®

REG1118

4

APPLICATIONS INFORMATION

Figure 1 shows the basic hookup diagram for the REG1118.

An output capacitor is required for proper operation and to

improve high frequency load regulation. A high quality

capacitor should be used to assure that the ESR (effective

series resistance) is less than 0.5

Ω. A capacitor of at least

1

µF is recommended. Increasing C

voltage deviation, see the typical curve “Load Transient

Response.” It is suggested that the input be decoupled with

10

µF low ESR capacitor.

ambient temperature above that expected in normal opera-

tion until the device’s thermal shutdown is activated. If this

occurs at more than 40

°C above the maximum expected

ambient temperature, then the T

during normal operation.

SOLDERING METHODS

The REG1118 package is suitable for infrared reflow and

vapor-phase reflow soldering techniques. The high rate of

temperature change that occurs with wave soldering, or hand

soldering can damage the REG1118.

FIGURE 1. Basic Connections.

THERMAL CONSIDERATIONS

The REG1118 has current limit and thermal shutdown

circuits that protect it from overload. The thermal shutdown

activates at approximately T

eration, however, the junction temperature should not be

allowed to exceed 125

°C. Any tendency to activate the

thermal shutdown in normal use is an indication of an

inadequate heat sink or excessive power dissipation. The

power dissipation is equal to:

The junction temperature can be calculated by:

where T

θ

The REG1118 derives heat sinking from conduction through

its copper leads, especially the large mounting tab. These

must be soldered to a circuit board with a substantial amount

of copper remaining (see Figure 2). Circuit board traces

connecting to the tab and the leads should be made as large

as practical. Other nearby circuit traces, including those on

the back side of the circuit board, help conduct heat away

from the device, even though they are not electrically connected.

Make all nearby copper traces as wide as possible and leave

only narrow gaps between traces. In addition, multiple

contacts to internal power and ground planes increase heat

sinking with minimal increase in circuit board area.

Table I shows approximate values of

θ

board and copper areas. Nearby heat dissipating compo-

nents, circuit board mounting conditions and ventilation can

dramatically affect the actual

θ

A simple experiment will determine whether the maximum

recommended junction temperature is exceeded in an actual

circuit board and mounting configuration: Increase the

FIGURE 2. Circuit Board Layout Example.

Total Area: 50 x 50mm

Without back-side copper:

35 x 17 mm

16 x 10 mm

16 x 10 mm

θ

With solid back-side copper:

θ

BACKSIDE

TOTAL PC BOARD

COPPER

COPPER

THERMAL RESISTANCE

AREA

AREA

AREA

JUNCTION-TO-AMBIENT

46

°C/W

47

°C/W

49

°C/W

051

°C/W

053

°C/W

55

°C/W

058

°C/W

059

°C/W

067

°C/W

72

°C/W

085

°C/W

NOTE: (1) Tab is attached to the topside copper.

TABLE I.

INSPEC Abstract Number: B91007604, C91012627

Kelly, E.G. “Thermal Characteristics of Surface 5WK9

Ω

Packages.” The Proceedings of SMTCON. Surface Mount

Technology Conference and Exposition: Competitive Surface

Mount Technology, April 3-6, 1990, Atlantic City, NJ, USA.

Abstract Publisher: IC Manage, 1990, Chicago, IL, USA.

REG1118

V

O

V

IN

10µF

4.7µF