LM4890 datasheet(18/31 Pages) NSC

18 / 31 page

Application Information

BRIDGED CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4890 has two operational

amplifiers internally, allowing for a few different amplifier

configurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of R

f to RIN

while the second amplifier’s gain is fixed by the two internal

20k

Ω resistors. Figure 1 shows that the output of amplifier

one serves as the input to amplifier two which results in both

amplifiers producing signals identical in magnitude, but out

of phase by 180˚. Consequently, the differential gain for the

IC is

VD= 2 *(Rf/RIN)

By driving the load differentially through outputs Vo1 and

Vo2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of the load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same con-

ditions. This increase in attainable output power assumes

that the amplifier is not current limited or clipped. In order to

choose an amplifier’s closed-loop gain without causing ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

A bridge configuration, such as the one used in the LM4890,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, Vo1 and Vo2, are biased

at half-supply, no net DC voltage exists across the load. This

eliminates the need for an output coupling capacitor which is

required in a single supply, single-ended amplifier configura-

tion. Without an output coupling capacitor, the half-supply

bias across the load would result in both increased internal

IC power dissipation and also possible loudspeaker damage.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS FOR THE LM4890LD

The LM4890LD’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

LM4890LD package should have its DAP soldered to the

grounded copper pad (heatsink) under the LM4890LD (the

NC pins, no connect, and ground pins should also be directly

connected to this copper pad-heatsink area). The area of the

copper pad (heatsink) can be determined from the LD Power

Derating graph. If the multiple layer copper heatsink areas

are used, then these inner layer or backside copper heatsink

areas should be connected to each other with 4 (2 x 2) vias.

The diameter for these vias should be between 0.013 inches

and 0.02 inches with a 0.050inch pitch-spacing. Ensure

efficient thermal conductivity by plating through and solder-

filling the vias. Further detailed information concerning PCB

layout, fabrication, and mounting an LLP package is avail-

able from National Semiconductor’s Package Engineering

Group under application note AN1187.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

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

internal power dissipation. Since the LM4890 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given application can

be derived from the power dissipation graphs or from Equa-

tion 1.

DMAX = 4*(VDD)

2/(2

π2R

(1)

It is critical that the maximum junction temperature T

JMAX of

150˚C is not exceeded. T

JMAX can be determined from the

power derating curves by using P

DMAX and the PC board foil

area. By adding additional copper foil, the thermal resistance

of the application can be reduced, resulting in higher P

DMAX.

Additional copper foil can be added to any of the leads

connected to the LM4890. Refer to the APPLICATION IN-

FORMATION on the LM4890 reference design board for an

example of good heat sinking. If T

JMAX still exceeds 150˚C,

then additional changes must be made. These changes can

include reduced supply voltage, higher load impedance, or

reduced ambient temperature. Internal power dissipation is a

function of output power. Refer to the Typical Performance

Characteristics curves for power dissipation information for

different output powers and output loading.

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 the bypass and power supply pins

should be as close to the device as possible. Typical appli-

cations employ a 5V regulator with 10 µF tantalum or elec-

trolytic capacitor and a ceramic bypass capacitor which aid

in supply stability. This does not eliminate the need for

bypassing the supply nodes of the LM4890. The selection of

a bypass capacitor, especially C

BYPASS, is dependent upon

PSRR requirements, click and pop performance (as ex-

plained in the section, Proper Selection of External Com-

ponents), system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4890 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

amplifier off when a logic low is placed on the shutdown pin.

By switching the shutdown pin to ground, the LM4890 supply

current draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages less than

0.5V

DC, the idle current may be greater than the typical

value of 0.1µA. (Idle current is measured with the shutdown

pin grounded).

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 in conjunction with an

external pull-up resistor. When the switch is closed, the

shutdown pin is connected to ground and disables the am-

plifier. If the switch is open, then the external pull-up resistor

will enable the LM4890. This scheme guarantees that the

shutdown pin will not float thus preventing unwanted state

changes.

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