6

UCC283-3/-5/-ADJ

UCC383-3/-5/-ADJ

Voltage Programming and Shutdown Feature for

Adjustable Version

A typical application circuit based on the UCC383 adjust-

able version is shown in Figure 5. The output voltage is

externally programmed through a resistive divider at the

ADJ pin.

V

R

R

volts

æ

è

ç

ö

ø

÷

125

1

2

1

.

(4)

The maximum programmed output voltage is constrained

by the 9V absolute rating of the IC (this includes the

charge pump voltage) and its ability to enhance the N-

Channel MOSFET. Unless the load current is below the

3A rating of the device, output voltages above 7V are not

recommended. The minimum output voltage can be pro-

grammed down to 1.25V, however, the input voltage must

always be greater than the UVLO of the part.

The adjustable version includes a shutdown feature, lim-

iting quiescent current to 40uA typical. The UCC383 is

shutdown by pulling the CT pin to below 0.25V. As shown

in Figure 4, a small logic level MOSFET or BJT transistor

in parallel with the timing capacitor can be driven with a

digital signal, putting the device in shutdown. If the CT

pin is not pulled low, the IC will internally pull up on the

pin enabling the regulator. The CT pin should not be

forced high, as this will interfere with the short circuit pro-

tection feature. Selection of the timing capacitor is ex-

plained in

Short Circuit Protection.

The adjustable version can be used in applications re-

quiring remote voltage sensing (i.e. monitoring a voltage

other than or not directly tied to the VOUT pin). This is

possible since the inverting input of the voltage amplifier

(see Block Diagram) is brought out to the ADJ pin.

Thermal Design

The Packing Information section of the data book con-

tains 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 includes two modes of

operation, normal and pulsed mode. In normal opera-

tion, the linear regulator and heat sink must dissipate

power equal to the maximum forward voltage drop multi-

plied by the maximum load current. Assuming a constant

current load, the expected heat rise at the regulator’s

junction can be calculated as follows:

()

TP

RISE

DISS

jc

ca

()

qq

q

=·

+

°C

(5)

Where theta, (

power dissipated. The thermal resistance of both the

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

grees Celsius 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 degrees Celsius per Watt. If a

lower thermal resistance is required by the application,

the device heat sinking would need to be improved.

0.2

0.3

0.4

0.5

0.6

-40

10

60

TEMPERATURE (°C)

Vout = 3.3v

Vout = 5v

Figure 4. Typical dropout voltage vs. case

temperature with a 3A load

0

0.1

0.2

0.3

0.4

0.5

01

23

LOAD CURRENT (AMPS)

Figure 5. Typical application for the 5 pin adjustable

version.