LM4864

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SNAS109F – SEPTEMBER 1999 – REVISED MAY 2013

EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION

The LM4864's exposed-dap (die attach paddle) package (NGY) provides a low thermal resistance between the

die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the

surrounding PCB copper traces, ground plane, and surrounding air.

The NGY package should have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad may

be connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink, and

radiation area.

Further detailed and specific information concerning PCB layout, fabrication, and mounting an NGY (WSON)

package is available from Texas Instruments's Package Engineering Group under application note AN1187.

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 0.1 μF bypass capacitors which aid in

supply stability, but do not eliminate the need for bypassing the supply nodes of the LM4864. The selection of

performance as explained in PROPER SELECTION OF EXTERNAL COMPONENTS, system cost, and size

constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4864 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

LM4864 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin

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 enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4864. 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 LM4864 is tolerant to a variety of external component combinations,

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

The LM4864 is unity-gain stable, giving a designer maximum system flexibility. The LM4864 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 toAUDIO 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.

Selection of Input Capacitor Size

Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized

capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have little ability to reproduce signals below 150 Hz. In

this case using a large input capacitor may not increase system performance.

Copyright © 1999–2013, Texas Instruments Incorporated

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