LM4841 datasheet(16/31 Pages) NSC

16 / 31 page

Application Information (Continued)

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. For a given supply voltage, bridge

mode has a distinct advantage over the single-ended con-

figuration: its differential output doubles the voltage

swing across the load. This produces four times the output

power when compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or that the

output signal is not clipped. To ensure minimum output sig-

nal clipping when choosing an amplifier’s closed-loop gain,

refer to the Audio Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

channel A’s and channel B’s outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a single-ended configuration forces a single-supply

amplifier’s half-supply bias voltage across the load. This

increases internal IC power dissipation and may perma-

nently damage loads such as speakers.

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 LM4841 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 LM4841’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 LM4841’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 LM4841’s

JA is 20˚C/W. In the MH and LQ pack-

ages soldered to a DAP pad that expands to a copper area

of 2in

2 on a PCB, the LM4841MH’s and LQ’s

JA is 41˚C/W.

For the LM4841MT package,

JA = 80˚C/W. At any given

ambient temperature T

A, use Equation (4) to find the maxi-

mum internal power dissipation supported by the IC packag-

ing. Rearranging Equation (4) and substituting P

DMAX for

DMAX' results in Equation (5). This equation gives the maxi-

mum ambient temperature that still allows maximum stereo

power dissipation without violating the LM4841’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 45˚C for the MH

package.

JMAX =PDMAX

JA +TA

(6)

Equation (6) gives the maximum junction temperature

JMAX. If the result violates the LM4841’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 1.0 µF

tantalum bypass capacitance connected between the

LM4841’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 LM4841’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.

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