LM4910MM/NOPB - NATIONAL SEMICONDUCTOR

LM4910

LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier

Literature Number: SNAS151G

January 2007

LM4910

Output Capacitor-less Stereo 35mW Headphone Amplifier

General Description

The LM4910 is an audio power amplifier primarily designed

for headphone applications in portable device applications. It

is capable of delivering 35mW of continuous average power

to a 32Ω load with less than 1% distortion (THD+N) from a

3.3VDC power supply.

The LM4910 utilizes a new circuit topology that eliminates

output coupling capacitors and half-supply bypass capacitors.

The LM4910 contains advanced pop & click circuitry which

eliminates noises caused by transients that would otherwise

occur during turn-on and turn-off.

Boomer audio power amplifiers were designed specifically to

provide high quality output power with a minimal amount of

external components. Since the LM4910 does not require any

output coupling capacitors, half-supply bypass capacitors, or

bootstrap capacitors, it is ideally suited for low-power portable

applications where minimal space and power consumption

are primary requirements.

The LM4910 features a low-power consumption shutdown

mode, activated by driving the shutdown pin with logic low.

Additionally, the LM4910 features an internal thermal shut-

down protection mechanism. The LM4910 is also unity-gain

stable and can be configured by external gain-setting resis-

tors.

Key Specifications

■PSRR at f = 217Hz 65dB (typ)

■Power Output at VDD = 3.3V, RL = 32Ω, and

THD ≤ 1%

35mW

(typ)

■Shutdown Current 0.1µA (typ)

Features

■Eliminates headphone amplifier output coupling

capacitors

■Eliminates half-supply bypass capacitor

■Advanced pop & click circuitry eliminates noises during

turn-on and turn-off

■Ultra-low current shutdown mode

■Unity-gain stable

■2.2V - 5.5V operation

■Available in space-saving MSOP, LLP, and SOIC

packages

Applications

■Mobile Phones

■PDAs

■Portable electronics devices

■Portable MP3 players

Typical Application

20030565

FIGURE 1. Typical Audio Amplifier Application Circuit

Boomer® is a registered trademark of National Semiconductor Corporation.

LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier

Connection Diagrams

MSOP/SO Package

20030502

Top View

Order Number LM4910MM or LM4910MA

See NS Package Number MUA08A or M08A

MSOP Marking

20030566

Top View

G - Boomer Family

C2 - LM4910MM

SO Marking

20030567

Top View

TT - Die Traceability

Bottom 2 lines - Part Number

LLP Package

20030595

Top View

Order Number LM4910LQ

See NS package Number LQB08A

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LM4910

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 9) 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 Pin 6 (Note 10) 10kV

ESD Susceptibility (Note 4) 2000V

ESD Susceptibility (Note 5) 200V

Junction Temperature 150°C

Thermal Resistance

θJC (MSOP) 56°C/W

θJA (MSOP) 190°C/W

θJC (SOP) 35°C/W

θJA (SOP) 150°C/W

θJC (LQ) 57°C/W

θJA (LQ) 140°C/W

Operating Ratings

Temperature Range

TMIN ≤ TA ≤ TMAX −40°C ≤ T A ≤ 85°C

Supply Voltage (VDD) 2.2V ≤ VCC ≤ 5.5V

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

The following specifications apply for VDD = 3.3V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25°C.

Symbol Parameter Conditions LM4910 Units

(Limits)

Typ

(Note 6)

Limit

(Notes 7,

IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.5 6 mA (max)

ISD Standby Current VSHUTDOWN = GND 0.1 1.0 µA (max)

VOS Output Offset Voltage 5 30 mV (max)

POOutput Power THD = 1% (max); f = 1kHz 35 30 mW (min)

THD+N Total Harmonic Distortion + Noise PO = 30mWrms; f = 1kHz 0.3 %

PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p sinewave

Input terminated with 10Ω to ground

65 (f =

217Hz)

65 (f =

1kHz)

VIH Shutdown Input Voltage High 1.5 V (min)

VIL Shutdown Input Voltage Low 0.4 V (max)

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

The following specifications apply for VDD = 3V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25°C.

