LM4876
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SNAS054E –FEBRUARY 2000–REVISED MAY 2013
For the VSSOP10A package, θJA = 210°C/W. Equation 6 shows that TJMAX , for the VSSOP10 package, is 158°C
for an ambient temperature of 25°C and using the same 5V power supply and an 8Ω load. This violates the
LM4876's 150°C maximum junction temperature when using the VSSOP10A package. Reduce the junction
temperature by reducing the power supply voltage or increasing the load resistance. Further, allowance should
be made for increased ambient temperatures. To achieve the same 61°C maximum ambient temperature found
for the SOIC8 package, the VSSOP10 packaged part should operate on a 4.1V supply voltage when driving an
8Ω load. Alternatively, a 5V supply can be used when driving a load with a minimum resistance of 12Ω for the
same 61°C maximum ambient temperature.
Fully charged Li-ion batteries typically supply 4.3V to portable applications such as cell phones. This supply
voltage allows the LM4876 to drive loads with a minimum resistance of 9Ω without violating the maximum
junction temperature when the maximum ambient temperature is 61°C.
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 3 is greater than that of Equation 4, 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 θJA. 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. When adding a heat sink, the θJA is the sum of θJC, θCS
,
and θSA. ( θJC is the junction-to-case thermal impedance, θCS is the case-to-sink thermal impedance, and θSA is
the sink-to-ambient thermal impedance.) Refer to the Typical Performance Characteristics curves for power
dissipation information at lower output power levels.
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 10µF 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 local bypass capacitance at the LM4876's supply pins.
Keep the length of leads and traces that connect capacitors between the LM4876's power supply pin and ground
as short as possible. Connecting a 1µF capacitor 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, and the amplifier's click and pop performance can be
compromised. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements,
click and pop performance (as explained in the section, SELECTING PROPER EXTERNAL COMPONENTS),
system cost, and size constraints.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the LM4876's shutdown function. Activate micro-power
shutdown by applying a voltage below 400mV to the SHUTDOWN pin. When active, the LM4876's micro-power
shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. Though the LM4876 is in
shutdown when 400mV is applied to the SHUTDOWN pin, the supply current may be higher than 0.01µA (typ)
shutdown current. Therefore, for the lowest supply current during shutdown, connect the SHUTDOWN pin to
ground. The relationship between the supply voltage, the shutdown current, and the voltage applied to the
SHUTDOWN pin is shown in Typical Performance Characteristics curves.
There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw
switch, a microprocessor, or a microcontroller. When using a switch, connect an external pull-down resistor
between the SHUTDOWN pin and GND. Connect the switch between the SHUTDOWN pin and VCC. Select
normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to GND
through the pull-down resistor, activating micro-power shutdown. The switch and resistor ensure that the
SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or a
microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN
pin with active circuitry eliminates the pull down resistor.
Copyright © 2000–2013, Texas Instruments Incorporated
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