LM4809 Boomer ® Audio Power Amplifier Series Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode

LM4809

Dual 105mW Headphone Amplifier with Active-Low

Shutdown Mode

General Description

The LM4809 is a dual audio power amplifier capable of

delivering 105mW per channel of continuous average power

into a 16

Ω load with 0.1% (THD+N) from a 5V power supply.

Boomer audio power amplifiers were designed specifically to

provide high quality output power with a minimal amount of

external components. Since the LM4809 does not require

bootstrap capacitors or snubber networks, it is optimally

suited for low-power portable systems.

The unity-gain stable LM4809 can be configured by external

gain-setting resistors.

The LM4809 features an externally controlled, active-low,

micropower consumption shutdown mode, as well as an

internal thermal shutdown protection mechanism.

Key Specifications

THD+N at 1kHz at 105mW continuous average power

into 16

Ω

0.1% (typ)

THD+N at 1kHz at 70mW continuous average power

into 32

Ω

0.1% (typ)

Shutdown Current

0.4µA (typ)

Features

Active-low shutdown mode

"Click and Pop" reduction circuitry

Low shutdown current

LLP, MSOP, and SO surface mount packaging

No bootstrap capacitors required

Unity-gain stable

Applications

Headphone Amplifier

Personal Computers

Microphone Preamplifier

PDA’s

Typical Application

Boomer

is a registered trademark of National Semiconductor Corporation.

20009001

*Refer to the Application Information Section for information concerning proper selection of the input and output coupling capacitors.

FIGURE 1. Typical Audio Amplifier Application Circuit

November 2002

LM4809

Dual

105mW

Headphone

Amplifier

with

Active-Low

Shutdown

Mode

DS200090

www.national.com

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

ESD Susceptibility (Note 4)

3.5kV

ESD Machine model (Note 8)

250V

Junction Temperature (T

)

150˚C

Soldering Information (Note 1)

Small Outline Package

Vapor Phase (60 sec.)

215˚C

Infrared (15 sec.)

220˚C

Thermal Resistance

(SO)

170˚C/W

(SO)

35˚C/W

(MSOP)

210˚C/W

(MSOP)

56˚C/W

(LLP)

117˚C/W (Note 9)

(LLP)

150˚C/W (Note 10)

(LLP)

15˚C/W

Operating Ratings

Temperature Range

MIN

≤ T

MAX

−40˚C

≤ T

≤ 85˚C

Supply Voltage (V

)

2.0V

≤ V

≤ 5.5V

Note 1: See AN-450 “Surface Mounting and their Effects on Product Reli-

ability” for other methods of soldering surface mount devices.

Electrical Characteristics

(Notes 2, 3)

The following specifications apply for V

= 5V unless otherwise specified, limits apply to T

= 25˚C.

Symbol

Parameter

Conditions

LM4809

Units

(Limits)

Typ

(Note 5)

Limit

(Note 7)

Supply Voltage

2.0

V (min)

5.5

V (max)

Supply Current

= 0V, I

= 0A

1.4

mA (max)

Shutdown Current

= 0V, V

SHUTDOWN

= GND

0.4

µA(max)

Output Offset Voltage

= 0V

4.0

mV(max)

Output Power

THD+N = 0.1%, f = 1kHz

= 16

Ω

105

= 32

Ω

mW (min)

THD+N

Total Harmonic Distortion

= 50mW, R

= 32

Ω

f = 20Hz to 20kHz

0.3

Crosstalk

Channel Separation

= 32

Ω; P

= 70mW

PSRR

Power Supply Rejection Ratio

= 1.0µF; V

RIPPLE

= 200mV

f = 1kHz; Input terminated into 50

Ω

SDIH

Shutdown Voltage Input High

0.8 x V

V (min)

SDIL

Shutdown Voltage Input Low

0.2 x V

V (max)

Electrical Characteristics

(Notes 2, 3)

The following specifications apply for V

= 3.3V unless otherwise specified, limits apply to T

= 25˚C.

Symbol

Parameter

Conditions

LM4809

Units

(Limits)

Typ

(Note 5)

Limit

(Note 7)

Supply Current

= 0V, I

= 0A

1.1

Shutdown Current

= 0V, V

SHUTDOWN

= GND

0.4

µA

Output Offset Voltage

= 0V

4.0

Output Power

THD+N = 0.1%, f = 1kHz

= 16

Ω

= 32

Ω

THD+N

Total Harmonic Distortion

= 25mW, R

= 32

Ω

f = 20Hz to 20kHz

0.4

Crosstalk

Channel Separation

= 32

Ω; P

= 25mW

LM4809

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Electrical Characteristics

(Notes 2, 3) (Continued)

The following specifications apply for V

= 3.3V unless otherwise specified, limits apply to T

= 25˚C.

Symbol

Parameter

Conditions

LM4809

Units

(Limits)

Typ

(Note 5)

Limit

(Note 7)

PSRR

Power Supply Rejection Ratio

= 1.0µF; V

RIPPLE

= 200mV

f = 1kHz; Input terminated into 50

Ω

SDIH

Shutdown Voltage Input High

0.8 x V

V (min)

SDIL

Shutdown Voltage Input Low

0.2 x V

V (max)

Electrical Characteristics

(Notes 2, 3)

The following specifications apply for V

= 2.6V unless otherwise specified, limits apply to T

= 25˚C.

