LM4780 datasheet(16/24 Pages) NSC

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Application Information (Continued)

DETERMINING THE CORRECT HEAT SINK

The choice of a heat sink for a high-power audio amplifier is

made entirely to keep the die temperature at a level such

that the thermal protection circuitry is not activated under

normal circumstances.

The thermal resistance from the die to the outside air,

(junction to ambient), is a combination of three thermal re-

sistances,

JC (junction to case),

CS (case to sink), and

(sink to ambient). The thermal resistance,

JC (junction to

case), of the LM4780T is 0.8˚C/W. Using Thermalloy Ther-

macote thermal compound, the thermal resistance,

(case to sink), is about 0.2˚C/W. Since convection heat flow

(power dissipation) is analogous to current flow, thermal

resistance is analogous to electrical resistance, and tem-

perature drops are analogous to voltage drops, the power

dissipation out of the LM4780 is equal to the following:

DMAX =(TJMAX−TAMB)/

(3)

where T

JMAX = 150˚C, TAMB is the system ambient tempera-

ture and

JA =

JC +

CS +

SA.

20058652

Once the maximum package power dissipation has been

calculated using Equation (2), the maximum thermal resis-

tance,

SA, (heat sink to ambient) in ˚C/W for a heat sink can

be calculated. This calculation is made using Equation (4)

which is derived by solving for

SA in Equation (3).

SA = [(TJMAX−TAMB)−PDMAX(

JC +

CS)]/PDMAX

(4)

Again it must be noted that the value of

SA is dependent

upon the system designer’s amplifier requirements. If the

ambient temperature that the audio amplifier is to be working

under is higher than 25˚C, then the thermal resistance for the

heat sink, given all other things are equal, will need to be

smaller.

SUPPLY BYPASSING

The LM4780 has excellent power supply rejection and does

not require a regulated supply. However, to improve system

performance as well as eliminate possible oscillations, the

LM4780 should have its supply leads bypassed with low-

inductance capacitors having short leads that are located

close to the package terminals. Inadequate power supply

bypassing will manifest itself by a low frequency oscillation

known as “motorboating” or by high frequency instabilities.

These instabilities can be eliminated through multiple by-

passing utilizing a large tantalum or electrolytic capacitor

(10µF or larger) which is used to absorb low frequency

variations and a small ceramic capacitor (0.1µF) to prevent

any high frequency feedback through the power supply lines.

If adequate bypassing is not provided, the current in the

supply leads which is a rectified component of the load

current may be fed back into internal circuitry. This signal

causes distortion at high frequencies requiring that the sup-

plies be bypassed at the package terminals with an electro-

lytic capacitor of 470µF or more.

BRIDGED AMPLIFIER APPLICATION

The LM4780 has two operational amplifiers internally, allow-

ing for a few different amplifier configurations. One of these

configurations is referred to as “bridged mode” and involves

driving the load differentially through the LM4780’s outputs.

This configuration is shown in Figure 2. Bridged mode op-

eration is different from the classical single-ended amplifier

configuration where one side of its load is connected to

ground.

A bridge amplifier design has a distinct advantage over the

single-ended configuration, as it provides differential drive to

the load, thus doubling output swing for a specified supply

voltage. Theoretically, four times the output power is pos-

sible as compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or clipped.

A direct consequence of the increased power delivered to

the load by a bridge amplifier is an increase in internal power

dissipation. For each operational amplifier in a bridge con-

figuration, the internal power dissipation will increase by a

factor of two over the single ended dissipation. Thus, for an

audio power amplifier such as the LM4780, which has two

operational amplifiers in one package, the package dissipa-

tion will increase by a factor of four. To calculate the

LM4780’s maximum power dissipation point for a bridged

load, multiply Equation (2) by a factor of four.

This value of P

DMAX can be used to calculate the correct size

heat sink for a bridged amplifier application. Since the inter-

nal dissipation for a given power supply and load is in-

creased by using bridged-mode, the heatsink’s

SA will have

to decrease accordingly as shown by Equation (4). Refer to

the section, Determining the Correct Heat Sink, for a more

detailed discussion of proper heat sinking for a given appli-

cation.

PARALLEL AMPLIFIER APPLICATION

Parallel configuration is normally used when higher output

current is needed for driving lower impedance loads (i.e. 4

Ω

or lower) to obtain higher output power levels. As shown in

Figure 3 , the parallel amplifier configuration consist of de-

signing the amplifiers in the IC to have identical gain, con-

necting the inputs in parallel and then connecting the outputs

in parallel through a small external output resistor. Any num-

ber of amplifiers can be connected in parallel to obtain the

needed output current or to divide the power dissipation

across multiple IC packages. Ideally, each amplifier shares

the output current equally. Due to slight differences in gain

the current sharing will not be equal among all channels. If

current is not shared equally among all channels then the

power dissipation will also not be equal among all channels.

It is recommended that 0.1% tolerance resistors be used to

set the gain (R

i and Rf) for a minimal amount of difference in

current sharing.

When operating two or more amplifiers in parallel mode the

impedance seen by each amplifier is equal to the total load

impedance multiplied by the number of amplifiers driving the

load in parallel as shown by Equation (5) below:

L(parallel) =RL(total) * Number of amplifiers

(5)

Once the impedance seen by each amplifier in the parallel

configuration is known then Equation (2) can be used with

this calculated impedance to find the amount of power dis-

sipation for each amplifier. Total power dissipation (P

DMAX)

within an IC package is found by adding up the power

dissipation for each amplifier in the IC package. Using the

calculated P

DMAX the correct heat sink size can be deter-

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