LM4838 datasheet(16/36 Pages) NSC

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Application Information (Continued)

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful single-ended or bridged amplifier. Equation (2)

states the maximum power dissipation point for a single-

ended amplifier operating at a given supply voltage and

driving a specified output load.

DMAX =(VDD)

2/(2

π2R

Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

power dissipation for the same conditions.

The LM4838 has two operational amplifiers per channel. The

maximum internal power dissipation per channel operating in

the bridge mode is four times that of a single-ended ampli-

fier. From Equation (3), assuming a 5V power supply and a

Ω load, the maximum single channel power dissipation is

1.27W or 2.54W for stereo operation.

DMAX =4 * (VDD)

2/(2

π2R

Bridge Mode

(3)

The LM4838’s power dissipation is twice that given by Equa-

tion (2) or Equation (3) when operating in the single-ended

mode or bridge mode, respectively. Twice the maximum

power dissipation point given by Equation (3) must not ex-

ceed the power dissipation given by Equation (4):

DMAX'=(TJMAX −TA)/

(4)

The LM4838’s T

JMAX = 150˚C. In the LQ package soldered

to a DAP pad that expands to a copper area of 5in

2 on a

PCB, the LM4838’s

JA is 20˚C/W. In the MTE package

soldered to a DAP pad that expands to a copper area of 2in

on a PCB, the LM4838MTE’s

is 41˚C/W. For the

LM4838MT package,

JA = 80˚C/W. At any given ambient

temperature T

A, use Equation (4) to find the maximum inter-

nal power dissipation supported by the IC packaging. Rear-

ranging Equation (4) and substituting P

DMAX for PDMAX' re-

sults in Equation (5). This equation gives the maximum

ambient temperature that still allows maximum stereo power

dissipation without violating the LM4838’s maximum junction

temperature.

A =TJMAX – 2*PDMAX

(5)

For a typical application with a 5V power supply and an 4

Ω

load, the maximum ambient temperature that allows maxi-

mum stereo power dissipation without exceeding the maxi-

mum junction temperature is approximately 99˚C for the LQ

package and 45˚C for the MTE package.

JMAX =PDMAX

JA +TA

(6)

Equation (6) gives the maximum junction temperature

JMAX. If the result violates the LM4838’s 150˚C TJMAX,

reduce the maximum junction temperature by reducing the

power supply voltage or increasing the load resistance. Fur-

ther allowance should be made for increased ambient tem-

peratures.

The above examples assume that a device is a surface

mount part operating around the maximum power dissipation

point. Since internal power dissipation is a function of output

power, higher ambient temperatures are allowed as output

power or duty cycle decreases.

If the result of Equation (2) is greater than that of Equation

(3), then decrease the supply voltage, increase the load

impedance, or reduce the ambient temperature. If these

measures are insufficient, a heat sink can be added to

reduce

JA. The heat sink can be created using additional

copper area around the package, with connections to the

ground pin(s), supply pin and amplifier output pins. External,

solder attached SMT heatsinks such as the Thermalloy

7106D can also improve power dissipation. When adding a

heat sink, the

JA is the sum of

JC,

CS, and

SA.(

JC is the

junction-to-case thermal impedance,

CS is the case-to-sink

thermal impedance, and

SA is the sink-to-ambient thermal

impedance.) Refer to the Typical Performance Character-

istics curves for power dissipation information at lower out-

put power levels.

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 10 µF in parallel with a 0.1 µF filter capacitor to

stabilize 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

LM4838’s supply pins and ground. Do not substitute a ce-

ramic capacitor for the tantalum. Doing so may cause oscil-

lation. Keep the length of leads and traces that connect

capacitors between the LM4838’s power supply pin and

ground as short as possible. Connecting a 1µF capacitor,

B, between the BYPASS pin and ground improves the

internal bias voltage’s stability and the amplifier’s PSRR. The

PSRR improvements increase as the BYPASS pin capacitor

value increases. Too large a capacitor, however, increases

turn-on time and can compromise the amplifier’s click and

pop performance. The selection of bypass capacitor values,

especially C

B, depends on desired PSRR requirements,

click and pop performance (as explained in the following

section, Selecting Proper External Components), system

cost, and size constraints.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4838’s performance requires properly se-

lecting external components. Though the LM4838 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4838 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 circuits demand input

signals with greater voltage swings to achieve maximum

output power. Fortunately, many signal sources such as

audio CODECs have outputs of 1V

RMS (2.83VP-P). Please

refer to the Audio Power Amplifier Design section for more

information on selecting the proper gain.

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