LM4892M/NOPB - NATIONAL SEMICONDUCTOR

LM4892

LM4892 1 Watt Audio Power Amplifier with Headphone Sense

Literature Number: SNAS130D

LM4892 OBSOLETE

October 4, 2011

1 Watt Audio Power Amplifier with Headphone Sense

General Description

The LM4892 is an audio power amplifier primarily designed

for demanding applications in mobile phones and other

portable communication device applications. It is capable of

delivering 1 watt of continuous average power to an 8Ω BTL

load with less than 1% distortion (THD+N) from a 5VDC power

supply. Switching between bridged speaker mode and head-

phone (single-ended) mode is accomplished using the head-

phone sense pin.

Boomer audio power amplifiers are designed specifically to

provide high quality output power with a minimal amount of

external components. The LM4892 does not require output

coupling capacitors or bootstrap capacitors, and therefore is

ideally suited for mobile phone and other low voltage appli-

cations where minimal power consumption is a primary re-

quirement.

The LM4892 features a low-power consumption shutdown

mode, which is achieved by driving the shutdown pin with

logic low. Additionally, the LM4892 features an internal ther-

mal shutdown protection mechanism.

The LM4892 contains advanced pop & click circuitry which

eliminates noise which would otherwise occur during turn-on

and turn-off transitions.

The LM4892 is unity-gain stable and can be configured by

external gain-setting resistors.

Key Specifications

■ PSRR at 217Hz, VDD = 5V, 8Ω Load 62dB (typ)

■ Power Output at 5.0V & 1% THD 1.0W (typ)

■ Power Output at 3.3V & 1% THD 400mW (typ)

■ Shutdown Current 0.1µA (typ)

Features

■Available in space-saving packages: LLP, micro SMD,

MSOP, and SOIC

■Ultra low current shutdown mode

■BTL output can drive capacitive loads up to 500pF

■Improved pop & click circuitry eliminates noise during turn-

on and turn-off transitions

■2.2 - 5.5V operation

■No output coupling capacitors, snubber networks or

bootstrap capacitors required

■Thermal shutdown protection

■Unity-gain stable

■External gain configuration capability

■Headphone amplifier mode

Applications

■Mobile Phones

■PDAs

■Portable electronic devices

Typical Application

20012701

FIGURE 1. Typical Audio Amplifier Application Circuit (Pin #'s apply to M & MM packages)

Boomer® is a registered trademark of National Semiconductor Corporation.

200127 Version 5 Revision 4 Print Date/Time: 2011/10/04 15:35:47

LM4892 1 Watt Audio Power Amplifier with Headphone Sense

Connection Diagrams

8 Bump micro SMD

20012723

Top View

Order Number LM4892IBP, LM4892IBPX

See NS Package Number BPA08DDB

Small Outline (SO) Package

20012735

Top View

Order Number LM4892M

See NS Package Number M08A

Mini Small Outline (MSOP) Package

20012736

Top View

Order Number LM4892MM

See NS Package Number MUA08A

micro SMD Marking

20012770

Top View

X - Date Code

T - Die Traceability

G - Boomer Family

H - LM4892IBP

SO Marking

20012772

Top View

XY - Date Code

TT - Die Traceability

Bottom 2 lines - Part Number

MSOP Marking

20012771

Top View

G - Boomer Family

92 - LM4892MM

LLP Package

20012789

Top View

Order Number LM4892LD

See NS Package Number LDA10B

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LM4892

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) 2500V

ESD Susceptibility (Note 5) 250V

Junction Temperature 150°C

Thermal Resistance

θJC (SOP) 35°C/W

θJA (SOP) 150°C/W

θJA (micro SMD) 220°C/W

θJC (MSOP) 56°C/W

θJA (MSOP) 190°C/W

θJA (LLP) 220°C/W (Note 9)

Soldering Information

See AN-1112 "microSMD Wafers Level Chip Scale

Package".

See AN-1187 "Leadless Leadframe Package (LLP)".

Operating Ratings

Temperature Range

TMIN ≤ TA ≤ TMAX −40°C ≤ TA ≤ 85°C

Supply Voltage 2.2V ≤ VDD ≤ 5.5V

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

The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.

