LM4898 datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

function of output power. If typical operation is not around the

maximum power dissipation point, the LM4898 can operate

at higher ambient temperatures. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation informa-

tion.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor location on both the

bypass and power supply pins should be as close to the

device as possible. A larger half-supply bypass capacitor

improves PSRR because it increases half-supply stability.

Typical applications employ a 5V regulator with 10µF

and0.1µF bypass capacitors that increase supply stability.

This, however, does not eliminate the need for bypassing the

supply nodes of the LM4898. Although the LM4898 will

operate without the bypass capacitor C

B, the PSRR may

decrease. A 1µF capacitor is recommended for C

B. This

value maximizes PSRR performance. Lesser values may be

used, but PSRR decreases at frequencies below 1kHz. The

issue of C

B selection is thus dependant upon desired PSRR

and click and pop performance as explained in the section

Proper Selection of External Components.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4898 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. In addition, the LM4898 con-

tains a Shutdown Mode pin, allowing the designer to desig-

nate whether the part will be driven into shutdown with a high

level logic signal or a low level logic signal. This allows the

designer maximum flexibility in device use, as the Shutdown

Mode pin may simply be tied permanently to either V

DD or

GND to set the LM4898 as either a "shutdown-high" device

or a "shutdown-low" device, respectively. The device may

then be placed into shutdown mode by toggling the Shut-

down Select pin to the same state as the Shutdown Mode

pin. For simplicity’s sake, this is called "shutdown same", as

the LM4898 enters shutdown mode whenever the two pins

are in the same logic state. The trigger point for either

shutdown high or shutdown low is shown as a typical value

in the Supply Current vs. Shutdown Voltage graphs in the

Typical Performance Characteristics section. It is best to

switch between ground and supply for maximum perfor-

mance. While the device may be disabled with shutdown

voltages in between ground and supply, the idle current

maybe greater than the typical value of 0.1µA. In either case,

the shutdown pin should be tied to a definite voltage to avoid

unwanted state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry, which pro-

vides a quick, smooth transition to shutdown. Another solu-

tion is to use a single-throw switch in conjunction with an

external pull-up resistor (or pull-down, depending on shut-

down high or low application). This scheme guarantees that

the shutdown pin 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 is critical when optimizing

device and system performance. Although the LM4898 is

tolerant to a variety of external component combinations,

consideration of component values must be made when

maximizing overall system quality.

The LM4898 is unity-gain stable, giving the designer maxi-

mum system flexibility. The LM4898 should be used in low

closed-loop gain configurations to minimize THD+N values

and maximize signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1Vrms are available

from sources such as audio codecs. Please refer to the

Audio Power Amplifier Design section for a more complete

explanation of proper gain selection. When used in its typical

application as a fully differential power amplifier the LM4898

does not require input coupling capacitors for input sources

with DC common-mode voltages of less than V

DD. Exact

allowable input common-mode voltage levels are actually a

function of V

DD,Ri, and Rf and may be determined by

Equation 5:

CMi<(VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri/2Rf)

(5)

f/Ri=AVD

(6)

Special care must be taken to match the values of the

feedback resistors (R

f1 and Rf2) to each other as well as

matching the input resistors (R

i1 and Ri2) to each other (see

Figure 1). Because of the balanced nature of differential

amplifiers, resistor matching differences can result in net DC

currents across the load. This DC current can increase

power consumption, internal IC power dissipation, reduce

PSRR, and possibly damaging the loudspeaker. The chart

below demonstrates this problem by showing the effects of

differing values between the feedback resistors while as-

suming that the input resistors are perfectly matched. The

results below apply to the application circuit shown in Figure

1, and assumes that V

DD =5V, RL =8

Ω, and the system has

DC coupled inputs tied to ground.

Tolerance

f1Rf2

Vo2-Vo1

LOAD

20%

0.8R

1.2R

-0.5V

62.5mA

10%

0.9R

1.1R

-0.250V

31.25mA

0.95R

1.05R

-0.125V

15.63mA

0.99R

1.01R

-0.025V

3.125mA

RRR

Similar results would occur if the input resistors were not

carefully matched. Adding input coupling capacitors in be-

tween the signal source and the input resistors will eliminate

this problem, however, to achieve best performance with

minimum component count it is highly recommended that

both the feedback and input resistors matched to 1% toler-

ance or better.

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