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

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

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.

P

(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

a32

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:

P

θ

JA

(2)

For package MUA08A,

θ

the LM4809. Depending on the ambient temperature, T

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

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

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

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

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