Symbol Parameter Conditions LM4910 Units

(Limits)

Typ

(Note 6)

Limit

(Notes 7,

IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.3 6 mA (max)

ISD Standby Current VSHUTDOWN = GND 0.1 1.0 µA (max)

VOS Output Offset Voltage 5 30 mV (max)

POOutput Power THD = 1% (max); f = 1kHz 30 25 mW (min)

THD+N Total Harmonic Distortion + Noise PO = 25mWrms; f = 1kHz 0.3 %

PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p sinewave

Input terminated with 10Ω to ground

65 (f = 217

Hz)

65 (f =

1kHz)

VIH Shutdown Input Voltage High 1.5 V (min)

VIL Shutdown Input Voltage Low 0.4 V (max)

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LM4910

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

The following specifications apply for VDD = 2.6V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25°C.

Symbol Parameter Conditions LM4910 Units

(Limits)

Typ

(Note 6)

Limit

(Notes 7,

IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.0 mA (max)

ISD Standby Current VSHUTDOWN = GND 0.1 µA (max)

VOS Output Offset Voltage 5 mV (max)

POOutput Power THD = 1% (max); f = 1kHz 13 mW

THD+N Total Harmonic Distortion + Noise PO = 10mWrms; f = 1kHz 0.3 %

PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p sinewave

Input terminated with 10Ω to ground

55 (f =

217Hz)

55 (f =

1kHz)

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

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

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

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

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

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

allowable power dissipation is PDMAX = (TJMAX - TA)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4910, see power

derating currents for more information.

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

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

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

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

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

Note 9: If the product is in shutdown mode and VDD exceeds 6V (to a max of 8V VDD) then most of the excess current will flow through the ESD protection circuits.

If the source impedance limits the current to a max of 10ma then the part will be protected. If the part is enabled when VDD is above 6V circuit performance will

be curtailed or the part may be permanently damaged.

Note 10: Human body model, 100pF discharged through a 1.5kΩ resistor, Pin 6 to ground.

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/(2πRiCi).

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

filter with Ri at fc = 1/(2πRiCi). Refer to the section Proper Selection of External Components, for an explanation

of how to determine the value of Ci.

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

4. CSSupply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for

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

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LM4910

Typical Performance Characteristics

THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Frequency

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LM4910

THD+N vs Output Power

20030516

THD+N vs Output Power

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THD+N vs Output Power

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THD+N vs Output Power

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THD+N vs Output Power

20030519

THD+N vs Output Power

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LM4910

Output Power vs

Load Resistance

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Output Power vs

Load Resistance

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Output Power vs

Load Resistance

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Output Power vs

Supply Voltage

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Output Power vs

Supply Voltage

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Power Dissipation vs

Output Power

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LM4910

Power Dissipation vs

Output Power

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Power Dissipation vs

Output Power

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Channel Separation

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Power Supply Rejection Ratio

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Power Supply Rejection Ratio

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Power Supply Rejection Ratio

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LM4910

Open Loop Frequency Response

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Noise Floor

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Frequency Response vs

Input Capacitor Size

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Supply Current vs

Supply Voltage

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LM4910

Application Information

ELIMINATING OUTPUT COUPLING CAPACITORS

Typical single-supply audio amplifiers that drive single-ended

(SE) headphones use a coupling capacitor on each SE out-

put. This output coupling capacitor blocks the half-supply

voltage to which the output amplifiers are typically biased and

couples the audio signal to the headphones. The signal return

to circuit ground is through the headphone jack's sleeve.

The LM4910 eliminates these output coupling capacitors.

Amp3 is internally configured to apply a bandgap referenced

voltage (VREF = 1.58V) to a stereo headphone jack's sleeve.

This voltage matches the quiescent voltage present on the

Amp1 and Amp2 outputs that drive the headphones. The

headphones operate in a manner similar to a bridge-tied-load

(BTL). The same DC voltage is applied to both headphone

speaker terminals. This results in no net DC current flow

through the speaker. AC current flows through a headphone

speaker as an audio signal's output amplitude increases on

the speaker's terminal.

The headphone jack's sleeve is not connected to circuit

ground. Using the headphone output jack as a line-level out-

put will place the LM4910's bandgap referenced voltage on a

plug's sleeve connection. This presents no difficulty when the

external equipment uses capacitively coupled inputs. For the

very small minority of equipment that is DC-coupled, the

LM4910 monitors the current supplied by the amplifier that

drives the headphone jack's sleeve. If this current exceeds

500mAPK, the amplifier is shutdown, protecting the LM4910

and the external equipment.

