LM4980 datasheet(14/20 Pages) NSC

14 / 20 page

Application Information (Continued)

tivating shutdown. As C

COUPLING’s value increases, output

transient magnitude can also increase. This increase can be

mitigated by a corresponding increase in C

MIDCAP’s value. A

minimum starting point when selecting C

MIDCAP’s value is

6.8µF when using 220µF output coupling capacitors.

SELECTING THE INPUT CAPACITOR VALUE

Amplifiying the lowest audio frequencies requires a relatively

high value input coupling capacitor, (C

IN in Figure 2). A high

value capacitor can be expensive and may compromise

space efficiency in portable designs. In many cases, how-

ever, the headphones used in portable systems have limited

ability to reproduce signals below 60Hz. Applications using

headphones with this limited frequency response reap little

improvement by using a high value input capacitor. A small

value of Ci (in the range of 0.1µF to 1.0µF), is recommended.

DRIVING POWERED SPEAKERS

Though the LM4980 is design primarily to drive headphones,

in many cases, it may be called on to act as a line level driver

when powered speakers or other devices may be connected

to the amplifier outputs. For powered speakers or other

devices with typical input resistances (10k

Ω) that are signifi-

cantly higher than the typical headphone resistance (32

Ω),

the output transients may not sufficiently suppressed when

using the Figure 2 circuit. If this is anticipated, a minor

modification of an additional resistor (a nominal value of

Ω) between each output and ground in the Figure 2 circuit

is needed to ensure that the output transient suppression is

not compromised. This reduces both the load resistance

seen by the LM4980 and the magnitude of power-on and

shutdown output transients.

POWER DISSIPATION

Power dissipation has to be evaluated and considered when

designing a successful amplifier. A direct consequence of the

power delivered to a load an amplifier is internal power

dissipation. The maximum per-amplifier power dissipation

for a given application can be derived from the power dissi-

pation graphs or from Equation 2.

PDMAX = V

2 / 2

πR

LOAD

(2)

It is critical that the maximum junction temperature T

JMAX of

150˚C is not exceeded. Since the typical application is for

headphone operation (16

Ω impedance) using a 3.0V supply

the maximum power dissipation is less than 29mW. There-

fore, in the case of this headphone amplifier, the power

dissipation is not a major concern.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be

as close to the device as possible. Typical applications em-

ploy a 3.0V regulator with 10µF tantalum or electrolytic ca-

pacitor and a ceramic bypass capacitor which aid in supply

stability. This does not eliminate the need for local power

supply bypassing connected as close as possible to the

LM4980’s supply pin. A power supply bypass capacitor value

in the range of 1.0µF to 10µF is recommended.

MICRO POWER SHUTDOWN

The voltage applied to the shutdown (SHDN) pin controls the

LM4980’s shutdown function. Activate micro-power shut-

down by applying a logic-low voltage to the SHDN pin. When

active, the LM4980’s micro-power shutdown feature turns off

the amplifier’s bias circuitry, reducing the supply current. The

trigger point is 0.4V (max) for a logic-low level, and 1.4V

(min) for a logic-high level. The low 0.1µA (typ) shutdown

current is achieved by applying a voltage that is as near as

ground as possible to the SHDN pin. A voltage that is higher

than ground may increase the shutdown current.

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external 100k

Ω pull-up resistor between the

SHDN pin and GND. Connect the switch between the SHDN

pin and V

DD. Select normal amplifier operation by closing the

switch. Opening the switch connects the SHDN pin to

ground, activating micro-power shutdown. The switch and

resistor guarantee that the SHDN pin will not float. This

prevents unwanted state changes. In a system with a micro-

processor or microcontroller, use a digital output to apply the

control voltage to the SHDN pin. Driving the SHDN pin with

active circuitry eliminates the pull-up resistor.

SUGGESTED PCB SCHEMATIC

Figure 5 is the schematic for the suggested PCB Layout.

This schematic and its associated PCB provide both a lean

tested layout and platform that can be used to verify the

LM4980’s outstanding audio performance.

Suggested PCB Design and Layout

Figures 6 through 9 show a suggested PCB layout for a

headphone amplifier circuit using the LM4980 .

PCB Layout Guidelines

This section provides practical guidelines for mixed signal

PCB layout that involves various digital/analog power and

ground traces. Designers should note that these are only

"rule-of-thumb" recommendations and the actual results will

depend heavily on the final layout.

MINIMIZING THD+N

PCB trace impedance on the power, ground, and all output

traces should be minimized to achieve optimal THD perfor-

mance. Therefore, use PCB traces that are as wide as

possible for these connections. As the gain of the amplifier is

increased, the trace impedance will have an ever increasing

adverse affect on THD performance. At unity-gain (0dB) the

parasitic trace impedance effect on THD performance is

reduced but still a negative factor in the THD performance of

the LM4980 in a given application.

GENERAL MIXED SIGNAL LAYOUT

RECOMMENDATION

Power and Ground Circuits

For two layer mixed signal design, it is important to isolate

the digital power and ground trace paths from the analog

power and ground trace paths. Star trace routing techniques

(bringing individual traces back to a central point rather than

daisy chaining traces together in a serial manner) can

greatly enhance low level signal performance. Star trace

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