Symbol Parameter Conditions

LM4892 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IDD Quiescent Power Supply Current VIN = 0V, Io = 0A, HP sense = 0V 4 10 mA (max)

VIN = 0V, Io = 0A, HP sense = 5V 2.5 mA (max)

ISD Shutdown Current Vshutdown = GND (Note 8) 0.1 µA (max)

PoOutput Power

THD = 2% (max), f = 1kHz,

RL = 8Ω, HP Sense < 0.8V 1 W

THD = 1% (max), f = 1kHz,

RL = 32Ω, HP Sense > 4V 90 mW

VIH HP Sense high input voltage 4 V (min)

VIL HP Sense low input voltage 0.8 V (max)

THD+N Total Harmonic Distortion+Noise Po = 0.4 Wrms; f = 1kHz 10Hz ≤ BW

≤ 80kHz

0.1 %

PSSR Power Supply Rejection Ratio Vripple = 200mV sine p-p 62 (f = 217Hz)

66 (f = 1kHz)

Electrical Characteristics VDD = 3.3V (Note 1, Note 2)

The following specifications apply for VDD = 3.3V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.

Symbol Parameter Conditions

LM4892 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IDD Quiescent Power Supply Current VIN = 0V, Io = 0A, HP sense = 0V 3.5 mA (max)

VIN = 0V, Io = 0A, HP sense = 3.3V 2.0 mA (max)

ISD Shutdown Current Vshutdown = GND (Note 8) 0.1 µA (max)

PoOutput Power

THD = 1% (max), f = 1kHz,

RL = 8Ω, HP Sense < 0.8V 0.4 W

THD = 1% (max), f = 1kHz,

RL = 32Ω, HP Sense > 3V 35 mW

VIH HP Sense high input voltage 2.6 V (min)

VIL HP Sense low input voltage 0.8 V (max)

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LM4892

Symbol Parameter Conditions

LM4892 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

THD+N Total Harmonic Distortion+Noise Po = 0.15 Wrms; f = 1kHz 10Hz ≤ BW

≤ 80kHz

0.1 %

PSSR Power Supply Rejection Ratio Vripple = 200mV sine p-p 60(f = 217Hz)

62 (f = 1kHz)

Electrical Characteristics VDD = 2.6V (Note 1, Note 2)

The following specifications apply for VDD = 2.6V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.

Symbol Parameter Conditions

LM4892 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IDD Quiescent Power Supply Current VIN = 0V, Io = 0A, HP sense = 0V 2.6 mA (max)

VIN = 0V, Io = 0A, HP sense = 2.6V 1.5 mA (max)

ISD Shutdown Current Vshutdown = GND (Note 8)0.1 µA (max)

PoOutput Power

THD = 1% (max), f = 1kHz,

RL = 8Ω, HP Sense < 0.8V 0.25 W

THD = 1% (max), f = 1kHz,

RL = 4Ω, HP Sense < 0.8V 0.28 W

THD = 1% (max), f = 1kHz, RL =

32Ω, HP Sense > 2.5V 20 mW

VIH HP Sense high input voltage 2.0 V (min)

VIL HP Sense low input voltage 0.8 V (max)

THD+N Total Harmonic Distortion+Noise Po = 0.1 Wrms; f = 1kHz 10Hz ≤ BW

≤ 80kHz

0.1 %

PSSR Power Supply Rejection Ratio Vripple = 200mV sine p-p 44(f = 217Hz)

44 (f = 1kHz)

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 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 LM4892, see power derating

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

Note 8: For micro SMD only, shutdown current is measured in a Normal Room Environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA.

Note 9: The Exposed-DAP of the LDA10B package should be electrically connected to GND or an electrically isolated copper area. The LM4892LD demo board

(views featured in the Application Information section) has the Exposed-DAP connected to GND with a PCB area of 353mils x 86.7mils (8.97mm x 2.20mm)

on the copper top layer and 714.7mils x 368mils (18.15mm x 9.35mm) on the copper bottom layer.

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LM4892

External Components Description

(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/(2π RiCi).

2. CiInput coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a highpass filter

with Ri at fc = 1/(2π RiCi). 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 External

Components, for information concerning proper placement and selection of CB.

6. COUT This output coupling capacitor blocks DC voltage while coupling the AC audio signal to the headphone speaker.

Combined with RL, the headphone impedance, it creates a high pass filter at fc = 1/(2πRLCOUT). Refer to the section,

Proper Selection of External Components for an explanation of how to determine the value of COUT.

7. RPU This is the pull up resistor to activate headphone operation when a headphone plug is plugged into the headphone

jack.

8. RSThis is the current limiting resistor for the headphone input pin.

9. RPD This is the pull down resistor to de-activate headphone operation when no headphone is plugged into the headphone

jack.