ELIMINATING THE HALF-SUPPLY BYPASS CAPACITOR

Typical single-supply audio amplifers are normally biased to

1/2VDD in order to maximize the output swing of the audio

signal. This is usually achieved with a simple resistor divider

network from VDD to ground that provides the proper bias

voltage to the amplifier. However, this scheme requires the

use of a half-supply bypass capacitor to improve the bias

voltage's stability and the amplifier's PSRR performance.

The LM4910 utilizes an internally generated, buffered

bandgap reference voltage as the amplifier's bias voltage.

This bandgap reference voltage is not a direct function of

VDD and therefore is less susceptible to noise or ripple on the

power supply line. This allows for the LM4910 to have a stable

bias voltage and excellent PSRR performance even without

a half-supply bypass capacitor.

OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED

The LM4910 contains advanced circuitry that virtually elimi-

nates output transients ('clicks and pops'). This circuitry pre-

vents all traces of transients when the supply voltage is first

applied or when the part resumes operation after coming out

of shutdown mode. The LM4910 remains in a muted condition

until there is sufficient input signal magnitude (>5mVRMS, typ)

to mask any remaining transient that may occur. Figure 2

shows the LM4910's lack of transients in the differential signal

(Trace B) across a 320 load. The LM4910's active-low SHUT-

DOWN pin is driven by the logic signal shown in Trace A.

Trace C is the VO1 output signal and Trace D is the VO3 output

signal.

To ensure optimal click and pop performance under low gain

configurations (less than 0dB), it is critical to minimize the RC

combination of the feedback resistor RF and stray input ca-

pacitance at the amplifier inputs. A more reliable way to lower

gain or reduce power delivered to the load is to place a current

limiting resistor in series with the load as explained in the

Minimizing Output Noise / Reducing Output Power sec-

tion.

20030592

FIGURE 2.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4910 has three operational am-

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

figurable gain while the other amplifier is internally fixed at the

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

of the two configurable amplifiers is set by selecting the ratio

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

AV = -(Rf/Ri)

By driving the loads through outputs VO1 and VO2 with VO3

acting as a buffered bias voltage the LM4910 does not require

output coupling capacitors. The typical single-ended amplifier

configuration where one side of the load is connected to

ground requires large, expensive output coupling capacitors.

A configuration such as the one used in the LM4910 has a

major advantage over single supply, single-ended amplifiers.

Since the outputs VO1, VO2, and VO3 are all biased at VREF =

1.58V, no net DC voltage exists across each load. This elim-

inates the need for output coupling capacitors that are re-

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

tion. Without output coupling capacitors in a typical single-

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

across the load resulting in both increased internal IC power

dissipation and possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifier. A direct consequence of the increased pow-

er delivered to the load by a bridge amplifier is an increase in

internal power dissipation. The maximum power dissipation

for a given application can be derived from the power dissi-

pation graphs or from Equation 1.

PDMAX = 4(VDD) 2 / (π2RL) (1)

It is critical that the maximum junction temperature TJMAX of

150°C is not exceeded. Since the typical application is for

headphone operation (32Ω impedance) using a 3.3V supply

the maximum power dissipation is only 138mW. Therefore,

power dissipation is not a major concern.

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LM4910

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be as

close to the device as possible.

Typical applications employ a 3.3V regulator with 10µF tan-

talum 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 LM4910. A bypass ca-

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

for CS.

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4910's shutdown function. Activate micro-power shutdown

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

active, the LM4910's micro-power shutdown feature turns off

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

trigger point is 0.4V(max) for a logic-low level, and 1.5V(min)

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 mi-

croprocessor, or a microcontroller. When using a switch,

connect an external 100kΩ pull-up resistor between the

SHUTDOWN pin and VDD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by opening the switch. Closing the switch connects the

SHUTDOWN pin to ground, activating micro-power shut-

down. The switch and resistor guarantee that the SHUT-

DOWN pin will not float. This prevents unwanted state

changes. In a system with a microprocessor or microcon-

troller, 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.

