LP2953AI datasheet(13/22 Pages) NSC

13 / 22 page

Application Hints (Continued)

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

R(max). This is calculated

by using the formula:

R(max) = TJ(max) − TA(max)

(J–A) =TR(max)/P(max)

where: T

J(max)

the

maximum

allowable

junction

temperature

A(max) is the maximum ambient temperature

Using the calculated values for T

R(max) and P(max), the

required value for junction-to-ambient thermal resistance,

(J–A), can now be found:

The heatsink is made using the PC board copper. The heat

is conducted from the die, through the lead frame (inside the

part), and out the pins which are soldered to the PC board.

The pins used for heat conduction are given in Table 1.

TABLE 1. Heat Conducting Pins

Part

Package

Pins

LP2952IN, LP2952AIN,

14-Pin DIP

3, 4, 5,

LP2952IN-3.3,

LP2952AIN-3.3

10, 11, 12

LP2953IN, LP2953AIN,

16-Pin DIP

4, 5, 12, 13

LP2953IN-3.3,

LP2953AIN-3.3

LP2952IM, LP2952AIM,

16-Pin Surface

Mount

1, 8, 9, 16

LP2952IM-3.3,

LP2952AIM-3.3,

LP2953IM, LP2953AIM,

LP2953IM-3.3,

LP2953AIM-3.3

Figure 2 shows copper patterns which may be used to

dissipate heat from the LP2952 and LP2953. Table 2 shows

some values of junction-to-ambient thermal resistance (

J–A)

for values of L and W for 1 oz. copper.

TABLE 2. Thermal Resistance for Various Copper

Heatsink Patterns

Package

L (in.)

H (in.)

J–A (˚C/W)

16-Pin DIP

0.5

1.5

0.19

14-Pin DIP

0.5

1.5

Surface Mount

0.5

1.5

0.19

HEATSINK REQUIREMENTS (Military Temperature

Range Devices)

The

maximum

allowable

power

dissipation

for

the

LP2953AMJ is limited by the maximum junction temperature

(+150˚C) and the two parameters that determine how quickly

heat flows away from the die: the ambient temperature and

the junction-to-ambient thermal resistance of the part.

01112707

FIGURE 1. P

TOTAL =(VIN −VOUT)IL +(VIN)IG

Current/Voltage Diagram

01112708

* For best results, useL=2H

** 14-Pin DIP is similar, refer to Table 1 for pins designated for

heatsinking.

FIGURE 2. Copper Heatsink Patterns

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