LM4940

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SNAS219C – OCTOBER 2003 – REVISED MAY 2013

TA = TJMAX - PDMAX-SEθJA

(3)

For a typical application with a 12V power supply and two 4

Ω SE loads, the maximum ambient temperature that

allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately

113°C for the KTW package.

TJMAX = PDMAX-SEθJA + TA

(4)

Equation 4 gives the maximum junction temperature TJMAX. If the result violates the LM4940'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 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. Further, ensure that speakers rated at a nominal 4

Ω do not fall

below 3

Ω. 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 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 LM4940'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

the LM4940's power supply pin and ground as short as possible. Connecting a 10µF capacitor, CBYPASS, 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 CBYPASS, depends on desired PSRR requirements, click and pop performance (as explained in

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

MICRO-POWER SHUTDOWN

The LM4940 features an active-low shutdown mode that disables the amplifier's bias circuitry, reducing the

supply current to 40

μA (typ). Connect SHUTDOWN to a voltage between 2V to VDD/2 for normal operation.

Connect SHUTDOWN to GND to disable the device. A voltage that is greater than GND can increase shutdown

current.

Copyright © 2003–2013, Texas Instruments Incorporated

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