LM4894 datasheet(13/19 Pages) NSC

13 / 19 page

Application Information (Continued)

mum power dissipation point obtained from Equation 3 must

not be greater than the power dissipation results from Equa-

tion 4:

DMAX =(TJMAX -TA)/

(4)

The LM4894’s

JA in an MUA10A package is 190˚C/W.

Depending on the ambient temperature, T

A, of the 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 decreased, the load

impedance increased, the ambient temperature reduced, or

the

JA reduced with heatsinking. In many cases, larger

traces near the output, V

DD, and GND pins can be used to

lower the

JA. The larger areas of copper provide a form of

heatsinking allowing higher power dissipation. For the typical

application of a 5V power supply, with an 8

Ω load, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 30˚C pro-

vided that device operation is around the maximum power

dissipation point. Recall that internal power dissipation is a

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

maximum power dissipation point, the LM4894 can operate

at higher ambient temperatures. Refer to the Typical Per-

formance Characteristics curves for power dissipation in-

formation.

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 and

0.1µF bypass capacitors that increase supply stability. This,

however, does not eliminate the need for bypassing the

supply nodes of the LM4894. Although the LM4894 will

operate without the bypass capacitor C

B, although 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

LM4894 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. In addition, the LM4894 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 LM4894 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 LM4894 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 may

be 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 LM4894 is

tolerant to a variety of external component combinations,

consideration of component values must be made when

maximizing overall system quality.

The LM4894 is unity-gain stable, giving the designer maxi-

mum system flexibility. The LM4894 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 LM4894

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)

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

(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) more infront. Because of the balanced nature of

differential amplifiers, resistor matching differences can re-

sult 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 ef-

fects of differing values between the feedback resistors while

assuming 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.

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