National Semiconductor LM4915 Technical data

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LM4915
Pseudo-Differential Mono Headphone Amplifier with
Fixed 6dB Gain

LM4915 Pseudo-Differential Mono Headphone Amplifier with Fixed 6dB Gain

May 2003

General Description

The LM4915 is a pseudo-differential audio power amplifier
primarily designed for demanding applications in mobile
phones and other portable audio device applications with
mono headphones. It is capable of delivering 90 miliwatts of
continuous average power to a 32Ω BTL load with less than
1% distortion (THD+N) from a 3V

Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4915 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other low voltage appli­cations where minimal power consumption is a primary re­quirement.

The LM4915 features a low-power consumption shutdown
mode. To facilitate this, Shutdown may be enabled by driving
the shutdown pin low. Additionally, the LM4915 features an
internal thermal shutdown protection mechanism.

The LM4915 contains advanced pop & click circuitry which
virtually eliminates noises which would otherwise occur dur­ing turn-on and turn-off transitions.

The LM4915 has an internally fixed gain of 6dB.

power supply.

DC

Typical Application

Key Specifications

n Improved PSRR at 217Hz and 1kHz 75dB (typ)
n Power Output at 5.0V & 1% THD into 32Ω 280mW (typ)
n Power Output at 3.0V & 1% THD into 32Ω 90mW (typ)
n Output Noise, A-weighted 20µV (typ)

Features

n Pseudo-differential amplification
n Internal gain-setting resistors
n Available in space-saving LLP package
n Ultra low current shutdown mode
n Can drive capacitive loads up to 500pF
n Improved pop & click circuitry virtually eliminates noises

during turn-on and turn-off transitions

n 2.2 - 5.5V operation
n No output coupling capacitors, snubber networks,

bootstrap capacitors or gain-setting resistors required

n Ultra low noise

Applications

n Mobile phones
n PDAs
n Portable electronics devices

200482B4

FIGURE 1. Typical Audio Amplifier Application Circuit

Boomer®is a registered trademark of National Semiconductor Corporation.

© 2003 National Semiconductor Corporation DS200482 www.national.com

Connection Diagrams

LM4915

LQ Package

Top View

Order Number LM4915LQ

See NS Package Number LQB08A

8 Pin LQ Marking

200482E7

X - Date Code

TT - Die Traceability

G - Boomer

A5 - LM4915LQ

200482B5

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LM4915

Absolute Maximum Ratings (Note 2)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Junction Temperature 150˚C

Thermal Resistance

θ

(LQ) 57˚C/W

JC

θ

(LQ) 140˚C/W

JA

Supply Voltage 6.0V

Storage Temperature −65˚C to +150˚C

Input Voltage -0.3V to V

DD

+ 0.3V

Power Dissipation (Note 3) Internally Limited

ESD Susceptibility (Note 4) 2000V

ESD Susceptibility (Note 5) 200V

Operating Ratings

Temperature Range

T

≤ TA≤ T

MIN

Supply Voltage (V

MAX

) 2.2V ≤ VCC≤ 5.5V

DD

−40˚C ≤ TA≤ +85˚C

Electrical Characteristics VDD=5V(Notes 1, 2, 8)

The following specifications apply for VDD= 5V, RL=16Ω unless otherwise specified. Limits apply to TA= 25˚C.

Symbol Parameter Conditions LM4915 Units

I

DD

I

SD

V

SDIH

V

SDIL

P

O

V

NO

Quiescent Power Supply Current VIN= 0V, IO= 0A 2 3.5 mA (max)

Shutdown Current V

Shutdown Voltage Input High 1.8 V

Shutdown Voltage Input Low 0.4 V

Output Power THD = 1% (max); f = 1kHz

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

PSRR Power Supply Rejection Ratio V

V

OS

Output Offset Voltage VIN= 0V 2 20 mV (max)

SHUTDOWN

=16

R

L

=32

R

L

RIPPLE

Typ

(Note 6)

= GND 0.1 Note 9 µA(max)

400
280

= 200mV sine p-p 75 dB

Limit

(Note 7)

375
250

(Limits)

mW

Electrical Characteristics VDD= 3.0V (Notes 1, 2, 8)

The following specifications apply for VDD= 3.0V, RL=16Ω unless otherwise specified. Limits apply to TA= 25˚C.

Symbol Parameter Conditions LM4915 Units

Typ

(Note 6)

I

DD

I

SD

V

SDIH

V

SDIL

P

O

V

NO

PSRR Power Supply Rejection Ratio V

V

OS

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 T
allowable power dissipation is P
curves 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 specifications are guaranteed by design, test, or statistical analysis.

