LM4951 datasheet(11/18 Pages) NSC

11 / 18 page

Application Information

HIGH VOLTAGE BOOMER

Unlike previous 5V Boomer® amplifiers, the LM4951 is de-

signed to operate over a power supply voltages range of

2.7V to 9V. Operating on a 7.5V power supply, the LM4951

will deliver 1.8W into an 8

Ω BTL load with no more than 1%

THD+N.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4951 consists of two opera-

tional amplifiers that drive a speaker connected between

their outputs. The value of input and feedback resistors

determine the gain of each amplifier. External resistors R

and R

f set the closed-loop gain of AMPA, whereas two 20k

Ω

internal resistors set AMP

B’s gain to -1. The LM4951 drives

a load, such as a speaker, connected between the two

amplifier outputs, V

O+ and VO -. Figure 1 shows that AMPA’s

output serves as AMP

B’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 AMP

A and AMPB and driven differentially

(commonly referred to as "bridge mode"). This results in a

differential, or BTL, gain of

VD = 2(Rf / Ri)

(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. Theoretically, 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 and

that the output signal is not clipped. To ensure minimum

output signal clipping when choosing an amplifier’s closed-

loop gain, refer to the AUDIO POWER AMPLIFIER DESIGN

section. Under rare conditions, with unique combinations of

high power supply voltage and high closed loop gain set-

tings, the LM4951 may exhibit low frequency oscillations.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

AMP1’s and AMP2’s outputs at half-supply. This eliminates

the coupling capacitor that single supply, single-ended am-

plifiers require. Eliminating an output coupling capacitor in a

typical single-ended configuration forces a single-supply am-

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

creases internal IC power dissipation and may permanently

damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful bridged amplifier.

The LM4951’s dissipation when driving a BTL load is given

by Equation (2). For a 7.5V supply and a single 8

Ω BTL load,

the dissipation is 1.42W.

DMAX-MONOBTL = 4(VDD)

2 / 2

π2R

Bridge Mode (2)

The maximum power dissipation point given by Equation (2)

must not exceed the power dissipation given by Equation

(3):

DMAX’= (TJMAX -TA) /

(3)

The LM4951’s T

JMAX = 150˚C. In the SD package, the

LM4951’s

JA is 73˚C/W when the metal tab is soldered to a

copper plane of at least 1in

2. This plane can be split between

the top and bottom layers of a two-sided PCB. Connect the

two layers together under the tab with an array of vias. At any

given ambient temperature T

A, use Equation (3) to find the

maximum internal power dissipation supported by the IC

packaging. Rearranging Equation (3) and substituting P

DMAX

for P

DMAX’ results in Equation (4). This equation gives the

maximum ambient temperature that still allows maximum

stereo power dissipation without violating the LM4951’s

maximum junction temperature.

A =TJMAX -PDMAX-MONOBTL

(4)

For a typical application with a 7.5V power supply and a BTL

Ω load, the maximum ambient temperature that allows

maximum stereo power dissipation without exceeding the

maximum junction temperature is approximately 46˚C for the

TS package.

JMAX =PDMAX-MONOBTL

JA +TA

(5)

Equation (5) gives the maximum junction temperature

JMAX. If the result violates the LM4951’s 150˚C, reduce the

maximum junction temperature by reducing the power sup-

ply voltage or increasing the load resistance. Further allow-

ance should be made for increased ambient temperatures.

The above examples assume that a device is operating

around the maximum power dissipation point. Since internal

power dissipation is a function of output power, higher am-

bient 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. Further,

ensure that speakers rated at a nominal 8

Ω do not fall below

Ω. 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 connec-

tions to the ground pins, supply pin and amplifier output pins.

Refer to the Typical Performance Characteristics curves

for power dissipation information at lower output power lev-

els.

POWER SUPPLY VOLTAGE LIMITS

Continuous proper operation is ensured by never exceeding

the voltage applied to any pin, with respect to ground, as

listed in the Absolute Maximum Ratings section.

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 voltage regulator typi-

cally use a 10µF in parallel with a 0.1µF filter capacitors 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

LM4951’s supply pins and ground. Do not substitute a ce-

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

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