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LM4892

Typical Performance Characteristics

THD+N vs Frequency

at VDD = 5V, 8Ω RL, and PWR = 250mW

20012737

THD+N vs Frequency

at VDD = 3.3V, 8Ω RL, and PWR = 150mW

20012738

THD+N vs Frequency

at VDD = 2.6V, 8Ω RL, and PWR = 100mW

20012739

THD+N vs Frequency

at VDD = 2.6V, 4Ω RL, and PWR = 100mW

20012740

THD+N vs Power Out

at VDD = 5V, 8Ω RL, 1kHz

20012784

THD+N vs Power Out

at VDD = 3.3V, 8Ω RL, 1kHz

20012742

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LM4892

THD+N vs Power Out

at VDD = 2.6V, 8Ω RL, 1kHz

20012785

THD+N vs Power Out

at VDD = 2.6V, 4Ω RL, 1kHz

20012786

Power Supply Rejection Ratio (PSRR) vs Frequency

at VDD = 5V, 8Ω RL

20012745

Input terminated with 10Ω R

Power Supply Rejection Ratio (PSRR) vs Frequency

at VDD = 5V, 8Ω RL

20012773

Input Floating

Power Supply Rejection Ratio (PSRR) vs Frequency

at VDD = 2.6V, 8Ω RL

20012747

Input terminated with 10Ω R

Power Supply Rejection Ratio (PSRR) vs Frequency

at VDD = 3.3V, 8Ω RL

20012746

Input terminated with 10Ω R

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LM4892

Power Dissipation vs

Output Power

VDD = 3.3V

20012749

Power Dissipation vs

Output Power

VDD = 5V

20012748

Output Power vs

Load Resistance

20012751

Power Dissipation vs

Output Power

VDD = 2.6V

20012750

Supply Current vs

Shutdown Voltage

20012753

Clipping (Dropout) Voltage vs

Supply Voltage

20012752

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LM4892

Open Loop Frequency Response

VDD = 5V No Load

20012787

Open Loop Frequency Response

VDD = 3V No Load

20012782

Power Derating Curves

20012788

Power Derating Curves vs

for 8 Bump microSMD

20012783

Frequency Response vs

Input Capacitor Size

20012754

Noise Floor

20012756

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LM4892

THD+N vs Frequency

at VDD = 5V, RL = 32Ω, PWR = 70mW, Headphone mode

20012776

THD+N vs Power Out

at VDD = 5V, RL = 32Ω, 1kHz, Headphone mode

20012777

Output Power vs Supply Voltage

RL = 8Ω

20012778

Output Power vs Supply Voltage

RL = 16Ω

20012779

Output Power vs Supply Voltage

RL = 32Ω

20012780

Output Power vs Supply Voltage

Headphone Output, RL = 32Ω

20012781

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LM4892

Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4892 has two operational am-

plifiers internally, allowing for a few different amplifier config-

urations. The first amplifier's gain is externally configurable,

while the second amplifier is internally fixed in a unity-gain,

inverting configuration. The closed-loop gain of the first am-

plifier is set by selecting the ratio of Rf to Ri while the second

amplifier's gain is fixed by the two internal 20kΩ resistors.

Figure 1 shows that the output of amplifier one serves as the

input to amplifier two which results in both amplifiers produc-

ing signals identical in magnitude, but out of phase by 180°.

Consequently, the differential gain for the IC is

AVD= 2 *(Rf/Ri)

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 ampli-

fier is not current limited or clipped. In order to choose an

amplifier's closed-loop gain without causing excessive clip-

ping, please refer to the Audio Power Amplifier Design

section.

A bridge configuration, such as the one used in LM4892, 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 elimi-

nates the need for an output coupling capacitor which is

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

tion. 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 suc-

cessful 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 LM4892 has two opera-

tional 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

PDMAX = 4*(VDD)2/(2π2RL) (1)

It is critical that the maximum junction temperature TJMAX of

150°C is not exceeded. TJMAX can be determined from the

power derating curves by using PDMAX and the PC board foil

area. By adding additional copper foil, the thermal resistance

of the application can be reduced from a free air value of 150°

C/W, resulting in higher PDMAX. Additional copper foil can be

added to any of the leads connected to the LM4892. It is es-

pecially effective when connected to VDD, GND, and the

output pins. Refer to the application information on the

LM4892 reference design board for an example of good heat

sinking. If TJMAX still exceeds 150°C, then additional changes

must be made. These changes can include reduced supply

voltage, higher load impedance, or reduced ambient temper-

ature. Internal power dissipation is a function of output power.

