LM4982TL datasheet(16/21 Pages) NSC

16 / 21 page

Application Information (Continued)

2C INTERFACE POWER SUPPLY PIN (I2CV

DD)

The LM4982’s I

2C interface is powered up through the

2CV

DD pin. The LM4982’s I

2C interface operates at a volt-

age level set by the I

2CV

DD pin which can be set indepen-

dent to that of the main power supply pin V

DD. This is ideal

whenever logic levels for the I

2C interface are dictated by a

microcontroller or microprocessor that is operating at a lower

supply voltage than the main battery of a portable system.

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

LM4982’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4982’s

power supply pins and ground as short as possible.

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM4982 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows

the outputs of the LM4982 to be biased about GND instead

of a nominal DC voltage, like traditional headphone amplifi-

ers. Because there is no DC component, the large DC

blocking capacitors (typically 220µF) are not necessary. The

coupling capacitors are replaced by two, small ceramic

charge pump capacitors, saving board space and cost.

Eliminating the output coupling capacitors also improves low

frequency response. In traditional headphone amplifiers, the

headphone impedance and the output capacitor form a high

pass filter that not only blocks the DC component of the

output, but also attenuates low frequencies, impacting the

bass response. Because the LM4982 does not require the

output coupling capacitors, the low frequency response of

the device is not degraded by external components.

In addition to eliminating the output coupling capacitors, the

ground referenced output nearly doubles the available dy-

namic range of the LM4982 when compared to a traditional

headphone amplifier operating from the same supply volt-

age.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4982 contains advanced circuitry that virtually elimi-

nates output transients (’clicks and pops’). This circuitry

prevents all traces of transients when the supply voltage is

first applied or when the part resumes operation after coming

out of shutdown mode.

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.

DMAX = (2VDD)

2 /(2

π2R

(1)

Since the LM4982 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 large internal power dissipation, the LM4982 does not

require heat sinking over a large range of ambient tempera-

tures. The maximum power dissipation point obtained must

not be greater than the power dissipation that results from

Equation 2:

DMAX =(TJMAX -TA)/(

JA)

(2)

For the micro SMD package,

JA = 105˚C/W. TJMAX = 150˚C

for the LM4982. Depending on the ambient temperature, T

of 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

reduced.

Power dissipation is a function of output power and thus, if

typical operation is not around the maximum power dissipa-

tion point, the ambient temperature may be increased ac-

cordingly.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4982’s performance requires properly se-

lecting external components. Though the LM4982 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

Charge Pump Capacitor Selection

Use low ESR (equivalent series resistance) (<100m

Ω) ce-

ramic capacitors with an X7R dielectric for best perfor-

mance. Low ESR capacitors keep the charge pump output

impedance to a minimum, extending the headroom on the

negative supply. Higher ESR capacitors result in reduced

output power from the audio amplifiers.

Charge pump load regulation and output impedance are

affected by the value of the flying capacitor (C1). A larger

valued C1 (up to 3.3uF) improves load regulation and mini-

mizes charge pump output resistance. Beyond 3.3uF, the

switch-on resistance dominates the output impedance for

capacitor values above 2.2uF.

The output ripple is affected by the value and ESR of the

output capacitor (C2). Larger capacitors reduce output ripple

on the negative power supply. Lower ESR capacitors mini-

mize the output ripple and reduce the output impedance of

the charge pump.

The LM4982 charge pump design is optimized for 2.2uF, low

ESR, ceramic, flying, and output capacitors.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C

i in Figure 1). A high value ca-

pacitor can be expensive and may compromise space effi-

ciency in portable designs. In many cases, however, the

speakers used in portable systems, whether internal or ex-

ternal, have little ability to reproduce signals below 150Hz.

Applications using speakers with this limited frequency re-

sponse reap little improvement by using high value input and

output capacitors.

Besides affecting system cost and size, C

i has an effect on

the LM4982’s click and pop performance. The magnitude of

the pop is directly proportional to the input capacitor’s size.

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