National Semiconductor LM4906 Technical data

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LM4906
1W, Bypass-Capacitor-less Audio Amplifier with Internal
Selectable Gain

LM4906 1W, Bypass-Capacitor-less Audio Amplifier with Internal Selectable Gain

May 2003

General Description

The LM4906 is an audio power amplifier primarily designed
for demanding applications in mobile phones and other por­table communication device applications. It is capable of
delivering 1W of continuous average power to an 8Ω BTL
load with less than 1% distortion (THD+N) from a +5V power
supply.

The LM4906 is the first National Semiconductor Boomer
Power Amplifier that does not require an external PSRR
bypass capacitor. The LM4906 also has an internal select­able gain of either 6dB or 12dB. In addition, no output
coupling capacitors or bootstrap capacitors are required
which makes the LM4906 ideally suited for cell phone and
other low voltage portable applications.

The LM4906 contains advanced pop and click circuitry that
eliminates noise, which would otherwise occur during
turn-on and turn-off transitions.

Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4906 features a low -power
consumption shutdown mode (the part is enabled by pulling
the SD pin high). Additionally, the LM4906 features an inter­nal thermal shutdown protection mechanism.

Typical Application

Key Specifications

j

Improved PSRR at 217Hz for +3V 71dB

j

Power Output at +5V, THD+N = 1%, 8Ω 1.0W (typ)

j

Power Output at +3V, THD+N = 1%, 8Ω 390mW (typ)

j

Total shutdown power supply current 0.1µA (typ)

Features

n Selectable gain of 6dB (2V/V) or 12dB (4V/V)
n No output or PSRR bypass capacitors required
n Improved “Click and Pop” suppression circuitry
n Very fast turn on time: 5ms (typ)
n Minimum external components
n 2.6 - 5.5V operation
n BTL output can drive capacitive loads
n Ultra low current shutdown mode (SD Low)

Applications

n Portable computers
n Desktop computers
n Multimedia monitors

200571B9

FIGURE 1. Typical Audio Amplifier Application Circuit

Boomer®is a registered trademark of National Semiconductor Corporation.

© 2003 National Semiconductor Corporation DS200571 www.national.com

Connection Diagrams

LM4906

MSOP Package MSOP Marking

Top View

Order Number LM4906MM

See NS Package Number MUB08A

LLP Package LD Marking

Top View

Order Number LM4906LD

See NS Package Number LDA10B

20057102

200571C3

Z - Plant Code

200571F1

X - Date Code

T - Die Traceability

200571F2

Z - Plant Code

XY - Date Code

T - Die Traceability

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LM4906

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 (Note 10) 6.0V

Thermal Resistance

θ

(MSOP) 56˚C/W

JC

θ

(MSOP) 190˚C/W

JA

θ

(LLP) 12˚C/W

JC

θ

(LLP) 63˚C/W

JA

Storage Temperature −65˚C to +150˚C

Input Voltage −0.3V to V

DD

+0.3V

Operating Ratings

Power Dissipation (Notes 3, 11) Internally Limited

ESD Susceptibility (Note 4) 2000V

ESD Susceptibility (Note 5) 200V

Junction Temperature 150˚C

Temperature Range

T

≤ TA≤ T

MIN

MAX

Supply Voltage 2.6V ≤ V

Electrical Characteristics VDD=5V (Notes 1, 2)

The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T

LM4906

Symbol Parameter Conditions

= 0V, Io= 0A, No Load 3.5 7 mA (max)

V

I

DD

I

SD

V

OS

P

o

T

WU

Quiescent Power Supply Current

Shutdown Current VSD= GND 0.1 2 µA (max)

Output Offset Voltage 7 35 mV (max)

Output Power

Wake-up time 5 ms

THD+N Total Harmonic Distortion+Noise P

PSRR Power Supply Rejection Ratio

IN

V

= 0V, Io= 0A, 8Ω Load 4 8 mA (max)

IN

THD+N = 1% (max);f=1kHz

=8Ω

R

L

= 0.4 Wrms; f = 1kHz 0.2 %

o

V

= 200mV sine p-p

ripple

Input terminated with 10Ω
Gain at 6dB

V

SDIH

V

SDIL

Shutdown Voltage Input High SD Pin High = Part On 1.5 V (min)

Shutdown Voltage Input Low SD Pin Low = Part Off 1.3 V (max)

Typical Limit

(Note 6) (Notes 7, 8)

1.0 0.9 W (min)

67 (f =
217Hz)
70 (f = 1kHz)

−40˚C ≤ TA≤ 85˚C

≤ 5.5V

DD

= 25˚C.