SELECTING EXTERNAL COMPONENTS

Selecting proper external components in applications using

integrated power amplifiers is critical to optimize device and

system performance. While the LM4910 is tolerant of external

component combinations, consideration to component values

must be used to maximize overall system quality.

The LM4910 is unity-gain stable which gives the designer

maximum system flexibility. The LM4910 should be used in

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

imize the signal to noise ratio. Low gain configurations require

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

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

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

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

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

Amplifier Design, for a more complete explanation of proper

gain selection

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

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

width is dictated by the choice of external components shown

in 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

and turn-on time.

SELECTION OF INPUT CAPACITOR SIZE

Amplifiying the lowest audio frequencies requires a high value

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

expensive and may compromise space efficiency in portable

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

portable systems have little ability to reproduce signals below

60Hz. Applications using headphones with this limited fre-

quency response reap little improvement by using a high

value input capacitor.

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

by the size of the input coupling capacitor Ci. A larger input

coupling capacitor requires more charge to reach its quies-

cent DC voltage. This charge comes from the output via the

feedback Thus, by minimizing the capacitor size based on

necessary low frequency response, turn-on time can be min-

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

is recommended.

USING EXTERNAL POWERED SPEAKERS

The LM4910 is designed specifically for headphone opera-

tion. Often the headphone output of a device will be used to

drive external powered speakers. The LM4910 has a differ-

ential output to eliminate the output coupling capacitors. The

result is a headphone jack sleeve that is connected to VO3

instead of GND. For powered speakers that are designed to

have single-ended signals at the input, the click and pop cir-

cuitry will not be able to eliminate the turn-on/turn-off click and

pop. Unless the inputs to the powered speakers are fully dif-

ferential the turn-on/turn-off click and pop will be very large.

AUDIO POWER AMPLIFIER DESIGN

A 30mW/32Ω Audio Amplifier

Given:

Power Output 30mWrms

Load Impedance 32Ω

Input Level 1Vrms

Input Impedance 20kΩ

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found.

Since 3.3V is a standard supply voltage in most applications,

it is chosen for the supply rail in this example. Extra supply

voltage creates headroom that allows the LM4910 to repro-

duce peaks in excess of 30mW without producing audible

distortion. At this time, the designer must make sure that the

power supply choice along with the output impedance does

no violate the conditions explained in the Power Dissipa-

tion section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 2.

(2)

From Equation 2, the minimum AV is 0.98; use AV = 1. Since

the desired input impedance is 20kΩ, and with AV equal to 1,

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

are chosen with Ri = 20kΩ and Rf = 20kΩ.

The last step in this design example is setting the amplifier's

−3dB frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency re-

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LM4910

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

both response limits is 0.17dB, well within the ±0.25dB de-

sired limit. The results are an

fL = 100Hz/5 = 20Hz (3)

and an

fH = 20kHz x 5 = 100kHz (4)

As mentioned in the Selecting Proper External Compo-

nents section, Ri and Ci create a highpass filter that sets the

amplifier's lower bandpass frequency limit. Find the coupling

capacitor's value using Equation (3).

Ci≥ 1/(2πR ifL) (5)

The result is

1/(2π*20kΩ*20Hz) = 0.397µF

Use a 0.39µF capacitor, the closest standard value.

The high frequency pole is determined by the product of the

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

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

which is much smaller than the LM4910 GBWP of 11MHz.

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

amplifier with higher differential gain, the LM4910 can still be

used without running into bandwidth limitations.

MINIMIZING OUTPUT NOISE / REDUCING OUTPUT POWER

20030568

FIGURE 3.

Output noise delivered to the load can be minimized with the

use of an external resistor, RSERIES, placed in series with each

load as shown in Figure 3. RSERIES forms a voltage divider

with the impedance of the headphone driver RL. As a result,

output noise is attenuated by the factor RL / (RL + RSERIES).

Figure 4 illustrates the relationship between output noise and

RSERIES for different loads. RSERIES also decreases output

power delivered to the load by the factor RL / (RL + RSERIES)

2. However, this may not pose a problem since most head-

phone applications require less than 10mW of output power.

Figure 5 illustrates output power (@1% THD+N) vs RSERIES

for different loads.

