LM4876

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SNAS054E – FEBRUARY 2000 – REVISED MAY 2013

For the VSSOP10A package,

for an ambient temperature of 25°C and using the same 5V power supply and an 8

Ω 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

8

Ω 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 9

Ω 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

the ground pin(s), supply pin and amplifier output pins. When adding a heat sink, the

and

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

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

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.

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