SIZE

Amplifying the lowest audio frequencies requires a high value

input coupling capacitor, Ci. A high value capacitor can be

expensive and may compromise space efficiency in portable

designs. In many cases, however, the headphones used in

portable systems have little ability to reproduce signals below

60Hz. Applications using headphones with this limited fre-

quency response reap little improvement by using a high

value input capacitor.

In addition to system cost and size, turn on time is affected by

the size of the input coupling capacitor Ci. A larger input cou-

pling capacitor requires more charge to reach its quiescent

DC voltage. This charge comes from the output via the feed-

back Thus, by minimizing the capacitor size based on nec-

essary low frequency response, turn-on time can be mini-

mized. A small value of Ci (in the range of 0.1µF to 0.39µF),

is recommended.

AUDIO POWER AMPLIFIER DESIGN

A 25mW/32Ω Audio Amplifier

Given:

Power Output 25mWrms

Load Impedance 32Ω

Input Level 1Vrms

Input Impedance 20kΩ

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found.

3V is a standard voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4912 to reproduce peak in excess of 25mW

without producing audible distortion. At this time, the designer

must make sure that the power supply choice along with the

output impedance does not violate the conditions explained

in the Power Dissipation section.

Once the power dissipation equations have been addressed,

the required gain can be determined from Equation 2.

(3)

From Equation 4, the minimum AV is 0.89; use AV = 1. Since

the desired input impedance is 20kΩ, and with a AV gain of 1,

a ratio of 1:1 results from Equation 1 for Rf to Ri. The values

are chosen with Ri = 20kΩ and Rf = 20kΩ. The final design

step is to address the bandwidth requirements which must be

stated as a pair of -3dB frequency points. Five times away

from a -3dB point is 0.17dB down from passband response

which is better than the required ± 0.25dB specified.

fL = 100Hz/5 = 20Hz

fH = 20kHz * 5 = 100kHz

As stated in the External Components section, Ri in con-

junction with Ci creates a

Ci ≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.

The high frequency pole is determined by the product of the

desired frequency pole, fH, and the differential gain, AV. With

an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz

which is much smaller than the LM4912 GBWP of 10MHz.

This figure displays that is a designer has a need to design

an amplifier with higher differential gain, the LM4912 can still

be used without running into bandwidth limitations.

Revision History

Rev Date Description

1.0 7/15/05 Fixed spelling typos.

1.01 06/16/08 Fixed a typo.

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LM4912

Physical Dimensions inches (millimeters) unless otherwise noted

MSOP

Order Number LM4912MM

NS Package Number MUB10A

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LM4912

Notes

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LM4912

Notes

LM4912 Stereo 40mW Low Noise Headphone Amplifier

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