Figure 4 shows an optional resistor connected between the

amplifier output that drives the headphone jack sleeve and

ground. This resistor provides a ground path that supressed

power supply hum. This hum may occur in applications such

as notebook computers in a shutdown condition and con-

nected to an external powered speaker. The resistor's 100Ω

value is a suggested starting point. Its final value must be de-

termined based on the tradeoff between the amount of noise

suppression that may be needed and minimizing the addi-

tional current drawn by the resistor (25mA for a 100Ω resistor

and a 5V supply).

ESD PROTECTION

As stated in the Absolute Maximum Ratings, pin 6 (Vo3) on

the LM4910 has a maximum ESD susceptibility rating of

10kV. For higher ESD voltages, the addition of a PCDN042

dual transil (from California Micro Devices), as shown in Fig-

ure 4, will provide additional protection.

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LM4910

20030594

FIGURE 4. The PCDN042 provides additional ESD protection beyond the 10kV shown in the Absolute Maximum Ratings

for the Vo3 output

Output Noise vs RSERIES

20030590

FIGURE 5.

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LM4910

Output Power vs RSERIES

20030591

FIGURE 6.

HIGHER GAIN AUDIO AMPLIFIER

20030593

FIGURE 7.

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LM4910

The LM4910 is unity-gain stable and requires no external

components besides gain-setting resistors, input coupling ca-

pacitors, and proper supply bypassing in the typical applica-

tion. However, if a very large closed-loop differential gain is

required, a feedback capacitor (Cf) may be needed as shown

in Figure 6 to bandwidth limit the amplifier. This feedback ca-

pacitor creates a low pass filter that eliminates possible high

frequency oscillations. Care should be taken when calculating

the -3dB frequency in that an incorrect combination of Rf and

Cf will cause frequency response roll off before 20kHz. A typ-

ical combination of feedback resistor and capacitor that will

not produce audio band high frequency roll off is Rf = 20kΩ

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

approximately 320kHz.

REFERENCE DESIGN BOARD and LAYOUT GUIDELINES

MSOP & SO BOARDS

20030569

FIGURE 8.

(Note: RPU2 is not required. It is used for test measurement purposes only.)

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LM4910

LM4910 SO DEMO BOARD ARTWORK

Composite View

20030570

Silk Screen

20030571

Top Layer

20030572

Bottom Layer

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LM4910

LM4910 MSOP DEMO BOARD ARTWORK

Composite View

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Silk Screen

20030575

Top Layer

20030576

Bottom Layer

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LM4910

LM4910 LLP DEMO BOARD ARTWORK

Composite View

20030597

Silk Screen

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Top Layer

20030599

Bottom Layer

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LM4910

LM4910 Reference Design Boards

Bill of Materials

Part Description Qty Ref Designator

LM4910 Mono Reference Design Board 1

LM4910 Audio AMP 1 U1

Tantalum Cap 1µF 16V 10 1 Cs

Ceramic Cap 0.39µF 50V Z50 20 2 Ci

Resistor 20kΩ 1/10W 5 4 Ri, Rf

Resistor 100kΩ 1/10W 5 1 Rpu

Jumper Header Vertical Mount 2X1, 0.100 1 J1

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.

Minimization of THD

PCB trace impedance on the power, ground, and all output

traces should be minimized to achieve optimal THD perfor-

mance. Therefore, use PCB traces that are as wide as pos-

sible for these connections. As the gain of the amplifier is

increased, the trace impedance will have an ever increasing

adverse affect on THD performance. At unity-gain (0dB) the

parasitic trace impedance effect on THD performance is re-

duced but still a negative factor in the THD performance of

the LM4910 in a given application.

GENERAL MIXED SIGNAL LAYOUT RECOMMENDATION

Power and Ground Circuits

For two 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 greatly en-

hance 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 may 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 ana-

log and digital sections. Further, place 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.

Revision History

Rev Date Description

1.0 7/12/05 Released to the WEB.

1.1 01/16/07 Deleted the phrase “patent pending” on

page 1.

19 www.national.com

LM4910

Physical Dimensions inches (millimeters) unless otherwise noted

MSOP

Order Number LM4910MM

NS Package Number MUA08A

Order Number LM4910MA

NS Package Number M08A

www.national.com 20

LM4910

Order Number LM4910LQ

NS Package Number LQB08A

21 www.national.com

LM4910

Notes

LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier

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