LM4902

SNAS150D – DECEMBER 2001 – REVISED APRIL 2013

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Since the LM4902 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 increase in power dissipation, the LM4902 does

not require heatsinking. From Equation 2, 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 3 must not be greater

than the power dissipation that results from Equation 4:

(4)

For package DGK,

of the system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by

the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage

must be decreased, the load impedance increased, the ambient temperature reduced, or the

larger areas of copper provide a form of heatsinking allowing a higher power dissipation. 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 30°C provided that device operation is around the maximum

power dissipation point. Internal power dissipation is a function of output power. If typical operation is not around

the maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical

Performance Characteristics

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. The effect of a larger half supply bypass capacitor is improved PSRR due to increased half-supply

stability. Typical applications employ a 5V regulator with 10

μF and a 0.1μF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the supply nodes of the LM4902. The selection of bypass

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 LM4902 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. The trigger point between a logic low and logic high level is typically half supply. It is best to switch

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

the LM4902 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown

pin voltages greater than GND, the idle current may be greater than the typical value of 0.1

μA. 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

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 disables the amplifier. If the switch is open, then the external pull-up resistor will enable the LM4902. This

scheme ensures that the shutdown pin will not float, thus preventing unwanted state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications using integrated power amplifiers is critical to optimize

device and system performance. While the LM4902 is tolerant to a variety of external component combinations,

consideration to component values must be used to maximize overall system quality.

The LM4902 is unity-gain stable, giving a designer maximum system flexibility. The LM4902 should be used in

low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain

configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1

Vrms are available from sources such as audio codecs. Please refer to the section, AUDIO POWER AMPLIFIER

DESIGN, for a more complete explanation of proper gain selection.

Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the

forms a first order high pass filter which limits low frequency response. This value should be chosen based on

needed frequency response for a few distinct reasons.

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