Agamem Microelectronics Inc.

AA4838

PRELIMINARY

AUDIO POWER AMPLIFIER

©Copyright Agamem Microelectronics Inc.

www.agamem.com.tw

2008/8/26

AGAMEM MICROELECTRONICS INCOPERATION RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE

TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. AGAMEM DOES NOT ASSUME ANY

LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCTS OR CIRCUIT DESRIBED HEREIN; NEITHER

DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.

10

41˚C/W. For the AA4838MT package, θJA=80˚C/W. At any given ambient temperature TA,

use Equation (4) to find the maximum internal power dissipation supported by the IC

packaging. Rearranging Equation (4) and substituting PDMAX for PDMAX' results in Equation

(5). This equation gives the maximum ambient temperature that still allows maximum stereo

power dissipation without violating the AA4838’s maximum junction temperature.

TA=TJMAX–2*PDMAXθJA…(5).

For a typical application with a 5V power supply and a 4Ω load, the maximum ambient

temperature that allows maximum stereo power dissipation without exceeding the maximum

junction temperature is approximately 99˚C for the LQ package and 45˚C for the MTE

package.

TJMAX=PDMAXθJA+TA…(6).

Equation (6) gives the maximum junction temperature TJMAX. If the result violates the

AA4838’s 150˚C TJMAX, 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 (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 θJA. 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 heat sinks such as the Thermally

7106D can also improve power dissipation. When adding a heat sink, the θJA is the sum of

θ

JC, θCS, and θSA. (θJC is the junction-to-case thermal impedance, θCS is the

case-to-sink thermal impedance, and θSA is the sink-to-ambient thermal impedance.) Refer

to the Typical Performance Characteristics curves for power dissipation 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 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 1.0µF tantalum bypass capacitance connected between the

AA4838’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum.

Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors

between the AA4838’s power supply pin and ground as short as possible. Connecting a 1µF