Symbol

Parameter

Conditions

LM4809

Units

(Limits)

Typ

(Note 5)

Limit

(Note 7)

Supply Current

= 0V, I

= 0A

0.9

Shutdown Current

= 0V, V

SHUTDOWN

= GND

0.2

µA

Output Offset Voltage

= 0V

4.0

Output Power

THD+N = 0.1%, f = 1kHz

= 16

Ω

= 32

Ω

THD+N

Total Harmonic Distortion

= 15mW, R

= 32

Ω

f = 20Hz to 20kHz

0.6

Crosstalk

Channel Separation

= 32

Ω; P

= 15mW

PSRR

Power Supply Rejection Ratio

= 1.0µF; V

RIPPLE

= 200mV

f = 1kHz; Input terminated into 50

Ω

SDIH

Shutdown Voltage Input High

0.8 x V

V (min)

SDIL

Shutdown Voltage Input Low

0.2 x V

V (max)

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

Note 3: All voltages are measured with respect to the ground pin, unless otherwise specified.

Note 4: Human body model, 100pF discharged through a 1.5k

Ω resistor.

Note 5: Typical specifications are specified at +25OC and represent the most likely parametric norm.

Note 6: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.

Note 8: Machine Model ESD test is covered by specification EIAJ IC-121-1981. A 200pF cap is charged to the specified voltage, then discharged directly into the

IC with no external series resistor (resistance of discharge path must be under 50Ohms).

Note 9: The given

is for an LM4809 packaged in an LDA08B wit the Exposed-Dap soldered to a printed circuit board copper pad with an area equivalent to that

of the Exposed-Dap itself.

Note 10: The given

is for an LM4809 packaged in an LDA08B with the Exposed-Dap not soldered to any printed circuit board copper.

External Components Description

(Figure 1)

Components

Functional Description

1. R

The inverting input resistance, along with R

, set the closed-loop gain. R

, along with C

, form a high

pass filter with f

= 1/(2

πR

2. C

The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. C

, along with R

create a highpass filter with f

= 1/(2

πR

). Refer to the section, Selecting Proper External

Components, for an explanation of determining the value of C

3. R

The feedback resistance, along with R

, set closed-loop gain.

4. C

This is the supply bypass capacitor. It provides power supply filtering. Refer to the Application

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

5. C

This is the BYPASS pin capacitor. It provides half-supply filtering. Refer to the section, Selecting

Proper External Components, for information concerning proper placement and selection of C

6. C

This is the output coupling capacitor. It blocks the DC voltage at the amplifier’s output and forms a high

pass filter with R

at f

= 1/(2

πR

)

LM4809

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Application Information

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4809’s shutdown function. Activate micro-power shut-

down by applying a logic low voltage to the SHUTDOWN pin.

The logic threshold is typically V

/2. When active, the

LM4809’s micro-power shutdown feature turns off the ampli-

fier’s bias circuitry, reducing the supply current. The low

0.4µA typical shutdown current is achieved by applying a

voltage that is as near as GND as possible to the SHUT-

DOWN pin. A voltage that is above GND 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

Ω pull-down resistor between the

SHUTDOWN pin and GND. Connect the switch between the

SHUTDOWN pin and V

. Select normal amplifier operation

by closing the switch. Opening the switch connects the

SHUTDOWN pin to GND through the pull-down resistor,

activating micro-power shutdown. The switch and resistor

guarantee that the SHUTDOWN pin will not float. This pre-

vents unwanted state changes. In a system with a micropro-

cessor or a microcontroller, use a digital output to apply the

control voltage to the SHUTDOWN pin. Driving the SHUT-

DOWN pin with active circuitry eliminates the pull-down re-

sistor.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATION

The LM4809’s exposed-Dap (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane, and surrounding air.

The LD package should have its DAP soldered to a copper

pad on the PCB. The DAP’s PCB copper pad may be con-

nected to a large plane of continuous unbroken copper. This

plane forms a thermal mass, heat sink, and radiation area.

However, since the LM4809 is designed for headphone ap-

plications, connecting a copper plane to the DAP’s PCB

copper pad is not required. The LM4809’s Power Dissipation

vs Output Power Curve in the Typical Performance Char-

acteristics shows that the maximum power dissipated is just

45mW per amplifier with a 5V power supply and a 32

Ω load.

Further detailed and specific information concerning PCB

layout, fabrication, and mounting an LD (LLP) package is

available from National Semiconductor’s Package Engineer-

ing Group under application note AN1187.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. 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.

