Datasheet

LM4864
www.ti.com 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
bypass capacitors, especially C
B
, is thus dependent upon desired PSRR requirements, click and pop
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
between ground and supply to provide maximum device performance. By switching the shutdown pin to V
DD
, the
LM4864 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin
voltages less than V
DD
, 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 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
bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, C
i
,
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.
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