LP2956 datasheet(12/20 Pages) NSC

12 / 20 page

Application Hints

HEATSINK REQUIREMENTS

A heatsink may be required with the LP2956 depending on

the maximum power dissipation and maximum ambient tem-

perature of the application. Under all expected operating

conditions, the junction temperature must be within the range

specified under Absolute Maximum Ratings.

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 speci-

fied AC input voltage must be used (since this produces 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 dissipated in the regulator

is also shown in Figure 1 (the currents and power due to ex-

ternal resistive dividers are not included, and are typically

negligible).

1133909

FIGURE 1. Current/Voltage Diagram

The next parameter which must be calculated is the maximum

allowable temperature rise, T

R(max). This is calculated by

using the formula:

R(max) = TJ(max) − T A(max)

where: T

J(max) is the maximum allowable junction temper-

ature

A(max) is the maximum ambient temperature

Using the calculated values for T

R(max) and P(max), the re-

quired value for junction-to-ambient thermal resistance,

(J-

A), can now be found:

θ(

J-A) = TR(max)/P(max)

The heatsink for the LP2956 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 shown

in Table 1.

TABLE 1.

Part

Package

Pins

LP2956IN

16-Pin Plastic DIP

4, 5, 12, 13

LP2956AIN

16-Pin Plastic DIP

4, 5, 12, 13

LP2956IM

16-Pin Surface Mt.

1, 8, 9, 16

LP2956AIM

16-Pin Surface Mt.

1, 8, 9, 16

Figure 2 shows copper patterns which may be used to dissi-

pate heat from the LP2956:

1133910

*For best results, use L = 2H

FIGURE 2. Copper Heatsink Patterns

Table 2 shows some typical values of junction-to-ambient

thermal resistance (

J-A) for values of L and W (1 oz. copper).

TABLE 2.

Package

L (In.)

H (In.)

J-A (°C/W)

16-Pin

Plastic

0.5

DIP

1.5

0.19

16-Pin

0.5

Surface

Mount

1.5

0.19

EXTERNAL CAPACITORS

A 2.2

μF (or greater) capacitor is required between the main

output pin and ground to assure stability. The auxiliary output

requires 10

μF to ground. Without these capacitors, the part

may oscillate. Most types of tantalum or aluminum electrolyt-

ics will work here. Film types will work, but are more expen-

sive. Many aluminum electrolytics contain electrolytes which

freeze at −30°C, which requires the use of solid tantalums

below −25°C. The important characteristic of the capacitors

is an ESR of 5

Ω (or less) on the main regulator output and an

ESR of 1

Ω (or less) on the auxiliary regulator output (the ESR

may increase by a factor of 20 or 30 as the temperature is

reduced from +25°C to −30°C). The value of these capacitors

may be increased without limit.

www.national.com

11339 Version 8 Revision 4

Print Date/Time: 2009/12/07 17:53:15

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