A

Units

(Limits)

dB

Electrical Characteristics VDD=3V (Notes 1, 2)

The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T

LM4906

Symbol Parameter Conditions

= 0V, Io= 0A, No Load 2.6 6 mA (max)

V

I

DD

I

SD

V

OS

P

o

T

WU

Quiescent Power Supply Current

Shutdown Current VSD= GND 0.1 2 µA (max)

Output Offset Voltage 7 35 mV (max)

Output Power

Wake-up time 4 ms

THD+N Total Harmonic Distortion+Noise P

PSRR Power Supply Rejection Ratio

IN

V

= 0V, Io= 0A, 8Ω Load 3 7 mA (max)

IN

THD+N = 1% (max);f=1kHz

=8Ω

R

L

= 0.15 Wrms; f = 1kHz 0.1 %

o

V

= 200mV sine p-p

ripple

Input terminated with 10Ω
Gain at 6dB

V

SDIH

V

SDIL

Shutdown Voltage Input High SD Pin High = Part On 1.1 V (min)

Shutdown Voltage Input Low SD Pin Low = Part Off 0.9 V (max)

Typical Limit

(Note 6) (Notes 7, 8)

390 mW

71 (f =
217Hz)
73 (f = 1kHz)

= 25˚C.

A

Units

(Limits)

dB

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Note 1: All voltages are measured with respect to the ground 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 andAC electrical specifications under particular test conditions which

LM4906

guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given; however, the typical value is a good indication of device performance.

Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
allowable power dissipation is P
curves for additional 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: R

Note 10: If the product is in Shutdown mode and V

If the source impedance limits the current to a max of 10mA, then the device will be protected. If the device is enabled when V

6.5V, no damage will occur, although operation life will be reduced. Operation above 6.5V with no current limit will result in permanent damage.

Note 11: Maximum power dissipation in the device (P
Equation 1 shown in the Application Information section. It may also be obtained from the power dissipation graphs.

is measured from the output pin to ground. This value represents the parallel combination of the 10kΩ output resistors and the two 20kΩ resistors.

OUT

DMAX

=(T

)/θJAor the number given inAbsolute Maximum Ratings, whichever is lower. For the LM4906, see power derating

JMAX–TA

exceeds 6V (to a max of 8V VDD), then most of the excess current will flow through the ESD protection circuits.

DD

) occurs at an output power level significantly below full output power. P

DMAX

, θJA, and the ambient temperature TA. The maximum

JMAX

is greater than 5.5V and less than

DD

can be calculated using

DMAX

External Components Description

Components Functional Description

1. C

2. C

Input coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a

2

highpass filter with R
for an explanation of how to determine the value of C

Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing

1

at fc=1/ (2πRiCi). Refer to the section, Proper Selection of External Components,

i

.

i

section for information concerning proper placement and selection of the supply bypass capacitor.

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Typical Performance Characteristics

LM4906

THD+N vs Frequency

= 5V, RL=8Ω,

V

DD

f = 1kHz, PWR = 500mW

THD+N vs Power Out

= 5V, RL=8Ω, f = 1kHz

V

DD

THD+N vs Frequency

VDD= 3V, RL=8Ω,

f = 1kHz, PWR = 250mW

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THD+N vs Power Out

VDD= 3V, RL=8Ω, f = 1kHz

Power Supply Rejection Ratio

vs Frequency

= 5V, RL=8Ω

V

DD

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Power Supply Rejection Ratio

vs Frequency

VDD= 3V, RL=8Ω

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Typical Performance Characteristics (Continued)

