LM4752 datasheet(17/22 Pages) NSC

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Application Information (Continued)

Device parameters from the datasheet:

J = 150˚C

Maximum junction temperature

JC = 2˚C/W

Junction-to-case thermal resistance

Calculations:

• P

DMAX =2 • [V S

2 /(2 •

π2 • R

L) ] = (24V)

2 /(2 •

π2 •

Ω) = 14.6W

≤ [(T

J −TA)/PDMAX]−

JC −

CS = [ (150˚C − 55˚C)

/ 14.6W ] − 2˚C/W − 0.2˚C/W = 4.3˚C/W

Conclusion: Choose a heatsink with

≤ 4.3˚C/W.

TO-263 Heatsink Design Examples

Example 1: (Stereo Single-Ended Output)

Given:

A=30˚C

J=150˚C

L=4

Ω

S=12V

JC=2˚C/W

DMAX from PD vs PO Graph:

DMAX

≈ 3.7W

Calculating P

DMAX:

DMAX =VCC

2 /(

π2R

L) = (12V)

2 /

π2(4Ω)) = 3.65W

Calculating Heatsink Thermal Resistance:

SA < [(T J −TA)/PDMAX]−

JC −

SA < 120˚C / 3.7W − 2.0˚C/W − 0.2˚C/W = 30.2˚C/W

Therefore the recommendation is to use 1.5 x 1.5 square

inch of single-sided copper clad.

Example 2: (Stereo Single-Ended Output)

Given:

A=50˚C

J=150˚C

L=4

Ω

S=12V

JC=2˚C/W

DMAX from PD vs PO Graph:

DMAX

≈ 3.7W

Calculating P

DMAX:

DMAX =VCC

2 /(

π2R

L) = (12V)

2 /(

π2(4Ω)) = 3.65W

Calculating Heatsink Thermal Resistance:

SA < [(TJ −TA)/PDMAX]−

JC −

SA < 100˚C / 3.7W − 2.0˚C/W − 0.2˚C/W = 24.8˚C/W

Therefore the recommendation is to use 2.0 x 2.0 square

inch of single-sided copper clad.

Example 3: (Bridged Output)

Given:

A=50˚C

J=150˚C

L=8

Ω

S=12V

JC=2˚C/W

Calculating P

DMAX:

DMAX = 4[VCC

2 /(2

π2R

L)] = 4(12V)

2 /(2

π2(8Ω)) = 3.65W

Calculating Heatsink Thermal Resistance:

SA < [(TJ −TA)/PDMAX]−

JC −

SA < 100˚C / 3.7W − 2.0˚C/W − 0.2˚C/W = 24.8˚C/W

Therefore the recommendation is to use 2.0 x 2.0 square

inch of single-sided copper clad.

Layout and Ground Returns

Proper PC board layout is essential for good circuit perfor-

mance. When laying out a PC board for an audio power

amplifer, particular attention must be paid to the routing of

the output signal ground returns relative to the input signal

and bias capacitor grounds. To prevent any ground loops,

the ground returns for the output signals should be routed

separately and brought together at the supply ground. The

input signal grounds and the bias capacitor ground line

should also be routed separately. The 0.1 µF high frequency

supply bypass capacitor should be placed as close as pos-

sible to the IC.

LM4752 Datasheet(PDF) 17 Page - National Semiconductor (TI)

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