LMH6321 datasheet(17/21 Pages) NSC

17 / 21 page

Application Hints (Continued)

THERMAL MANAGEMENT

Heatsinking

For some applications, a heat sink may be required with the

LMH6321. This depends on the maximum power dissipation

and maximum ambient temperature of the application. To

accomplish heat sinking, the tabs on TO-263 and PSOP

package may be soldered to the copper plane of a PCB for

heatsinking (note that these tabs are electrically connected

to the most negative point in the circuit, i. e.,V

−).

Heat escapes from the device in all directions, mainly

through the mechanisms of convection to the air above it and

conduction to the circuit board below it and then from the

board to the air. Natural convection depends on the amount

of surface area that is in contact with the air. If a conductive

plate serving as a heatsink is thick enough to ensure perfect

thermal conduction (heat spreading) into the far recesses of

the plate, the temperature rise would be simply inversely

proportional to the total exposed area. PCB copper planes

are, in that sense, an aid to convection, the difference being

that they are not thick enough to ensure perfect conduction.

Therefore, eventually we will reach a point of diminishing

returns (as seen in Figure 5). Very large increases in the

copper area will produce smaller and smaller improvement

in thermal resistance. This occurs, roughly, for a 1 inch

square of 1 oz copper board. Some improvement continues

until about 3 square inches, especially for 2 oz boards and

better, but beyond that, external heatsinks are required.

Ultimately, a reasonable practical value attainable for the

junction to ambient thermal resistance is about 30 ˚C/W

under zero air flow.

A copper plane of appropriate size may be placed directly

beneath the tab or on the other side of the board. If the

conductive plane is placed on the back side of the PCB, it is

recommended that thermal vias be used per JEDEC Stan-

dard JESD51-5.

Determining Copper Area

One can determine the required copper area by following a

few basic guidelines:

Determine the value of the circuit’s power dissipation,

Specify a maximum operating ambient temperature,

A(MAX). Note that when specifying this parameter, it

must be kept in mind that, because of internal tempera-

ture rise due to power dissipation, the die temperature,

J, will be higher than TA by an amount that is depen-

dent on the thermal resistance from junction to ambient,

JA. Therefore, TA must be specified such that TJ does

not exceed the absolute maximum die temperature of

150˚C.

Specify a maximum allowable junction temperature, T

J-

(MAX)

, which is the temperature of the chip at maximum

operating current. Although no strict rules exist, typically

one should design for a maximum continuous junction

temperature of 100˚C to 130˚C, but no higher than

150˚C which is the absolute maximum rating for the part.

Calculate the value of junction to ambient thermal resis-

tance,

Choose a copper area that will guarantee the specified

J(MAX) for the calculated

JA.

JA as a function of cop-

per area in square inches is shown in Figure 4.

The maximum value of thermal resistance, junction to ambi-

ent

JA, is defined as:

JA =(TJ(MAX) -TA(MAX) )/ PD(MAX)

(6)

where:

J(MAX) = the maximum recommended junction temperature

A(MAX) = the maximum ambient temperature in the user’s

environment

D(MAX) = the maximum recommended power dissipation

Note: The allowable thermal resistance is determined by the maximum

allowable heat rise , TRISE =TJ(MAX) -TA(MAX) =(θJA)(PD(MAX)). Thus,

if ambient temperature extremes force TRISE to exceed the design

maximum, the part must be de-rated by either decreasing PD to a safe

level, reducing

θJA, further, or, if available, using a larger copper area.

Procedure

First determine the maximum power dissipated by the

buffer, P

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

a resistive load, and assuming equal supplies, P

D(MAX)

is given by

D(MAX) =IS (2V

+)+V+2/4R

(7)

where: I

S = quiescent supply current

Determine the maximum allowable die temperature rise,

R(MAX) =TJ(MAX)-TA(MAX) =PD(MAX)

(8)

Using the calculated value of T

R(MAX) and PD(MAX) the

required value for junction to ambient thermal resistance

can be found:

JA =TR(MAX)/PD(MAX)

(9)

Finally, using this value for

JA choose the minimum

value of copper area from Figure 4.

Example

Assume the following conditions:

+ =V− = 15V, R

L =50

Ω,I

S =15mA TJ(MAX) = 125˚C,

A(MAX) = 85˚C.

From (7)

D(MAX)

(2V

+)+V+2/4R

= (15 mA)(30V) +

225V

2/200

Ω = 1.58W

From (8)

R(MAX) = 125˚C - 85˚C = 40˚C

From (9)

JA = 40˚C/1.58W = 25.3˚C/W

Examining the plot of Copper Area vs.

JA, we see that we

cannot attain this low of a thermal resistance for one layer of

1 oz copper. It will be necessary to derate the part by

decreasing either the ambient temperature or the power

dissipation. Other solutions are to use two layers of 1 oz foil,

or use 2 oz copper (see Table 1), or to provide forced air flow.

One should allow about an extra 15% heat sinking capability

for safety margin.

LMH6321 Datasheet(PDF) 17 Page - National Semiconductor (TI)

LMH6321 Datasheet(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL