LP2953AI datasheet(12/22 Pages) NSC

12 / 22 page

Schematic Diagram

01112706

Application Hints

HEATSINK REQUIREMENTS (Industrial Temperature

Range Devices)

The maximum allowable power dissipation for the LP2952/

LP2953 is limited by the maximum junction temperature

(+125˚C) and the external factors that determine how quickly

heat flows away from the part: the ambient temperature and

the junction-to-ambient thermal resistance for the specific

application.

The industrial temperature range (−40˚C

≤ T

≤ +125˚C)

parts are manufactured in plastic DIP and surface mount

packages which contain a copper lead frame that allows

heat to be effectively conducted away from the die, through

the ground pins of the IC, and into the copper of the PC

board. Details on heatsinking using PC board copper are

covered later.

To determine if a heatsink is required, the maximum power

dissipated by the regulator, P(max), must be calculated. It is

important to remember that if the regulator is powered from

a transformer connected to the AC line, the maximum

specified AC input voltage must be used (since this pro-

duces the maximum DC input voltage to the regulator).

Figure 1 shows the voltages and currents which are present

in the circuit. The formula for calculating the power dissi-

pated in the regulator is also shown in Figure 1:

LP2953AI 数据表(PDF) 12 Page - National Semiconductor (TI)