LM4805 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

nal power dissipation is 4 times that of a single-ended am-

plifier. The maximum power dissipation for a given BTL

application can be derived from Equation 1.

DMAX(AMP) = 4(VDD)

2 /(2

π2R

(1)

BOOST CONVERTER POWER DISSIPATION

At higher duty cycles, the increased ON-time of the switch

FET means the maximum output current will be determined

by power dissipation within the LM2731 FET switch. The

switch power dissipation from ON-time conduction is calcu-

lated by Equation 2.

DMAX(SWITCH) =DCxIIND(AVE)

2 xR

DS(ON)

(2)

where DC is the duty cycle.

There will be some switching losses as well, so some derat-

ing needs to be applied when calculating IC power dissipa-

tion.

TOTAL POWER DISSIPATION

The total power dissipation for the LM4805 can be calculated

by adding Equation 1 and Equation 2 together to establish

Equation 3:

DMAX(TOTAL) = [4*(VDD)

2/2

π2R

L]+[DCxIIND(AVE)

DS(ON)]

(3)

The result from Equation 3 must not be greater than the

power dissipation that results from Equation 4:

DMAX =(TJMAX -TA)/

θJA

(4)

For the LQA28A,

JA = 59˚C/W. TJMAX = 125˚C for the

LM4805. Depending on the ambient temperature, T

A,ofthe

system surroundings, Equation 4 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 3 is greater than that of

Equation 4, then either the supply voltage must be in-

creased, the load impedance increased or T

A reduced. For

the typical application of a 3V power supply, with V1 set to

5.5V and an 8

Ω load, the maximum ambient temperature

possible without violating the maximum junction temperature

is approximately 111˚C provided that device operation is

around the maximum power dissipation point. Thus, for typi-

cal applications, power dissipation is not an issue. Power

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

operation is not around the maximum power dissipation

point, the ambient temperature may be increased accord-

ingly. Refer to the Typical Performance Characteristics

curves for power dissipation information for lower output

levels.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4805’s exposed-DAP (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. The low

thermal resistance allows rapid heat transfer from the die to

the surrounding PCB copper traces, ground plane, and sur-

rounding air. The LD package should have its DAP soldered

to a copper pad on the PCB. The DAP’s PCB copper pad

may be connected to a large plane of continuous unbroken

copper. This plane forms a thermal mass, heat sink, and

radiation area. Further detailed and specific information con-

cerning PCB layout, fabrication, and mounting an LD (LLP)

package is found in National Semiconductor’s Package En-

gineering Group under application note AN1187.

SHUTDOWN FUNCTION

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch, and a pull-up resistor.

One terminal of the switch is connected to GND. The other

side is connected to the two shutdown pins and the terminal

of the pull-up resistor. The remaining resistance terminal is

connected to V

DD. If the switch is open, then the external

pull-up resistor connected to V

DD will enable the LM4805.

This scheme guarantees that the shutdown pins will not float

thus preventing unwanted state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers, and switching boost convert-

ers, is critical for optimizing device and system performance.

Consideration to component values must be used to maxi-

mize overall system quality.

The best capacitors for use with the switching converter

portion of the LM4805 are multi-layer ceramic capacitors.

They have the lowest ESR (equivalent series resistance)

and highest resonance frequency, which makes them opti-

mum for high frequency switching converters.

When selecting a ceramic capacitor, only X5R and X7R

dielectric types should be used. Other types such as Z5U

and Y5F have such severe loss of capacitance due to effects

of temperature variation and applied voltage, they may pro-

vide as little as 20% of rated capacitance in many typical

applications. Always consult capacitor manufacturer’s data

curves before selecting a capacitor. High-quality ceramic

capacitors can be obtained from Taiyo-Yuden, AVX, and

Murata.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for

low noise performance and high power supply rejection. The

capacitor location on both V1 and V

DD (Cs2 and Cs1) pins

should be as close to the device as possible.

SELECTING INPUT CAPACITOR FOR AUDIO

AMPLIFIER

One of the major considerations is the closedloop bandwidth

of the amplifier. To a large extent, the bandwidth is dictated

by the choice of external components shown in Figure 1. The

input coupling capacitor, C

i, forms a first order high pass filter

which limits low frequency response. This value should be

chosen based on needed frequency response for a few

distinct reasons.

High value input capacitors are both expensive and space

hungry in portable designs. Clearly, a certain value capacitor

is needed to couple in low frequencies without severe at-

tenuation. However, speakers used in portable systems,

whether internal or external, have little ability to reproduce

signals below 100Hz to 150Hz. Thus, using a high value

input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is affected by the value of the input coupling capaci-

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