LM49100 Datasheet, PDF(18/24 Page) National Semiconductor (TI) – Mono Class AB Audio Subsystem with a True-Ground Headphone Amplifier

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LM49100 Datasheet, PDF (18/24 Pages) National Semiconductor (TI) – Mono Class AB Audio Subsystem with a True-Ground Headphone Amplifier

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PDMAX = (TJMAX - TA) / Î¸JA

(4)

The LM49100's TJMAX = 150Â°C. In the GR package, the

LM49100's Î¸JA is 50.2Â°C/W. At any given ambient tempera-

ture TA, use Equation (4) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (4) and substituting PDMAX-TOTAL for PDMAX results in

Equation (5). This equation gives the maximum ambient tem-

perature that still allows maximum stereo power dissipation

without violating the LM49100's maximum junction tempera-

ture.

TA = TJMAX - PDMAX-TOTAL Î¸JA

(5)

For a typical application with a 5V power supply and an 8â¦

load, the maximum ambient temperature that allows maxi-

mum mono power dissipation without exceeding the maxi-

mum junction temperature is approximately 114Â°C for the GR

package.

TJMAX = PDMAX-TOTAL Î¸JA + TA

(6)

Equation (6) gives the maximum junction temperature

TJMAX. If the result violates the LM49100's 150Â°C, reduce the

maximum junction temperature by reducing the power supply

voltage or increasing the load resistance. Further allowance

should be made for increased ambient temperatures.

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 (3) is greater

than that of Equation (4), then decrease the supply voltage,

increase the load impedance, or reduce the ambient temper-

ature. If these measures are insufficient, a heat sink can be

added to reduce Î¸JA. The heat sink can be created using ad-

ditional copper area around the package, with connections to

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

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is crit-

ical for low noise performance and high power supply rejec-

tion. Applications that employ a 5V regulator typically use a

1ÂµF in parallel with a 0.1ÂµF filter capacitors to stabilize the

regulator's output, reduce noise on the supply line, and im-

prove the supply's transient response. However, their pres-

ence does not eliminate the need for a local 4.7ÂµF tantalum

bypass capacitor and a parallel 0.1ÂµF ceramic capacitor con-

nected between the LM49100's supply pin and ground. Keep

the length of leads and traces that connect capacitors be-

tween the LM49100's power supply pin and ground as short

as possible.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (CIN 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 and headphones with this limited

frequency response reap little improvement by using large in-

put capacitor.

The internal input resistor (Ri), typical 12.5kâ¦, and the input

capacitor (CIN) produce a high pass filter cutoff frequency that

is found using Equation (7).

fc = 1 / (2ÏRiCIN)

(7)

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of CB, the capacitor connected

to the BYPASS pin. Since CB determines how fast the

LM49100 settles to quiescent operation, its value is critical

when minimizing turn-on pops. Choosing CB equal to 2.2ÂµF

along with a small value of Ci (in the range of 0.1ÂµF to 0.33ÂµF),

produces a click-less and pop-less shutdown function. As dis-

cussed above, choosing CIN no larger than necessary for the

desired bandwidth helps minimize clicks and pops. CB's value

should be in the range of 4 to 5 times the value of CIN . This

ensures that output transients are eliminated when power is

first applied or the LM49100 resumes operation after shut-

down.

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