LM4652 datasheet(15/20 Pages) NSC

15 / 20 page

Application Information (Continued)

1 should be in the range of 0.1µF to 1µF or 2 - 20% of CBYP.

fl1 and Rfl2 are found by the ratio Rfl1 = 10Rfl2.

A lower ratio can be used if the application is for lower output

voltages than the 125Watt, 4

Ω solution show here.

The feedback RC filter’s pole location should be higher than

the output filter pole. The reason for two capacitors in paral-

lel instead of one larger capacitor is to reduce the possible

EMI from the feedback traces. C

fl1 is placed close as pos-

sible to the output of the LM4652 so that an audio signal is

present on the feedback trace instead of a high frequency

square wave. C

fl2 is then placed as close as possible to the

feedback inputs (pins 14, 19) of the LM4651 to filter off any

noise picked up by the feedback traces. The combination

lowers EMI and provides a cleaner audio feedback signal to

the LM4651. R

f should be in range of 100kΩ to1MΩ.Cf

controls the bandwidth of the error signal and should be in

the range of 100pF to 470pF.

Determine the Value for C

START (Start Up Delay)

The start-up delay is chosen to be 1 second to ensure

minimum pops or clicks when the amplifier is powered up.

Using Equation (2), the value of C

START is 11.7µF. A standard

value of 10µF is used.

Determine the Value of Gain, R

1, and R2

The gain is set to produce a 125W output at no more than

1% distortion with a 3V

RMS input. A dissipation of 125W in a

Ω load requires a 22.4V

RMS signal. To produce this output

signal, the LM4651/LM4652 amplifier needs an overall

closed-loop gain of 22.4V

RMS/3VRMS, or 7.5V/V (17.5db).

Equation (12) shows all the variables that affect the system

gain.

Gain = [(R

2/R1) x ((Rfl1 +Rfl2)/ Rfl2)−(R2/R1) + .5]. (3)

The values for R

fI1,RfI2, and Rf were found in the Determine

the Value of the Filters section above. Therefore, R

fI1 =

620k

Ω,R

fI2 = 62kΩ and Rf = 390kΩ. The value of VCC was

also found as the first step in this example to be ±20V.

Inserting these values into equation (12) and reducing gives

the equation below:

2 = .7R1

(4)

The input resistance is desired to be 20k

Ω so R

1 is set to

20k

Ω.R

2 is then found to be 14kΩ.

Lowering R

2 direcly affects the noise of the system. Chang-

ing R

1 to increase gain with the lower value for R2 has very

little affect on the noise level. The percent change in noise is

about what whould be expected with a higher gain. The

drawback to a lower R

1 value is a larger CIN value, neces-

sary to properly couple the lowest desired signal frequen-

cies. If a 20k

Ω input impedance is not required, then the

recommended values shown in

Figure 1, Typical Audio

Application Circuit should be used: with R

1’s value set to

4.7k

Ω and R

2’s value set to 3.5kΩ for a gain 7.5V/V.

Determine the Needed Heat Sink

The only remaining design requirement is a thermal design

that prevents activating the thermal protection circuitry. Use

Equations (9) - (11) to calculate the amount of power dissi-

pation for the LM4652. The appropriate heat sink size, or

thermal resistance in ˚C/W, will then be determined.

Equation (9) determines the percentage of loss caused by

the switching. Use the typical values given in the Electrical

Characteristics for the LM4651 and Electrical Character-

istics for the LM4652 tables for the rise time, fall time and

over modulation time:

%Loss = (25ns+26ns+350ns) * 125kHz

%Loss = 5.0%

This switching loss causes a maximum power dissipation,

using Equation (10), of:

DSWITCH = (5.0% * 125W) / (1−5.0%)

DSWITCH = 6.6W

Next the power dissipation caused by the R

DS(ON) of the

output FETs is found by multiplying the output current times

the R

DS(ON). Again, the value for RDS(ON) is found from the

Electrical Characteristics for the LM4652 table above.

The value for R

DS(ON) at 100˚C is used since we are calcu-

lating the maximum power dissipation.

OUTRMS = SQRT(125watts/4Ω) = 5.59 amps

RDS(ON) = (5.59A)

2 * (0.230

Ω*2)

RDS(ON) = 14.4W

The total power dissipation in the LM4652 is the sum of

these two power losses giving:

DTOTAL = 6.6W + 14.4W = 21W

The value for Maximum Power Dissipation given in the Sys-

tem Electrical Characteristics for the LM4651 and

LM4652 is 22 watts. The difference is due to approximately

1 watt of power loss in the LM4651. The above calculations

are for the power loss in the LM4652.

Lastly, use Equation (11) to determine the thermal resistance

of the LM4652’s heat sink. The values for

JC and TJMAX are

found in the Operating Ratings and the Absolute Maxi-

mum Ratings section above for the LM4652. The value of

JC is 2˚C/W for the isolated (TF) package or 1˚C/W for the

non-isolated (T) package. The value for T

JMAX is 150˚C. The

value for

CS is set to 0.2˚C/W since this is a reasonable

value when thermal grease is used. The maximum ambient

temperature from the design requirements is 50˚. The value

SA for the isolated (TF) package is:

SA = [(150˚C − 50˚C)/21W] − 2˚C/W − 0.2˚C/W

SA = 2.5˚C/W

and for the non-isolated (T) package without a mica washer

to isolate the heat sink from the package:

SA = [(150˚C − 50˚C)/21W] − 1˚C/W − 0.2˚C/W

SA = 3.5˚C/W

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