LM4928 datasheet(13/20 Pages) NSC

13 / 20 page

Application Information (Continued)

DMAX =(VDD)

2 /(2

π2R

L) Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier

operating at the same conditions.

DMAX = 4(VDD)

2/(2

π2R

L) Bridge Mode per channel (3)

DMAX = 8(VDD)

2/(2

π2R

L) Bridge Mode both channel (4)

Since the LM4928 has bridged outputs, the maximum inter-

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

plifier. Even with this substantial increase in power dissipa-

tion, the LM4928 does not require additional heatsinking

under most operating conditions and output loading. From

Equation 3, assuming a 5V power supply and an 8

Ω load,

the maximum power dissipation point is 625mW per chan-

nel. Then multiply by two or use equation 4 to get 1.25W total

power dissipation for both channels. The maximum power

dissipation point obtained from Equation 4 must not be

greater than the power dissipation results from Equation 5:

DMAX =(TJMAX -TA)/

(5)

Depending on the ambient temperature, T

A, of the system

surroundings, Equation 5 can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of Equation 4 is greater than that of Equation 5,

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 in the LLP

package, the maximum ambient temperature possible with-

out violating the maximum junction temperature is approxi-

mately 85˚C provided 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

LM4928 can operate at higher ambient temperatures. Refer

to the Typical Performance Characteristics curves for

power dissipation information.

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 LM4928. The LM4928 will operate with-

out the bypass capacitor C

B, although the PSRR may de-

crease. 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.

OPTIMIZING RF IMMUNITY

The internal circuitry of the LM4928 suppresses the amount

of RF signal that is coupled into the chip. However, certain

external factors, such as output trace length, output trace

orientation, distance between the chip and the antenna,

antenna strength, speaker type, and type of RF signal, may

affect the RF immunity of the LM4928. In general, the RF

immunity of the LM4928 is application specific. Neverthe-

less, optimal RF immunity can be achieved by using short

output traces and increasing the distance between the

LM4928 and the antenna.

SHUTDOWN FUNCTION

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

LM4928 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. The device may then be placed

into shutdown mode by toggling the Shutdown Select pin to

logic low. The trigger point for shutdown 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. 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 LM4928 is

tolerant to a variety of external component combinations,

consideration of component values must be made when

maximizing overall system quality.

The LM4928 is unity-gain stable, giving the designer maxi-

mum system flexibility. The LM4928 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 LM4928

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 6:

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

(6)

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