LM4951

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SNAS244N – AUGUST 2004 – REVISED MAY 2013

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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 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 2 is greater than that of Equation (3), then decrease the supply voltage, increase the

load impedance, or reduce the ambient temperature. Further, ensure that speakers rated at a nominal 8

Ω do not

fall below 6

created using additional copper area around the package, with connections to the ground pins, supply pin and

amplifier output pins. Refer to the Typical Performance Characteristics curves for power dissipation information at

lower output power levels.

POWER SUPPLY VOLTAGE LIMITS

Continuous proper operation is ensured by never exceeding the voltage applied to any pin, with respect to

ground, as listed in the Absolute Maximum Ratings section.

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 voltage 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 a local 1.0µF tantalum bypass capacitance

connected between the LM4951's supply pins and ground. Do not substitute a ceramic capacitor for the

tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between

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

explained in the section, SELECTING EXTERNAL COMPONENTS), system cost, and size constraints.

MICRO-POWER SHUTDOWN

The LM4951 features an active-low micro-power shutdown mode. When active, the LM4951's micro-power

shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The low 0.01µA typical

shutdown current is achieved by applying a voltage to the SHUTDOWN pin that is as near to GND as possible. A

voltage that is greater than GND may increase the shutdown current.

There are a few methods to control the micro-power shutdown. These include using a single-pole, single-throw

switch (SPST), a microprocessor, or a microcontroller. When using a switch, connect the SPST switch between

GND to the SHUTDOWN pin activating micro-power shutdwon.The switch and internal pull-down resistor

ensures 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 active-state voltage to the SHUTDOWN pin.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires

amplification and desired output transient suppression.

frequency that is found using Equation 6.

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