LM4990 datasheet(13/19 Pages) NSC

13 / 19 page

Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4990 has two internal opera-

tional amplifiers. The first amplifier’s gain is externally con-

figurable, 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 Ri 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/Ri)

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 LM4990,

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.

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 LM4990 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 copper foil, the thermal resistance of the

application can be reduced from the free air value of

JA,

resulting in higher P

DMAX values without thermal shutdown

protection circuitry being activated. Additional copper foil can

be added to any of the leads connected to the LM4990. It is

especially effective when connected to V

DD, GND, and the

output pins. Refer to the application information on the

LM4990 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 re-

duced supply voltage, higher load impedance, or reduced

ambient temperature. Internal power dissipation is a function

of output power. Refer to the Typical Performance Charac-

teristics curves for power dissipation information for differ-

ent 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 10µF 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 LM4990. The selection of

a bypass capacitor, especially C

B, is dependent upon PSRR

requirements, click and pop performance (as explained in

the section, Proper Selection of External Components),

system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4990 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. In addition, the LM4990 con-

tains a Shutdown Mode pin (LD and MH packages only),

allowing the designer to designate 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 flexibil-

ity in device use, as the Shutdown Mode pin may simply be

tied permanently to either V

DD or GND to set the LM4990 as

either a "shutdown-high" device or a "shutdown-low" device,

respectively. The device may then be placed into shutdown

mode by toggling the Shutdown pin to the same state as the

Shutdown Mode pin. For simplicity’s sake, this is called

"shutdown same", as the LM4990 enters shutdown mode

whenever the two pins are in the same logic state. The MM

package lacks this Shutdown Mode feature, and is perma-

nently fixed as a ‘shutdown-low’ device. 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 to optimize device

and system performance. While the LM4990 is tolerant of

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