10

UCC284-5/-12/-ADJ

UCC384-5/-12/-ADJ

R3 along with C2 set the time that Q1 is allowed to be

on. Since TM is the maximum amount of time that Q1

should be allowed to stay on, an added safety margin

may be to use 0.9

• TM instead. This will ensure that Q1

is turned off in the proper amount of time. With a chosen

value for C2, R3 can be calculated as follows:

()

()

()

()

R

T

CF

n

VV

VV

M

IN

IN

3

09

21

16

=

•

•• −

−

















.sec

.

l

Ohms

(8)

After the CT capacitor has charged up for a time equal to

0.9

• TM , Q1 will turn off and allow the SD/CT pin to be

pulled back to –1.5V with respect to GND through a 50k

resistor. At this point , the SD/CT pin can be used by the

UCC384 overcurrent timing control.

Although it may desirable to have as short a delay time

as possible, a small portion of this delay time is fixed by

the UCC384 and cannot be shortened. This is shown in

Fig. 13, where the CT capacitor has been removed from

the circuit completely, giving a fixed VIN to VOUT delay

of approximately 150

µs for a circuit with VIN = –6V and

VOUT = –5V.

Thermal Design

The Packaging Information section of this data book con-

tains reference material for the thermal ratings of various

packages. The section also includes an excellent article

entitled

Thermal Characteristics of Surface Mount Pack-

Thermal design for the UCC384 includes two modes of

operation, normal and pulsed. In normal mode, 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

P

jc

ca

C

RISE

DISS

=•

+

°

θθ

(9)

power dissipated. The junction to case thermal resis-

tance (

der to prevent the regulator from going into thermal

shutdown, the case to ambient thermal resistance (

must keep the junction temperature below 150

°C. If the

UCC384 is mounted on a 5 square inch pad of 1 ounce

copper, for example, the thermal resistance (

comes 40-70

°C/W. If a lower thermal resistance is re-

quired by the application, the device heat sinking would

need to be improved.

When the UCC384 is in a pulsed mode, due to an

overcurrent condition, the maximum average power dis-

sipation is calculated as follows:

()

()

[]

()

()

()

P

VV

V

V

I

A

T

T

Watt

AVE

IN

OUT

PEAK

ON

ON

=

−•

•

•

















sec

sec

40

s

(10)

As seen in equation 10, the average power during a fault

is reduced dramatically by the duty cycle, allowing the

heat sink to be sized for normal operation. Although the

significant, the thermal mass of the package will gener-

ally keep the junction temperature from rising unless the

APPLICATION INFORMATION (cont.)

Figure 13. VIN to VOUT delay with CT capacitor

removed.

UNITRODE CORPORATION

7 CONTINENTAL BLVD. • MERRIMACK, NH 03054

TEL. (603) 424-2410 FAX (603) 424-3460