LM4811
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SNAS119D –DECEMBER 2000–REVISED APRIL 2013
For the VSSOP package, θJA = 194°C/W, and for the WSON/SON package, θJA = 63°C/W. TJMAX = 150°C for the
LM4811. For a given ambient temperature, TA, of the system surroundings, Equation 1 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 TA
reduced. For the VSSOP package in a typical application of a 5V power supply and a 32Ωload, the maximum
ambient temperature possible without violating the maximum junction temperature is approximately 134.5°C. This
assumes the device operates at maximum power dissipation and uses surface mount packaging. Internal power
dissipation is a function of output power. If typical operation is not around the maximum power dissipation point,
operation at higher ambient temperatures is possible. Refer to Typical Performance Characteristics for power
dissipation information for lower output power levels.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4811's exposed-dap (die attach paddle) package (WSON/SON) 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 WSON/SON 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.
However, since the LM4811 is designed for headphone applications, connecting a copper plane to the DAP's
PCB copper pad is not required. Figure 32 in Typical Performance Characteristics shows that the maximum
power dissipated is just 45mW per amplifier with a 5V power supply and a 32Ωload.
Further detailed and specific information concerning PCB layout, fabrication, and mounting an WSON/SON
package is available from Texas Instruments' 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 value of the bypass capacitor directly affects the LM4811's half-supply voltage stability and PSRR.
The stability and supply rejection increase as the bypass capacitor's value increases. Typical applications employ
a 5V regulator with 10µF and a 0.1µF bypass capacitors which aid in supply stability, but do not eliminate the
need for bypassing the supply nodes of the LM4811. The selection of bypass capacitors, especially CB, is thus
dependent upon desired low frequency PSRR, click and pop performance, (explained in PROPER SELECTION
OF EXTERNAL COMPONENTS), system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not is use, the LM4811 features amplifier bias circuitry shutdown.
This shutdown function is activated by applying a logic high to the SHUTDOWN pin. The trigger point is 1.4V
minimum for a logic high level, and 0.4V maximum for a logic low level. It is best to switch between ground and
VDD to ensure optimal shutdown operation. By switching the SHUTDOWN pin to VDD, the LM4811 supply current
draw will be minimized in idle mode. Whereas the device will be disabled with shutdown voltages less than VDD,
the idle current may be greater than the typical value of 0.3µA. In either case, the SHUTDOWN pin should be
tied to a fixed voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry. This
provides a quick, smooth shutdown transition. 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, the external pull-up resistor, RPU, will disable the LM4811.
This scheme ensures that the SHUTDOWN pin will not float, thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated power amplifiers is critical for optimum device and
system performance. While the LM4811 is tolerant of external component combinations, consideration must be
given to the external component values that maximize overall system quality.
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