ing excessive clipping which will damage high frequency

transducers used in loudspeaker systems, please refer to

the Audio Power Amplifier Deslgn section.

A bridge configuration, such as the one used in Boomer Au-

dio Power Amplifiers, also creates a second advantage over

single-ended amplifiers. Since the differential outputs, V

O1

and V

across the load. This eliminates the need for an output cou-

pling capacitor which is required in a single supply,

single-ended amplifier configuration. Without a large output

coupling capacitor in a single supply single-ended amplifier,

the half-supply bias across the load would result in both in-

creased internal IC power dissipation.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful 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. Equation 1 states the maximum

power dissipation point for a bridge amplifier operating at a

given supply voltage and driving a specified output load.

P

L)

(1)

Since the LM4870 has two operational amplifiers in one

package, the maximum internal power dissipation is 4 times

that of a single-ended amplifier. Even with this substantial in-

crease in power dissipation, the LM4870 does not require

heatsinking. From Equation 1, assuming a 5V power supply

and an 8

Ω load, the maximum power dissipation point is 625

mW. The maximum power dissipation point obtained from

Equation 1 must not be greater than the power dissipation

that results from Equation 2:

P

(2)

For the LM4870 surface mount package,

θ

T

of the system surroundings, Equation 2 can be used to find

the maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased or the load impedance increased. For the typical ap-

plication of a 5V power supply, with an 8

Ω load, the maxi-

mum ambient temperature possible without violating the

maximum junction temperature is approximately 88˚C, pro-

vided that device operation is around the maximum power

dissipation point. Power dissipation is a function of output

power and thus, if typical operation is not around the maxi-

mum power dissipation point, the ambient temperature can

be increased. Refer to the Typical Performance Character-

istics curves for power dissipation information for lower out-

put powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance CharacterIstIcs sec-

tion, the effect of a larger half-supply bypass capacitor is im-

proved low frequency THD+N due to increased half-supply

stability. Typical applications employ a 5V regulator with

a10 µF tantalum and a 0.1 µF film bypass capacitors which

aid in supply stability, but do not eliminate the need for by-

passing the supply nodes of the LM4870. The selection of

bypass capacitors, especially C

desired low frequency THD+N, system cost, and size con-

straints.

SHUTDOWN FUNCTION

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

LM4870 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. The shutdown feature turns the am-

plifier off when a logic high is placed on the shutdown pin.

Upon going into shutdown, the output is immediately discon-

nected from the speaker. There is a built-in threshold which

produces a drop in quiescent current to 500 µA typically. For

a 5V power supply, this threshold occurs when 2V–3V is ap-

plied to the shutdown pin. A typical quiescent current of

0.6 µA results when the supply voltage is applied to the shut-

down pin. In many applications, a microcontroller or micro-

processor output is used to control the shutdown circuitry

which provides a quick, smooth transition into shutdown. An-

other solution is to use a single-pole, single-throw switch that

when closed, is connected to ground and enables the ampli-

fier. If the switch is open, then a soft pull-up resistor of 47 k

Ω

will disable the LM4870. There are no soft pull-down resis-

tors inside the LM4870, so a definite shutdown pin voltage

must be applied externally, or the internal logic gate will be

left floating which could disable the amplifier unexpectedly.

HEADPHONE CONTROL INPUTS

The LM4870 possesses two headphone control inputs that

disable the amplifier and reduce I

either one or both of these inputs have a logic-high voltage

placed on their pins.

Unlike the shutdown function, the headphone control func-

tion does not provide the level of current conservation that is

required for battery powered systems. Since the quiescent

current resulting from the headphone control function is

1000 times more than the shutdown function, the residual

currents in the device may create a pop at the output when

coming out of the headphone control mode. The pop effect

may be eliminated by connecting the headphone sensing

output to the shutdown pin input as shown in

Figure 2. This

solution will not only eliminate the output pop, but will also

utilize the full current conservation of the shutdown function

by reducing I

shutdown. This configuration also allows the designer to use

the control inputs as either two headphone control pins or a

headphone control pin and a shutdown pin where the lowest

level of current consumption is obtained from either function.

Figure 3 shows the implementation of the LM4870’s head-

phone control function using a single-supply headphone am-

plifier. The voltage divider of R1 and R2 sets the voltage at

the HP-IN1 pin to be approximately 50 mV when there are

no headphones plugged into the system. This logic-low volt-

age at the HP-IN1 pin enables the LM4870 to amplify AC sig-

nals. Resistor R3 limits the amount of current flowing out of

the HP-IN1 pin when the voltage at that pin goes below

ground resulting from the music coming from the headphone

amplifier. The output coupling cap protects the headphones

by blocking the amplifier’s half-supply DC voltage. The ca-

pacitor also protects the headphone amplifier from the low

voltage set up by resistors R1 and R2 when there aren’t any

headphones plugged into the system. The tricky point to this

setup is that the AC output voltage of the headphone ampli-

fier cannot exceed the 2.0V HP-IN1 voltage threshold when

there aren’t any headphones plugged into the system, as-

suming that R1 and R2 are 100k and 1k, respectively. The

LM4870 may not be fully shutdown when this level is ex-

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