Refer to the Typical Performance Characteristics 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 both the bypass and power supply pins

should be as close to the device as possible. Typical appli-

cations employ a 5V regulator with 10µF tantalum or elec-

trolytic capacitor and a ceramic bypass capacitor which aid in

supply stability. This does not eliminate the need for bypass-

ing the supply nodes of the LM4892. The selection of a bypass

capacitor, especially CB, is dependent upon PSRR require-

ments, click and pop performance (as explained in the sec-

tion, Proper Selection of External Components), system

cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4892 contains a shutdown pin to externally turn off the

amplifier's bias circuitry. This shutdown feature turns the am-

plifier off when a logic low is placed on the shutdown pin. By

switching the shutdown pin to ground, the LM4892 supply

current draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages less than

0.5VDC, the idle current may be greater than the typical value

of 0.1µA. (Idle current is measured with the shutdown pin

grounded).

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch in conjunction with an

external pull-up resistor. When the switch is closed, the shut-

down pin is connected to ground and disables the amplifier.

If the switch is open, then the external pull-up resistor will en-

able the LM4892. This scheme guarantees that the shutdown

pin will not float thus preventing unwanted state changes.

Table 1. Logic Level Truth Table for Shutdown and HP

Sense Operation

Shutdown HP Sense

Pin Operational Mode

Logic High Logic Low Bridged Amplifier

Logic High Logic High Single-Ended Amplifier

Logic Low Logic Low Micro-Power Shutdown

Logic Low Logic High Micro-Power Shutdown

HP SENSE FUNCTION

Applying a voltage between 4V and VCC to the LM4892's HP-

Sense headphone control pin turns off Amp2 and mutes a

bridged-connected load. Quiescent current consumption is

reduced when the IC is in the single-ended mode.

Figure 2 shows the implementation of the LM4892's head-

phone control function. With no headphones connected to the

headphone jack, the R4-R6 voltage divider sets the voltage

applied to the HP-Sense pin (pin3) at approximately 50mV.

This 50mV enables the LM4892 and places it in bridged mode

operation.

While the LM4892 operates in bridged mode, the DC potential

across the load is essentially 0V. Since the HP-Sense thresh-

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LM4892

old is set at 4V, even in an ideal situation, the output swing

can not cause a false single-ended trigger. Connecting head-

phones to the headphone jack disconnects the headphone

jack contact pin from V01 and allows R4 to pull the HP Sense

pin up to VCC. This enables the headphone function, turns off

Amp2, and mutes the bridged speaker. The amplifier then

drives the headphone whose impedance is in parallel with

R6. Resistor R6 has negligible effect on output drive capability

since the typical impedance of headphones is 32Ω. The out-

put coupling capacitor blocks the amplifier's half supply DC

voltage, protecting the headphones.

A microprocessor or a switch can replace the headphone jack

contact pin. When a microprocessor or switch applies a volt-

age greater than 4V to the HP Sense pin, a bridged-connect-

ed speaker is muted and Amp1 drives the headphones.

20012774

FIGURE 2. Headphone Circuit (Pin #'s apply to M & MM

packages)

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 LM4892 is tolerant of external

component combinations, consideration to component values

must be used to maximize overall system quality.

The LM4892 is unity-gain stable which gives the designer

maximum system flexibility. The LM4892 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 1 Vrms are available from

sources such as audio codecs. Please refer to the section,

Audio Power Amplifier Design, for a more complete expla-

nation 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 Figure 1. The input coupling capacitor, Ci, forms a first order

high pass filter which limits low frequency response. This val-

ue should be 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 attenua-

tion. But in many cases the speakers used in portable sys-

tems, 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, Ci. A

larger input coupling capacitor requires more charge to reach

its quiescent DC voltage (nominally 1/2 VDD). 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 consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, CB, is the most critical component to minimize turn-

on pops since it determines how fast the LM4892 turns on.

The slower the LM4892's outputs ramp to their quiescent DC

voltage (nominally 1/2 VDD), the smaller the turn-on pop.

Choosing CB equal to 1.0µF along with a small value of Ci (in

the range of 0.1µF to 0.39µF), should produce a virtually

clickless and popless shutdown function. While the device will

function properly, (no oscillations or motorboating), with CB

equal to 0.1µF, the device will be much more susceptible to

turn-on clicks and pops. Thus, a value of CB equal to 1.0µF is

recommended in all but the most cost sensitive designs.

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 ± 0.25 dB

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. A second way to determine the minimum supply

rail is to calculate the required Vopeak using Equation 2 and

add the output voltage. Using this method, the minimum sup-

ply voltage would be (Vopeak + (VODTOP + VODBOT)), where

VODBOT and VODTOP are extrapolated from the Dropout Voltage

vs Supply Voltage curve in the Typical Performance Char-

acteristics section.

