LM4936 datasheet(18/28 Pages) NSC

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Application Information (Continued)

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4936 output stage consists of

two pairs of operational amplifiers, forming a two-channel

(channel A and channel B) stereo amplifier. (Though the

following discusses channel A, it applies equally to channel

B.)

Figure 2 shows that the first amplifier’s negative (-) output

serves as the second amplifier’s input. This results in both

amplifiers producing signals identical in magnitude, but 180˚

out of phase. Taking advantage of this phase difference, a

load is placed between −OUTA and +OUTA and driven dif-

ferentially (commonly referred to as “bridge mode”). This

results in a differential gain of

VD =2*(Rf/R i)

(1)

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 LM4936 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 LM4936’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 due to stereo operation.

Twice the maximum power dissipation point given by Equa-

tion (3) must not exceed the power dissipation given by

Equation (4):

DMAX'=(TJMAX −TA)/

(4)

The LM4936’s T

JMAX = 150˚C. In the MH package soldered

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

2 on a

PCB, the LM4936MH’s

JA is 41˚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 LM4936’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 LM4936’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.

If the result of Equation (3) multiplied by 2 for stereo opera-

tion is greater than that of Equation (4), 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 ampli-

fier 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 im-

pedance,

CS is the case-to-sink thermal impedance, and

SA is the sink-to-ambient thermal impedance.) Refer to the

Typical Performance Characteristics curves for power dis-

sipation information at lower output 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 10µF in parallel with a 0.1µF 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 1µF tanta-

lum bypass capacitance connected between the LM4936’s

supply pins and ground. Do not substitute a ceramic capaci-

tor for the tantalum. Doing so may cause oscillation. Keep

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