Quiescent Power Supply Current VIN= 0V, IO= 0A 1.5 2.5 mA (max)

Shutdown Current V

SHUTDOWN

= GND 0.1 Note 9 µA(max)

Shutdown Voltage Input High 1.8 V

Shutdown Voltage Input Low 0.4 V

Output Power THD = 1% (max); f = 1kHz

=16

R

L

=32

R

L

125

90

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

= 200mV sine p-p 70 dB

RIPPLE

Output Offset Voltage VIN= 0V 2 20 mV (max)

, θJA, and the ambient temperature, TA. The maximum

DMAX

=(T

)/ θJAor the number given in Absolute Maximum Ratings, whichever is lower. For the LM4915, see power derating

JMAX-TA

JMAX

Limit

(Note 7)

100

80

(Limits)

mW (min)

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Electrical Characteristics VDD= 3.0V (Notes 1, 2, 8) (Continued)

Note 9: See ISDdistribution values shown in the ISDDistribution curve, VDD=5VandV=3V,shown in the Typical Performance Characteristics section.

LM4915

External Components Description (Figure 1)

Components Functional Description

1. C

2. C

Bypass pin capacitor that provides half-supply filtering. Refer to the section Proper Selection of External

B

Components for information concerning proper placement and selection of C

Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a

i

high-pass filter with the internal input resistance R

= 1/(2πRiCi). Refer to the section Proper Selection of External Components for an explanantion of

filter f

c

how to determine the value of C

.

i

. For the LM4915, Ri= 20kΩ, thus creating a high-pass

i

.

B

Typical Performance Characteristics

THD+N vs Frequency

= 5V, RL=16Ω

V

DD

THD+N vs Frequency

= 3V, RL=16Ω,PO= 100mW

V

DD

THD+N vs Frequency

VDD= 5V, RL=32Ω

200482C6 200482C7

THD+N vs Frequency

VDD= 3V, RL=32Ω,PO= 80mW

200482C4 200482C5

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Typical Performance Characteristics (Continued)

LM4915

THD+N vs Frequency

V

= 2.6V, RL=16Ω,PO= 50mW

DD

THD+N vs Output Power

= 5V, RL=16Ω

V

DD

THD+N vs Frequency

VDD= 2.6V, RL=32Ω,PO= 40mW

200482C2 200482C3

THD+N vs Output Power

VDD= 5V, RL=32Ω

THD+N vs Output Power

= 3V, RL=16Ω

V

DD

200482D2 200482D3

THD+N vs Output Power

VDD= 3V, RL=32Ω

200482D0 200482D1

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Typical Performance Characteristics (Continued)

LM4915

THD+N vs Output Power

V

= 2.6V, RL=16Ω

DD

PSRR vs Frequency

= 5V, RL=16Ω,PO= 375mW

V

DD

Input 10Ω Terminated

THD+N vs Output Power

VDD= 2.6V, RL=32Ω

200482C8 200482C9

PSRR vs Frequency

VDD= 5V, RL=32Ω,PO= 250mW

Input 10Ω Terminated

200482C0 200482C1

PSRR vs Frequency

= 3V, RL=16Ω

V

DD

Input 10Ω Terminated

200482B8 200482B9

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PSRR vs Frequency

VDD= 3V, RL=32Ω

Input 10Ω Terminated

Typical Performance Characteristics (Continued)

LM4915

PSRR vs Frequency

V

= 2.6V, RL=16Ω

DD

Input 10Ω Terminated

Output Power vs Load Resistance

= 2.6V, RL=32Ω

V

DD

PSRR vs Frequency

VDD= 2.6V, RL=32Ω

Input 10Ω Terminated

200482B6 200482B7

Output Power vs Supply Voltage

RL=16Ω

Output Power vs Supply Voltage

=32Ω

R

L

200482D9

200482E5

Power Dissipation vs Output Power

VDD=5V

200482E4 200482E1

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Typical Performance Characteristics (Continued)

LM4915

Power Dissipation vs Output Power

V

=3V

DD

Noise Floor Shutdown Hysterisis Voltage

200482E0

Frequency Response

vs Input Capacitor Size

=5V

V

DD

200482E6

200482D8

Shutdown Hysterisis Voltage

=3V

V

DD

200482E9

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I

Distribution

SD

V

DD

200482E8

=5V

200482F0

Typical Performance Characteristics (Continued)

I

Distribution

SD

=3V

V

DD

200482F1

LM4915

Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4915 is a pseudo-differential audio amplifier that
features a fixed gain of 6dB. Internally this is accomplished
by two separate sets of inverting amplifiers, each set to a
gain of 2. The LM4915 features precisely matched internal
gain-setting resistors set to R
eliminating the need for external resistors and fixing the
differential gain at A

VD

A differential amplifier works in a manner where the differ­ence between the two input signals is amplified. In most
applications, this would require input signals that are 180˚
out of phase with each other. The LM4915 works in a
pseudo-differential manner, so DC offset normally cancelled
by a fully differential amplifier needs to be blocked by input
coupling capacitors for the LM4915 to amplify the difference
between the inputs.