LM4906

Noise Floor

V

= 5V, RL=8Ω

DD

80kHz Bandwith, Input to GND

Power Dissipation

vs Output Power, V

DD

=5V

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Power Derating Curve

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Power Dissipation

vs Output Power, VDD=3V

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Shutdown Hysteresis Voltage

= 5V, SD Mode = VDD(High)

V

DD

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Shutdown Hysteresis Voltage

VDD= 5V, SD Mode = VDD(Low)

Typical Performance Characteristics (Continued)

LM4906

Shutdown Hysteresis Voltage

V

= 3V, SD Mode = VDD(High)

DD

Output Power

vs Supply Voltage, R

L

=8Ω

Shutdown Hysteresis Voltage

VDD= 3V, SD Mode = GND (Low)

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Output Power

vs Supply Voltage, RL=16Ω

Output Power

vs Supply Voltage, R

=32Ω

L

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Frequency Response

vs Input Capacitor Size

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Typical Performance Characteristics (Continued)

LM4906

PSRR Distribution

V

= 5V, f = 1kHz, RL=8Ω

DD

PSRR Distribution

= 3V, f = 1kHz, RL=8Ω

V

DD

PSRR Distribution

VDD= 5V, f = 217Hz, RL=8Ω

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PSRR Distribution

VDD= 3V, f = 217Hz, RL=8Ω

200571F6 200571F7

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Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4906 has two internal opera­tional amplifiers. The first amplifier’s gain is either 6dB or
12dB depending on the gain select input (Low = 6dB, High =
12dB). The second amplifier’s gain is fixed by the two inter­nal 20kΩ resistors. Figure 2 shows that the output of ampli­fier one serves as the input to amplifier two which results in
both amplifiers producing signals identical in magnitude, but
out of phase by 180˚. Consequently, the differential gain for
the IC is

= 2 * (20k / 20k) or 2 * (40k / 20k)

A

VD

By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura­tion where one side of the load is connected to ground.

A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
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 clipped. In order to
choose an amplifier’s closed-loop gain without causing ex­cessive clipping, please refer to the Audio Power Amplifier
Design section.

A bridge configuration, such as the one used in LM4906,
also creates a second advantage over single-ended amplifi­ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura­tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4906 has two opera­tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa­tion 1.

=4*(VDD)2/(2π2RL) (1)

P

DMAX

sinking. If T

still exceeds 150˚C, then additional

JMAX

changes must be made. These changes can include re­duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the Typical Performance Charac-
teristics curves for power dissipation information for differ­ent output powers and output loading.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on the power supply pin should be as
close to the device as possible. Typical applications employ
a 5V regulator with 10µF tantalum 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 LM4906.

TURNING ON THE LM4906

The power supply must first be applied before the application
of an input signal to the device and the ramp time to V

DD

must be less than 4ms, otherwise the wake-up time of the
device will be affected. After applying V

, the LM4906 will

DD

turn-on after an initial minimum threshold input signal of
7mV
An input signal of less than 7mV

, resulting in a generated output differential signal.

RMS

will result in a negligible

RMS

output voltage. Once the device is turned on, the input signal
can go below the 7mV
however, SHUTDOWN or V

without shutting the device off. If,

RMS

is cycled, the minimum

DD

threshold requirement for the input signal must first be met
again, with V

ramping first.

DD

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4906 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. The device is placed into shut­down mode by toggling the Shutdown pin Low/ground. The
trigger point for shutdown low is shown as a typical value in
the Supply Current vs Shutdown Voltage graphs in the Typi-
cal Performance Characteristics section. It is best to
switch between ground and supply for maximum perfor­mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current may
be greater than the typical value of 0.1µA. In either case, the
shutdown pin should be tied to a definite voltage to avoid
unwanted state changes.

In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro­vides a quick, smooth transition to shutdown. Another solu­tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut­down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.

