LME49610 datasheet(13/18 Pages) NSC

13 / 18 page

area attains a nominal value of 20°C/W junction to ambient

thermal resistance (

JA) under zero air flow.

30042562

FIGURE 6. Thermal Resistance (typ) for 5 lead TO-263

Package Mounted on 1oz. copper

A copper plane may be placed directly beneath the tab. Ad-

ditionally, a matching plane can be placed on the opposite

side. If a plane is placed on the side opposite of the

LME49610, connect it to the plane to which the buffer’s metal

tab is soldered with a matrix of thermal vias per JEDEC Stan-

dard JESD51-5.

Determining Copper Area

Find the required copper heat sink area using the following

guidelines:

1. Determine the maximum power dissipation of the

LME49610, P

2. Specify a maximum operating ambient temperature, T

(MAX). Note that the die temperature, TJ, will be higher than

A by an amount that is dependent on the thermal resistance

from junction to ambient,

JA. Therefore, TA must be specified

such that T

J does not exceed the absolute maximum die tem-

perature of 150°C.

3. Specify a maximum allowable junction temperature, T

(MAX), This is the LME49610’s die temperature when the buffer

is drawing maximum current (quiescent and load). It is pru-

dent to design for a maximum continuous junction tempera-

ture of 100°C to 130°C. Ensure, however, that the junction

temperature never exceeds the 150°C absolute maximum

rating for the part.

4. Calculate the value of junction to ambient thermal resis-

tance,

JA.

5. θ

JA as a function of copper area in square inches is shown

in Figure 6. Choose a copper area that will guarantee the

specified T

J(MAX) for the calculated θJA. The maximum value

of junction to ambient thermal resistance,

JA, is defined as:

JA = (TJ(MAX) - TA(MAX) ) / PD(MAX) (°C/W)

(1)

where:

J(MAX) = the maximum recommended junction temperature

A(MAX)

= the maximum ambient temperature in the

LME49610’s environment

D(MAX) = the maximum recommended power dissipation

Note: The allowable thermal resistance is determined by the

maximum allowable temperature increase:

RISE = TJ(MAX) - TA(MAX)

Thus, if ambient temperature extremes force T

RISE to exceed

the design maximum, the part must be de-rated by either de-

creasing P

D to a safe level, reducing θJA further, or, if avail-

able, using a larger copper area.

Procedure

1. First determine the maximum power dissipated by the

LME49610, P

D(MAX). For the simple case of the buffer driving

a resistive load, and assuming equal supplies, P

D(MAX) is giv-

en by:

DMAX(AC) = (IS x VS) + (VS)

2 / (2

π2R

(Watts)

(2)

DMAX(DC) = (IS x VS) + (VS)

2 / R

(Watts)

(3)

where:

S = |VEE| + VCC (V)

S = quiescent supply current (A)

Equation (2) is for sinusoidal output voltages and (3) is for DC

output voltages

2. Determine the maximum allowable die temperature rise,

RISE(MAX) = TJ(MAX) - TA(MAX)

°C

(4)

3. Using the calculated value of T

RISE(MAX) and PD(MAX), find

the required value of junction to ambient thermal resistance

combining equation 1 and equation 4 to derive equation 5:

JA = TRISE(MAX) / PD(MAX)

(°C/W)

(5)

4. Finally, choose the minimum value of copper area from

Figure 6 based on the value for

JA.

Example

Assume the following conditions: V

= |V

EE| + VCC = 30V,

L = 32Ω, IS = 15mA, sinusoidal output voltage, TJ(MAX) = 125°

C, T

A(MAX) = 85°C.

Applying Equation (2):

DMAX = (IS x VS) + (VS)

2 / 2

π2R

= (15mA)(30V) + 900V2

/ 632Ω

= 1.87W

Applying Equation (4):

RISE(MAX) = 125°C – 85°C

= 40°C

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