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LM4947 Datasheet(PDF) 28 Page - Texas Instruments
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SNAS349D – JUNE 2006 – REVISED MAY 2013
LM4947 and in the transducer load. The amount of power dissipation in the LM4947 is very low. This is because
the ON resistance of the switches used to form the output waveforms is typically less than 0.25
Ω. This leaves
only the transducer load as a potential "sink" for the small excess of input power over audio band output power.
The LM4947 dissipates only a fraction of the excess power requiring no additional PCB area or copper plane to
act as a heat sink.
The LM4947 also has a pair of single-ended amplifiers driving stereo headphones, R
. The maximum
internal power dissipation for R
is given by Equation 7 and Equation 8. From Equation 7 and
Equation 8, assuming a 5V power supply and a 32
Ω load, the maximum power dissipation for L
40mW, or 80mW total.
): Single-ended Mode
): Single-ended Mode
The maximum internal power dissipation of the LM4947 occurs when all 3 amplifiers pairs are simultaneously on;
and is given by Equation 9.
The maximum power dissipation point given by Equation 9 must not exceed the power dissipation given by
) / θ
The LM4947's T
= 150°C. In the ITL package, the LM4947's θ
is 65°C/W. At any given ambient
, use Equation 10 to find the maximum internal power dissipation supported by the IC packaging.
Rearranging Equation 10 and substituting P
' results in Equation 11. This equation gives the
maximum ambient temperature that still allows maximum stereo power dissipation without violating the LM4947's
maximum junction temperature.
For a typical application with a 5V power supply and an 8
Ω load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 104°C
for the ITL package.
Equation 12 gives the maximum junction temperature T
. If the result violates the LM4947's 150°C, reduce
the maximum junction temperature by reducing the power supply voltage or increasing the load resistance.
Further allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface mount part operating around the maximum power
dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are
allowed as output power or duty cycle decreases. If the result of Equation 9 is greater than that of Equation 10,
then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to reduce
. The heat sink can be created using additional
copper area around the package, with connections to the ground pin(s), supply pin and amplifier output pins.
External, solder attached SMT heatsinks such as the Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the
is the sum of θ
, and θ
is the junction-to-case thermal impedance,
is the case-to-sink thermal impedance, and θ
is the sink-to-ambient thermal impedance). Refer to the
TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation information at lower output power
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically use a 1µF in parallel with a 0.1µF filter capacitors to
stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response.
However, their presence does not eliminate the need for a local 1.1µF tantalum bypass capacitance connected
between the LM4947's supply pins and ground. Keep the length of leads and traces that connect capacitors
between the LM4947's power supply pin and ground as short as possible. Connecting a 2.2µF capacitor, C
between the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's
PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however,
increases turn-on time and can compromise the amplifier's click and pop performance. The selection of bypass
capacitor values, especially C
, depends on desired PSRR requirements, click and pop performance (as
explained in the section, SELECTING EXTERNAL COMPONENTS), system cost, and size constraints.
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