LM4906

It is critical that the maximum junction temperature T
150˚C is not exceeded. T
power derating curves by using P

can be determined from the

JMAX

and the PC board foil

DMAX

JMAX

area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of θ
resulting in higher P

values without thermal shutdown

DMAX

JA

protection circuitry being activated. Additional copper foil can
be added to any of the leads connected to the LM4906. It is
especially effective when connected to V

, GND, and the

DD

output pins. Refer to the application information on the
LM4906 reference design board for an example of good heat

of

SELECTION OF INPUT CAPACITOR SIZE

Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-

,

ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor­mance.

In addition to system cost and size, click and pop perfor­mance is effected by the size of the input coupling capacitor,

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Application Information (Continued)

C

LM4906

A larger input coupling capacitor requires more charge to

i.

reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8Ω Audio Amplifier

Given:

Power Output 1 Wrms

Load Impedance 8Ω

Input Level 1 Vrms

Input Impedance 20 kΩ

Bandwidth 100 Hz– 20 kHz

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.

DD

±

0.25 dB

). This

Extra supply voltage creates headroom that allows the
LM4906 to reproduce peaks in excess of 1W 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.

The gain of the LM4906 is internally set at either 6dB or
12dB.

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

= 20kHz*5=100kHz

f

H

As stated in the External Components section, Rin(20k) in
conjunction with C

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

C

2

create a highpass filter.

2

FIGURE 2. REFERENCE DESIGN BOARD SCHEMATIC

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200571C0

Application Information (Continued)

LM4906 MSOP DEMO BOARD ARTWORK

LM4906

Top Layer

Bottom Layer

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200571E7

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Application Information (Continued)

LM4906 LD DEMO BOARD ARTWORK

LM4906

Top Layer

Bottom Layer

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200571E9

Application Information (Continued)

Mono LM4906 Reference Design Boards

Bill of Material

Part Description Quantity Reference Designator

LM4906 Audio Amplifier 1 U1

Tantalum Capcitor, 1µF 1 C1

Ceramic Capacitor, 0.39µF 1 C2

Jumper Header Vertical Mount 2X1 0.100“ spacing 5 J1, J2, Input, Output, V

LM4906

DD

PCB LAYOUT GUIDELINES

This section provides practical guidelines for mixed signal
PCB layout that involves various digital/analog power and
ground traces. Designers should note that these are only
"rule-of-thumb" recommendations and the actual results will
depend heavily on the final layout.

GENERAL MIXED SIGNAL LAYOUT
RECOMMENDATION

Power and Ground Circuits

For 2 layer mixed signal design, it is important to isolate the
digital power and ground trace paths from the analog power
and ground trace paths. Star trace routing techniques (bring­ing individual traces back to a central point rather than daisy
chaining traces together in a serial manner) can have a
major impact on low level signal performance. Star trace
routing refers to using individual traces to feed power and
ground to each circuit or even device. This technique will
require a greater amount of design time but will not increase
the final price of the board. The only extra parts required will
be some jumpers.

Single-Point Power / Ground Connections

The analog power traces should be connected to the digital
traces through a single point (link). A "Pi-filter" can be helpful
in minimizing High Frequency noise coupling between the
analog and digital sections. It is further recommended to put
digital and analog power traces over the corresponding digi­tal and analog ground traces to minimize noise coupling.

Placement of Digital and Analog Components

All digital components and high-speed digital signal traces
should be located as far away as possible from analog
components and circuit traces.

Avoiding Typical Design / Layout Problems

Avoid ground loops or running digital and analog traces
parallel to each other (side-by-side) on the same PCB layer.
When traces must cross over each other do it at 90 degrees.
Running digital and analog traces at 90 degrees to each
other from the top to the bottom side as much as possible will
minimize capacitive noise coupling and cross talk.

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Physical Dimensions inches (millimeters) unless otherwise noted

LM4906

MSOP

Order Number LM4906MM

NS Package Number MUA08A

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

LM4906 1W, Bypass-Capacitor-less Audio Amplifier with Internal Selectable Gain

LLP

Order Number LM4906LD

NS Package Number LDA10B

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the

2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

labeling, can be reasonably expected to result in a
significant injury to the user.

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Support Center

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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.