LM4880

SNAS103C – NOVEMBER 1995 – REVISED MAY 2013

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APPLICATION INFORMATION

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4880 contains a shutdown pin to externally turn off

the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the

shutdown pin. The trigger point between a logic low and logic high level is typically half supply. It is best to switch

between ground and the supply to provide maximum device performance. By switching the shutdown pin to VDD,

the LM4880 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown

pin voltages less than VDD, the idle current may be greater than the typical value of 0.7 μA. In either case, the

shutdown pin should be tied to a definite voltage because leaving the pin floating may result in an unwanted

shutdown condition.

In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which

provides 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 enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4880. This

scheme ensures that the shutdown pin will not float which will prevent unwanted state changes.

POWER DISSIPATION

Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to

ensure a successful design. Equation 1 states the maximum power dissipation point for a single-ended amplifier

operating at a given supply voltage and driving a specified output load.

PDMAX = (VDD)

2/(2π2R

L)

(1)

Since the LM4880 has two operational amplifiers in one package, the maximum internal power dissipation point

is twice that of the number which results from Equation 1. Even with the large internal power dissipation, the

LM4880 does not require heat sinking over a large range of ambient temperatures. From Equation 1, assuming a

5V power supply and an 8

Ω load, the maximum power dissipation point is 158 mW per amplifier. Thus the

maximum package dissipation point is 317 mW. The maximum power dissipation point obtained must not be

greater than the power dissipation that results from Equation 2:

PDMAX = (TJMAX-TA)/θJA

(2)

For the LM4880 surface mount package,

θJA = 170° C/W and TJMAX = 150°C. Depending on the ambient

temperature, TA, 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 decreased, the load impedance increased, or the ambient temperature

reduced. 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 96°C provided 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 maximum power dissipation point, the ambient temperature may be

increased accordingly. Refer to Typical Performance Characteristics for power dissipation information for lower

output powers.

POWER SUPPLY BYPASSING

As with any power 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. As displayed in Typical Performance Characteristics, the effect of a larger half supply bypass capacitor

is improved low frequency PSRR due to increased half-supply stability. Typical applications employ a 5V

regulator with 10

μF and a 0.1 μF bypass capacitors which aid in supply stability, but do not eliminate the need

for bypassing the supply nodes of the LM4880. The selection of bypass capacitors, especially CB, is thus

dependant upon desired low frequency PSRR, click and pop performance as explained in PROPER SELECTION

OF EXTERNAL COMPONENTS, system cost, and size constraints.

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