(2)

5V is a standard voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4892 to reproduce peaks in excess of 1W with-

out 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 differential gain can be determined from Equa-

tion 3.

(3)

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LM4892

Rf/Ri = AVD/2

From Equation 3, the minimum AVD is 2.83; use AVD = 3.

Since the desired input impedance was 20kΩ, and with a

AVD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of

Ri = 20kΩ and Rf = 30kΩ. 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 create a highpass filter.

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

The high frequency pole is determined by the product of the

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

a AVD = 3 and fH = 100kHz, the resulting GBWP = 150kHz

which is much smaller than the LM4892 GBWP of 4 MHz.

This figure displays that if a designer has a need to design an

amplifier with a higher differential gain, the LM4892 can still

be used without running into bandwidth limitations.

20012724

FIGURE 3. Higher Gain Audio Amplifier

The LM4892 is unity-gain stable and requires no external

components besides gain-setting resistors, an input coupling

capacitor, and proper supply bypassing in the typical appli-

cation. However, if a closed-loop differential gain of greater

than 10 is required, a feedback capacitor (Cf) may be needed

as shown in Figure 3 to bandwidth limit the amplifier. This

feedback capacitor creates a low pass filter that eliminates

possible high frequency oscillations. Care should be taken

when calculating the -3dB frequency in that an incorrect com-

bination of Rf and Cf will cause rolloff before 20kHz. A typical

combination of feedback resistor and capacitor that will not

produce audio band high frequency rolloff is Rf = 20kΩ and

Cf = 25pF. These components result in a -3dB point of ap-

proximately 320 kHz.

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LM4892

20012775

FIGURE 4. Reference Design Schematic For Demo Boards

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LM4892

LM4892 micro SMD BOARD ARTWORK

Silk Screen

20012757

Top Layer

20012758

Bottom Layer

20012759

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LM4892

LM4892 SO DEMO BOARD ARTWORK

Silk Screen

20012762

Top Layer

20012763

Bottom Layer

20012764

LM4892 MSOP DEMO BOARD ARTWORK

Silk Screen

20012765

Top Layer

20012766

Bottom Layer

20012767

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LM4892

LM4892 LLP DEMO BOARD ARTWORK

Composite View

20012790

Silk Screen

20012791

Top Layer

20012792

Bottom Layer

20012793

17 www.national.com

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LM4892

Mono LM4892 Reference Design Boards

Bill of Material for all Demo Boards

Part Description Qty Ref Designator

LM4892 Audio Amplifier 1 U1

Tantalum Capacitor, 1µF 2 Cs, Cb

Ceramic Capacitor, 0.39µF 1 Ci

Capacitor, 100µF 1 Cout

Resistor, 1kΩ, 1/10W 1 Rpd

Resistor, 20kΩ, 1/10W 3 Ri, Rf, Rpu2

Resistor, 100kΩ, 1/10W 2 Rpu1, Rs

Jumper Header Vertical Mount 2X1, 0.100"

spacing

1 J1

3.5mm Audio Jack (PC mount, w/o nut),

PN# SJS-0357-B Shogyo International

Corp. (www.shogyo.com)

1 J2

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 (bring-

ing 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 com-

ponents and circuit traces.

Avoiding Typical Design / Layout Problems

Avoid ground loops or running digital and analog traces par-

allel 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 min-

imize capacitive noise coupling and cross talk.

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LM4892

Physical Dimensions inches (millimeters) unless otherwise noted

Note: Unless otherwise specified.

1. Epoxy coating.

2. 63Sn/37Pb eutectic bump.

3. Recommend non-solder mask defined landing pad.

4. Pin 1 is established by lower left corner with respect to text orientation pins are numbered counterclockwise.

5. Reference JEDEC registration MO-211, variation BC.

8-Bump micro SMD

Order Number LM4892IBP, LM4892IBPX

NS Package Number BPA08DDB

X1 = 1.361±0.03 X2 = 1.361±0.03 X3 = 0.850±0.10

19 www.national.com

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LM4892

MSOP

Order Number LM4892MM

NS Package Number MUA08A

Order Number LM4892M

NS Package Number M08A

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LM4892

LLP

Order Number LM4892LD

NS Package Number LDA10B

21 www.national.com

200127 Version 5 Revision 4 Print Date/Time: 2011/10/04 15:35:47

LM4892

Notes

LM4892 1 Watt Audio Power Amplifier with Headphone Sense

For more National Semiconductor product information and proven design tools, visit the following Web sites at:

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