DMAX

= (V

)

/ (2

)

(1)

Since the LM4809 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 1. Even

with the large internal power dissipation, the LM4809 does

not require heat sinking over a large range of ambient tem-

perature. From Equation 1, assuming a 5V power supply and

a 32

Ω load, the maximum power dissipation point is 40mW

per amplifier. Thus the maximum package dissipation point

is 80mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

DMAX

= (T

JMAX

− T

) /

(2)

For package MUA08A,

= 210˚C/W. T

JMAX

= 150˚C for

the LM4809. Depending on the ambient temperature, T

, 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 de-

creased, the load impedance increased or T

reduced. For

the typical application of a 5V power supply, with a 32

Ω load,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 133.2˚C

provided that device operation is around the maximum

power dissipation point. Power dissipation is a function of

output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature

may be increased accordingly. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for lower output powers.

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 stabi-

lize 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

LM4809’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4809’s

power supply pin and ground as short as possible. Connect-

ing a 4.7µF capacitor, C

, 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 the amplifier’s turn-on time. The

selection of bypass capacitor values, especially C

, 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 LM4809’s performance requires properly se-

lecting external components. Though the LM4809 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4809 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-to-noise ra-

tio. 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

LM4809

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Application Information

(Continued)

CODECs have outputs of 1V

RMS

(2.83V

P-P

). Please refer to

the Audio Power Amplifier Design section for more infor-

mation on selecting the proper gain.

Input and Output Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input and output coupling capacitors (C

and C

in Figure 1).

A high value capacitor can be expensive and may compro-

mise 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 150Hz. Applications using speakers with this limited

frequency response reap little improvement by using high

value input and output capacitors.

Besides affecting system cost and size, C

has an effect on

the LM4809’s click and pop performance. The magnitude of

the pop is directly proportional to the input capacitor’s size.

Thus, pops can be minimized by selecting an input capacitor

value that is no higher than necessary to meet the desired

−3dB frequency. Please refer to the Optimizing Click and

Pop Reduction Performance section for a more detailed

discussion on click and pop performance.

As shown in Figure 1, the input resistor, R

and the input

capacitor, C

, produce a −3dB high pass filter cutoff fre-

quency that is found using Equation (3). In addition, the

output load R

, and the output capacitor C

, produce a -3db

high pass filter cutoff frequency defined by Equation (4).

I-3db

=1/2

πR

(3)

O-3db

=1/2

πR

(4)

Also, careful consideration must be taken in selecting a

certain type of capacitor to be used in the system. Different

types of capacitors (tantalum, electrolytic, ceramic) have

unique performance characteristics and may affect overall

system performance.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the value of C

, the capacitor

connected to the BYPASS pin. Since C

determines how

fast the LM4809 settles to quiescent operation, its value is

critical when minimizing turn-on pops. The slower the

LM4809’s outputs ramp to their quiescent DC voltage (nomi-

nally 1/2 V

), the smaller the turn-on pop. Choosing C

equal to 4.7µF along with a small value of C

(in the range of

0.1µF to 0.47µF), produces a click-less and pop-less shut-

down function. As discussed above, choosing C

no larger

than necessary for the desired bandwith helps minimize

clicks and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4809 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. During turn-on, the

LM4809’s internal amplifiers are configured as unity gain

buffers. An internal current source charges up the capacitor

on the BYPASS pin in a controlled, linear manner. The gain

of the internal amplifiers remains unity until the voltage on

the BYPASS pin reaches 1/2 V

. As soon as the voltage on

the BYPASS pin is stable, the device becomes fully opera-

tional. During device turn-on, a transient (pop) is created

from a voltage difference between the input and output of the

amplifier as the voltage on the BYPASS pin reaches 1/2 V

For this discussion, the input of the amplifier refers to the

node between R

and C

. Ideally, the input and output track

the voltage applied to the BYPASS pin. During turn-on, the

buffer-configured amplifier output charges the input capaci-

tor, C

, through the input resistor, R

. This input resistor

delays the charging time of C

thereby causing the voltage

difference between the input and output that results in a

transient (pop). Higher value capacitors need more time to

reach a quiescent DC voltage (usually 1/2 V

) when

charged with a fixed current. Decreasing the value of C

and

will minimize turn-on pops at the expense of the desired

-3dB frequency.

Although the BYPASS pin current cannot be modified,

changing the size of C

alters the device’s turn-on time and

the magnitude of “clicks and pops”. Increasing the value of

reduces the magnitude of turn-on pops. However, this

presents a tradeoff: as the size of C

increases, the turn-on

time increases. There is a linear relationship between the

size of C

and the turn-on time. Here are some typical

turn-on times for various values of C

0.1µF

80ms

0.22µF

170ms

0.33µF

270ms

0.47µF

370ms

0.68µF

490ms

1.0µF

920ms

2.2µF

1.8sec

3.3µF

2.8sec

4.7µF

3.4sec

10µF

7.7sec

In order eliminate “clicks and pops”, all capacitors must be

discharged before turn-on. Rapidly switching V

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 C

. This capacitor usually has a

high value. C

discharges through internal 20k

Ω resistors.

Depending on the size of C

, the discharge time constant

can be relatively large. To reduce transients in single-ended

mode, an external 1k

Ω–5kΩ resistor can be placed in par-

allel with the internal 20k

Ω resistor. The tradeoff for using

this resistor is increased quiescent current.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70 mW

Load Impedance