The LM4915 provides what is known as a ’bridged mode’
output (bridge-tied-load, BTL). This results in output signals
at Vo1 and Vo2 that are 180˚ out of phase with respect to
each other. Bridged mode operation is different from the
single-ended amplifier configuration that connects the load
between the amplifier output and ground. A bridged amplifier
design has distinct advantages over the single-ended con­figuration: it provides differential drive to the load, thus dou­bling maximum possible output swing for a specific supply
voltage. Four times the output power is possible compared
with a single-ended amplifier under the same conditions.

This increase in attainable output power assumes that the
amplifier is not current limited or clipped. A bridged configu­ration, such as the one used in the LM4915, 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. BTL configuration
eliminates the output coupling capacitor required in single­supply, single-ended amplifier configurations. If an output
coupling capacitor is not used in a single-ended output con-

= 20kΩ and Rf= 40kΩ, thus

i

= 6dB.

figuration, the half-supply bias across the load would result
in both increased internal IC power dissipation as well as
permanent loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. 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.

=(VDD)2/(2π2RL) Single-Ended (1)

P

DMAX

However, a direct consequence of the increased power de­livered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.

= 4(VDD)2/(2π2RL) Bridge Mode (2)

P

DMAX

Since the LM4915 has bridged outputs, the maximum inter­nal power dissipation is 4 times that of a single-ended am­plifier.

Even with this substantial increase in power dissipation, the
LM4915 does not require additional heatsinking under most
operating conditions and output loading. From Equation 2,
assuming a 5V power supply and an 16Ω load, the maximum
power dissipation point is 316mW. The maximum power
dissipation point obtained from Equation 2 must not be
greater than the power dissipation results from Equation 3:

=(T

P

The LM4915’s θ

DMAX

in an LQB08A package is 140˚C/W. De-

JA

JMAX-TA

pending on the ambient temperature, T

)/θ

JA

, of the system

A

(3)

surroundings, Equation 3 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 2 is greater than that of Equation 3,

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Application Information (Continued)

then either the supply voltage must be decreased, the load

LM4915

impedance increased, the ambient temperature reduced, or
the θ
traces near the output, V
lower the θ
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with a 16Ω load power
dissipation is not an issue. Recall that internal power dissi­pation is a function of output power. If typical operation is not
around the maximum power dissipation point, the LM4915
can operate at higher ambient temperatures. Refer to the
Typical Performance Characteristics curves for power dis­sipation information.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.

Typical applications employ a 5V regulator with 10µF and

0.1µF bypass capacitors that increase supply stability. This,
however, does not eliminate the need for bypassing the
supply nodes of the LM4915. A 1µF capacitor is recom­mended for C
This value coupled with small input capacitors (0.1µF to

0.47µF) gives virtually zero click and pop with outstanding
PSRR performance.

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the
LM4915’s shutdown function. Activate micro-power shut­down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4915’s micro-power shutdown fea­ture turns off the amplifier’s bias circuitry, reducing the sup­ply current. The trigger point is 0.4V for a logic-low level, and

1.8V for a logic-high level. 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
microprocessor, or a microcontroller. When using a switch,
connect an external 100k. pull-up resistor between the
SHUTDOWN pin and V
SHUTDOWN pin and ground. Select normal amplifier opera­tion by opening the switch. Closing the switch connects the
SHUTDOWN pin to ground, activating micro-power shut­down.

reduced with heatsinking. In many cases, larger

JA

. The larger areas of copper provide a form of

JA

. A 4.7µF capacitor is recommended for CB.

S

, and GND pins can be used to

DD

. Connect the switch between the

DD

The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a
system 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.

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 LM4915 is tolerant of
external component combinations, and requires minimal ex­ternal components, consideration to component values must
be used to maximize overall system quality.

The input coupling capacitor, C
filter which limits low frequency response given by f
1/(2πR

). Riis internally set to 20kΩ. This value should be

iCi

, forms a first order high pass

i

c

chosen based on needed frequency response for a few
distinct reasons.

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 attenu­ation. 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 affected by the size of the input coupling capacitor,

. A larger input coupling capacitor requires more charge to

C

I

reach its quiescent DC voltage (nominally 1/2 V

DD

). This
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

Besides minimizing the input capacitor size, careful consid­eration should be paid to the bypass capacitor value. Bypass
capacitor, C

, is the most critical component to minimize

B

turn-on pops since it determines how fast the LM4915 turns
on. The slower the LM4915’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
Choosing C

equal to 4.7µF along with a small value of CI(in

B

), the smaller the turn-on pop.

DD

the range of 0.1µF to 0.47µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with

equal to 1.0µF, the device will be much more susceptible

C

B

to turn-on clicks and pops. Thus, a value of C

equal to

B

4.7µF is recommended in all but the most cost sensitive
designs.

=

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Physical Dimensions inches (millimeters) unless otherwise noted

LM4915 Pseudo-Differential Mono Headphone Amplifier with Fixed 6dB Gain

Order Number LM4915LQ

NS Package Number LQB08A

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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
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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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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.

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