UCC283-3, UCC283-5, UCC283-ADJ, UCC383-3, UCC383-5, UCC383-ADJ

LOW DROPOUT 3A LINEAR REGULATOR FAMILY

SLUS215B − OCTOBER 1998 − REVISED FEBRUARY 2004

9

POST OFFICE BOX 655303

APPLICATION INFORMATION

thermal design

The Package Information section of the Power Supply Control Products Data Book, Volume 3 (Literature

No. SLUD003) contains reference material for the thermal ratings of various packages. The section also

includes an excellent article Thermal Characteristics of Surface Mount Packages, that is the basis of the

following discussion.

Thermal design for the UCC383 family of linear regulators includes two modes of operation, normal and pulsed

mode. In normal operation, the linear regulator and heat sink must dissipate power equal to the maximum

forward voltage drop multiplied by the maximum load current. Assuming a constant current load, the expected

heat rise at the regulator’s junction can be calculated as follows:

t

RISE

(

q) + P

DISS

q

jc ) qca

°C

Where theta, (

TO−220 and TO−263 packages (junction to case) is 3

°C per Watt. In order to prevent the regulator from going

into thermal shutdown, the case to ambient theta must keep the junction temperature below 150

°C. If the LDO

is mounted on a 5-square inch pad of 1-ounce copper, for example, the thermal resistance from junction to

ambient becomes 60

°C per Watt. If a lower thermal resistance is required by the application, the device heat

sinking would need to be improved.

When the UCC383 regulator is in pulsed mode due to an overload or short-circuit in the application, the

maximum average power dissipation is calculated as follows:

P

PULSE(avg) +

V

IN *

V

OUT

I

CL

t

ON

33

t

ON

Watts

As seen in Equation 6, the average power during a fault is reduced dramatically by the duty cycle, allowing the

be significant, the thermal mass of the package generally keeps the junction temperature from rising unless the

ripple rejection

Even though the UCC383 family of linear regulators are not optimized for fast transient applications (Refer to

the UC182 Fast LDO Linear Regulator), they do offer significant power supply rejection at lower frequencies.

Figure 6 depicts ripple rejection performance in a typical application. The performance can be improved with

additional filtering.

(5)

(6)