LM4950

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SNAS174E – JULY 2003 – REVISED MAY 2013

POWER DISSIPATION

Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation 2

states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and

driving a specified output load.

PDMAX-SE = (VDD)

2/ (2π2R

L):

Single Ended

(2)

The LM4950's dissipation is twice the value given by Equation 2 when driving two SE loads. For a 12V supply

and two 8

Ω SE loads, the LM4950's dissipation is 1.82W.

The LM4950's dissipation when driving a BTL load is given by Equation 3. For a 12V supply and a single 8

Ω BTL

load, the dissipation is 3.65W.

PDMAX-MONOBTL = 4(VDD)

2/ 2π2R

L:

Bridge Mode

(3)

The maximum power dissipation point given by Equation 3 must not exceed the power dissipation given by

Equation 4:

PDMAX' = (TJMAX - TA) / θJA

(4)

The LM4950's TJMAX = 150°C. In the KTW package, the LM4950's θJA is 20°C/W when the metal tab is soldered

to a copper plane of at least 16in2. This plane can be split between the top and bottom layers of a two-sided

PCB. Connect the two layers together under the tab with a 5x5 array of vias. For the NEC package, use an

external heatsink with a thermal impedance that is less than 20°C/W. At any given ambient temperature TA, use

Equation 4 to find the maximum internal power dissipation supported by the IC packaging. Rearranging

Equation 4 and substituting PDMAX for PDMAX' results in Equation 5. This equation gives the maximum ambient

temperature that still allows maximum stereo power dissipation without violating the LM4950's maximum junction

temperature.

TA = TJMAX - PDMAX-MONOBTLθJA

(5)

For a typical application with a 12V power supply and a BTL 8

Ω load, the maximum ambient temperature that

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

77°C for the KTW package.

TJMAX = PDMAX-MONOBTLθJA + TA

(6)

Equation 6 gives the maximum junction temperature TJMAX. If the result violates the LM4950'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 (SE

operation) or 8

Ω (BTL operation) do not fall below 3Ω or 6Ω, respectively. 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 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 LM4950'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 LM4950's power supply pin and ground as short as possible. Connecting a 10µF capacitor, CBYPASS, between

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